Several examples document the inevitable and continuous interaction between humans and plants, primarily characterized by the human interest in using them for consumption, materials production, medical purposes, decoration, and bioremediation. An example of the most striking human manipulations on the plants’ life and environment is the establishment of urban green spaces, for instance, public parks and/or private residential gardens, with surfaces more or less limited, where plants are grown with objectives often different from those of agriculture [1]. In fact, if agriculture has aims that are almost exclusively at production, the creation of green areas within urban centers was born as a response to profoundly different problems, such as the demographic increase and the progressive urbanization of landscapes. The latter is among the dominant anthropogenic factors influencing the natural and adaptive evolution of living populations [2], and for this reason, it is the most important phenomenon affecting the environments in the 21st century ^[3][4][5][6][3,4,5,6].

Considering the multitude of urban contexts, purposes, and needs for which green spaces in cities are created, it is today very challenging to provide an exhaustive definition of ‘urban area’ and its relative ‘urban vegetation’, since the geographic, climatic, and resource-related opportunities, and constraints, are not equally distributed factors across the world and specific for each context. Furthermore, urban vegetation can also include cultural plant typology with agricultural interest related to food production, such as the horticultural species. Starting from these considerations, the present rentryview aims to provide a general overview of how genetics and genomics notions can help in the selection and improvement of plant species and/or suitable varieties for specific urban contexts. In detail, after a brief overview of the genomic tools available and potentially usable in genetic improvement plans, researcherswe focused on the evaluation of particular genetic traits for the selection of plants in relation to the urban area in which they are used and the environmental stress to which they are generally subject. Finally, researcherswe took into consideration the contribution of genetics in achieving specific goals that are usually set in urban contexts.

With a univocal classification of urban green spaces not being present ^[7][8][9][7,8,9], in order to allow a better understanding of some of the aspects covered, reswearchers judged it appropriate to simplify the classification of these environments by empirically dividing them into four macro areas, according to their surface availability per plant and their intended usage as follows:

(i)

 Urban/metropolitan parks: these are green areas in cities, and other incorporated places, that offer recreation and green spaces to residents of and visitors to the municipality;

(ii)

 Urban gardens: these are areas where urban vegetation is exploited to provide food products, especially employing horticultural species. Urban food production can be carried out by citizens or administrations in private buildings and public spaces for self-consumption, or can be performed by farms with commercial purposes, also using innovative outdoor or indoor growth systems;

(iii)

 Road verges: these are small, vegetated areas composed of grass or plants and sometimes also trees, mainly located between a roadway and a sidewalk or within roundabouts;

(iv)

 Roofs/terraces/balconies: these are small green areas located in private or public buildings. They include both surfaces partially or completely covered with vegetation (e.g., green roofs) and container gardens where plants are maintained in pots.

Each of these four categories is set up according to different purposes (ornamental, urban planning, bioremediation, soil consolidation purposes, etc.), that, in turn, affect the choice of the species/varieties to be used (i.e., woody perennial plants, meadows and turf grasses, horticultural and floricultural ornamental plants), and their adaptive characteristics (resistance to biotic, abiotic stress, hypogeal and epigeal habitus, minimum maintenance requirement, etc.). For this reason, as previously indicated, it is not easy if not impossible to define specific and common breeding programs for urban vegetation, since it includes both agricultural and not species with different genetic and physiological properties, and agronomical needs (Figure 1).

The bibliographic analysis for this work has been performed exploiting Scopus^® in order to search for articles and reviews of interest written in English by integrating different combinations of selected keywords for every paragraph, including terms defining:

(a)

The environment of interest: “urban areas”, “cities”, “green areas, “green gardens” “public green”, “public parks”, “urban agriculture”;

(b)

 The plant typologies: “plant”, “ornamental”, “flowering”, “horticultural”, “woody”, “trees”, “meadows”, “turfgrasses”;

(c)

 The genetic subject: “breeding” “molecular markers”, “marker-assisted selection” “marker-assisted breeding” “molecular selection” “genomic selection” “genomics” “genetic improvement” “variety” “cultivar”;

(d)

 Specific goals: “abiotic stress”, “heat shock”, “biotic stress”, “pathogen stress”, “water stress”, “drought”, “dwarf” “compacted”, “growth habit”, “edible flowers” “food production”, “leafy vegetation”, “baby leaf”, “phytoremediation”, “air purification”, “biodiversity”, “soil erosion”, “soil stability”, “psychosocial”, “ecosystem services”.

The lists of documents found have been then manually screened to select the most relevant works. In addition, the research has been deepened with a snowball sampling approach, exploiting citations and references included in the already screened papers.

Plant breeding is a science-driven creative process that involves the evaluation, selection and propagation of plant populations characterized by a combination of specific desirable traits [10]. Originally, conventional breeding was exclusively based on time-consuming and expensive phenotyping observations: plant selection was accomplished through massive screening of phenotypic parameters mainly related to the morphology of the plant (fruits, flowers, stem), to its tolerance to both biotic (insects, molds and viruses), and abiotic stresses (temperature, drought, salinity, heavy metals) [11].

Genetics made it possible to overcome many of the limitations associated with conventional breeding, facilitating, and speeding up the process of varietal constitution. New opportunities for crop improvement were generated thanks to the increasing availability of whole genome data that, in turn, enabled the discovery of candidate genes related to traits of interest, mutations responsible for phenotypic variability (e.g., single nucleotide polymorphisms, SNP or insertion/deletion, Indel) and molecular markers associated with traits of interest (e.g., simple sequence repeats, SSR) [12]. Overall, these molecular data are a powerful predictive tool for the targeted selection of superior genotypes. However, as their discovery is strictly dependent on the availability of sequenced genomes, for many years, their application in breeding processes has been limited to a few sequenced species of great agricultural and economic importance, such as rice, barley, tomato, and grapevine [13]. Moreover, before the advent of single molecule real-time (SMRT) sequencing (better known as third-generation sequencing platforms), the complex architecture of a genome represented another major obstacle to the assembly process. The set of sequenced genomes belonging to ornamental species and some tree families still only represents a small fraction of all genomes sequenced thus far for three main and often coexisting reasons [14]. First, in this category of plants, the high level of heterozygosity along with the abundance of GC and repetitive sequences represent a significant source of ambiguity during genome reconstruction. Second, some fruit tree species are often widespread in urban areas (e.g., the Citrus genus) [15], and several ornamental plants (e.g., Rosa, Chrysanthemum, Lilium, Mandevilla, and Primula) [16] are the result of intraspecific and interspecific crosses and ploidy manipulation. These aspects increase the probability of chimeric assemblies because of incorrect connections between scaffolds belonging to homologous chromosomes [17]. Finally, technical problems in the assembly phase are frequently encountered in species characterized by mega-genomes (i.e., genomes > 10 Gb), such as ornamental plants (e.g., peony, 13.79 Gb) [18] and tree genera typical of the urban context (e.g., Cedrus, 15–16 Gb, and Pinus, 31 Gb) [19].

The progressive collapse of sequencing costs and the release of tools able to overcome part of the technical challenges previously mentioned made HTS (High-Throughput Sequencing) platforms accessible also to minor crop species (also known as orphan crops, for review, see for example Simko et al. [20] and Bohra et al. [21]) and ornamental plants (for review, see Zheng et al. [14]). As a matter of example, the number of sequenced genomes completed yearly for ornamental plants significantly increased from 1 in 2012 to 17-20 in the last five years, and currently, the genome sequences of more than 70 ornamental species have been released [14]. Conversely, most of the tree species frequently found in urban green areas of the Mediterranean basin (e.g., Carpinus betulus, Tilia cordata, Acer platanoides, and Pinus pinea) remain unsequenced today.

Genomics and transcriptomics open a wealth of opportunities not only for large-scale plant breeding but also for small-scale cultivations, such as those in an urban environment. In Table 1 reswearchers reported the main molecular approaches currently used to connect phenotype to genotype. In fact, the ultimate goal in plant breeding is to use the genotypic information to predict phenotypes and select improved cultivars ^[22][23][22,23].

Considering the ratio between extragenic and genic regions, most variants identified through WGR or RRS are unlikely to underlie phenotypic effects. However, the combination of hundreds of noncoding allelic variants produces molecular profiles able to identify uniquely a given genotype. Such information is extensively used for screening populations and selecting parental genotypes in breeding programs (marker-assisted breeding, MAB), testing the purity and uniformity of commercial varieties, evaluating the stability of cultivars through generations, registering new varieties and for assessing their distinctiveness from preexisting varieties [33].

For large-scale crop breeding (open-field and greenhouses), the marker-assisted genetic improvement techniques mentioned above are well documented [34]. The development and incorporation of molecular markers in breeding processes could also be adopted for small-scale cultivation approaches (from urban parks to small private allotments). In the latter case, the goal would be the development of varieties with a genetic background suitable for urban needs. In this context, the contribution of genomics in identifying markers and genes responsible for plant adaptability in urban environments will play a crucial role.

In the development of any plant breeding program, it is necessary to define the desired variety’s ideotype, i.e., the ideal plant model endowed with every trait of interest for the specific aims and destinations [35]. While some adaptive characteristics of plants are shared by all types of urban vegetation, there are others that must respond to specific context conditions; thus, the ideotype can change according to the macro area considered. For example, the soil requirements of a tree grown on a roadside must necessarily be distinct from those of a tree grown in an urban park. This is both to allow the plant to grow by bypassing the stress of soil deficiency and to prevent it from causing damage to the road surface as it grows. Generally, plants must be able to cope with the environment in which they are introduced, not suffering from abiotic or biotic stresses. In fact, urban areas represent environments characterized by restrictive growing conditions due to a high density of buildings, infrastructures, and heavy traffic, which cause negative effects such as soil compaction, pollution, high temperatures, drought, lack of light, presence of heavy metals, high salinity, and nutrient deficiency [36]. The ability to cope with different kinds of stress has become even more important, considering the increasing impact that environmental changes and global warming have on plants [37]. In past years, it has been reported that abiotic stresses reduced by as much as 50% the yields of most major crops worldwide ^[37][38][39][37,38,39]. Providing varieties able to adapt to several different types of environments and resilient to stress factors is in general a major challenge in the immediate future for breeders to guarantee cultivation ^[40][41][42][40,41,42]. Moreover, plants must also be resilient to stresses to limit the usage of agronomic inputs. Modern cultivars have been mainly bred for performance under a high supply of factors such as irrigation water, fertilizers, pesticides, and interventions such as tillage and manutention [43]. Hence, the need to provide varieties environmentally and economically sustainable stands with a view of limiting costs and energy exploited for their cultivation, reducing limited resource usage, and avoiding the introduction of polluting chemical compounds into the environment ^[44][45][44,45]. In addition, to guarantee plant sustainability, it is important that they do not present potentially harmful traits for ecosystems and human health, such as invasiveness and high-allergenic compound production [46]. An important strategy to cope with the adaptability and sustainability problem is to recur to wild or traditionally used populations, diffused in areas with environmental conditions similar to those of the destination of interest. This is a key factor because, generally, these accessions are resilient to the biotic and abiotic stress typical of their diffusion area ^[47][48][47,48]. In addition, not or lowly genetically improved populations present a higher genetic variability in comparison to commercial varieties, allowing to have a wider range of alleles to be drawn on for the specific breeding aims. From this point of view, the creation and maintenance of germplasm banks containing wide collections of plant material (seeds or plants conserved in vivo, in vitro or cryo-conserved) derived from wild populations and local varieties is a strategy of great relevance for having a continued availability of genotypes with useful traits ^[49][50][49,50]. With regard to germplasm collections, it is important to point out that presenting a high genetic variability also means an increase in the possibility of having in the selected genotypes alleles linked to undesirable or even harmful traits, e.g., the susceptibility to a pathogen or the production of a toxic compound. These alleles can segregate with traits of interest (linkage drag), therefore, during the breeding process, careful work of selection against them is necessary [51,52,53].

In urban areas, the average temperature can be recorded as up to 10 °C warmer than the surrounding rural areas due to impervious surface cover, anthropogenic heat sources, and low vegetation cover. Warmer temperatures increase vapor pressure deficits, creating a greater atmospheric demand for water via transpiration and reducing soil moisture, which limits ^[51][52][53]the amount of water available to roots. In addition, heat stress negatively influences plant physiology as a consequence of protein damage, enzyme inactivation, photosynthesis inhibition, cell membrane deterioration, cell division interruption, and disturbance of reproductive phases [54,55,56]. Heat stress responses include the involvement of heat shock proteins (HSPs), which have a chaperone function, hence assisting the correct conformational folding of misformed proteins and helping in damaged protein movement and degradation [57]. In recent years, the molecular bases of high-temperature defense have been studied in the species of greatest horticultural interest and potentially usable in an urban context, including tomato (Solanum lycopersicum L.) [58,59], lettuce (Lactuca sativa L.) [60,61], spinach (Spinacia oleracea L.) [62,63], identifying specific transcription factors (heat stress factors—HSFs), HSPs or molecular markers related to them. Considering ornamental species, analogous results were obtained with Rododendron hainanense Merr. [64] and carnation (Dianthus caryophyllus L.) [65]. However, the use of molecular markers associated with HSPs or HSFs for breeding aims has received little attention outside of cereals. In chili pepper (Capsicum annuum L.) a study reported the introgression of two genes encoding HSP70 and HSP24 from a heat-tolerant breeding line to an elite commercial variety using markers and backcrosses with the ladder, without genetic transformation. For the tolerance-related gene selection, a MAS approach was conducted with two markers that were closely linked to the genes; while for the recovery of the recurrent parent line’s genome, to obtain the agronomic traits of interest, a MAB strategy was followed with a set of 250 paired SSRs [66].

Because of their generally small dimensions, the herbaceous plants for meadows and turfgrasses can be suitable for several surface availabilities, including buildings. Moreover, given the possibility of walking on them, the use of these species does not prohibit additional uses of the occupied space. The plant varieties that are the most selected for these uses generally belong to species of the Poaceae family and can be pure or derived from interspecific hybridizations [118]. Among the most common microthermal species, Lolium perenne L., Poa spp., Agrotis spp., and Festuca spp. can be found, while among macro therms, Buchloe dactiloides Nutt., Cynodon spp., Eremochloa ophiuroides Munro, and Paspalum vaginatum Sw. are present [119]. Traits of interest for turfgrasses varieties selection regard leaf fineness, rapidity in covering the surface, tolerance to trampling and mowing, slowness in sprout growing, and aesthetic aspects such as greenness and covering uniformity. Breeding programs regarding the species destinated for this use started in the 1970s and until today were carried out with continuous selection schemes, tests of progenies and intercrosses between the best-performing genotypes. For warm-season grasses, selection programs have been dominated by clonal propagation because stolons are easy for most species and interspecific progeny sterility is common [120]. Advances in genetic improvement have been relevantly focused on reducing the turf height increment rate, in finessing leaves, in wear tolerance, in environmental adaptation to different conditions and in crown-rust resistance. In contrast, the improvements that remain to be achieved are related to tolerance to other diseases, the preservation of greenness in winter and summer, and ground cover rapidity, all traits that seem to have high plasticity determined by genotype per environment (G × E) interactions. Sampoux et al. reported that the selection of plants to lessen the growth rate has probably limited the availability of the genetic resources necessary to improve the ground cover velocity and winter greenness [121].

Anthropogenic activities lead urban environments to be affected by many ecological issues of different natures, starting from pollution, which has an impact on soils as well as on water and air ^{[54][55][56][57]}[122,123,124,125]. Plants, in addition to withstanding environmental conditions and not contributing to pollution, can directly reduce the contamination already present and in general provide ecosystem services that, consequently, improve the environmental state and human health ^[58][59][126,127]. The ability to offer a beneficial effect on the territory is an aspect of great interest for urban contexts that should be considered in breeding programs and urbanistic management ^[55][123]. Indeed, the setting of green areas can improve slope stability reducing erosion ^[60][128], it can favor faunal resettlement enhancing biodiversity ^[61][129], and can reduce rainfall water surface runoff favoring infiltration into the soil ^[62][130]. In addition, it has been demonstrated that plants can limit the phenomenon of urban heat islands by mitigating thermal changes, by increasing evapotranspiration, shadowing, radiation reflection and moisture traps, hence saving energy used for cooling ^[63][64][131,132], keeping lower noise pollution ^[65][133] and reducing the CO₂ emissions absorbing the gas through leaves ^[66][134]. Another important aspect influencing the choice of a precise cultural species for specific purposes is the ability to provide plant products, particularly food, and their intended use, whether commercial or for private self-consumption. Strongly related to this last factor, surface availability is above the main determinants that can influence the possibility of having an adequate level of productivity. This is in general the largest bottleneck, as space in urban areas is often very limited and indirectly forces specific use choices and selection priorities ^[67][104]. Last, although it is not a primary concern for genetic selection plans, the supply of the psychosocial benefits that are derived from the creation of green urban spots assumes an important role in cities, where the high human presence and the reduced availability of cultivated areas are typical [135].

Aiming at exploiting limited urban spaces for commercial food production or private consumption, high attention should be given to leafy vegetables, which can provide food products needing on average less surfaces and in shorter times compared to fruit species [184,190,191]. A commercial typology of great interest nowadays and for which leafy vegetables are the most popular cultures is fresh-cut production. It consists of fruits or vegetables which have undergone minimal processing after harvest (e.g., trimming, washing, decontaminating, packaging), in order to provide ready-to-eat products maintaining their fresh state. The main issue, being the processing made before passage to the distribution chain, is the higher perishability compared to non-fresh-cut products [192]. The leafy vegetables most commonly used for fresh-cut production are spinach (Spinacia oleracea L.), kale (Brassica oleracea L. var. acephala D.C.), salad rocket (Eruca sativa Mill.), wild rocket (Diplotaxis tenuifolia L.), corn salad (Valerianella locusta L.), chicory (Cichorium intybus L.), curly endive (Cichorium endivia L. var. crispum), and lettuce, especially looseleaf varieties (Lactuca sativa L. var. acephala) [193,194]. To date, lettuce is the most popular species also in vertical farming [195]. In the last decade, breeding for fresh vegetables was mostly based on improving yields and postprocessing performance in terms of shelf life, leaving other aspects to lower priority, such as biotic and abiotic resistance. Moreover, the attention was focused mainly on tolerance to high plant density and hypoxia, post-cutting recovery ability, low core length, solid midrib and good organoleptic and nutritive profiles [192,196,197]. To select cultivars with less browning susceptibility after cutting, the use of browning activity-related enzymes and volatile molecules responsible for off-odors as metabolic markers have been proposed [196]. The market increment of the ready-to-eat products allows farms to harvest leafy vegetables at a very early maturation stage, even further reducing the crop cycle, strongly enhancing the sown density and obtaining higher yields per surface unit [194]. Baby leaf lettuces such as Green Last, althougeaf, Red Leaf, and Lollo Red, hence cultivars particularly suitable in providing quality products in a very limited growth time, have been developed and demonstrated to be better than whole-head lettuce in terms of harvest index, simplicity of being processed, oxidation after cutting reduction due to smaller stem diameter and appreciation by consumers [198]. Developing new baby leaf varieties can be very interesting to farms but also to private urban gardens for their it is not acultivation in pots and planters, especially in balconies and other tight spaces.

Innovative food primaroduction species suitable for urban areas, whose products are having a continuously increasing commercial interest, are edible flower plants [174]. Edible flowers present innovative and original organoleptic characteristics and can be highly nutritive for their nutraconceeutical values, such as the elevated content of health molecules such as antioxidants, e.g., flavonoids, phenolic acids, and alkaloids [199]. Flowerin for genetic seleg plants are already one of the first choices for small spaces in cities due to their high aesthetic effect, but species able to provide edible flowers can combine ornamental value with the supply of innovative food products. These species include begonia (Begonia x tuberhybrida Voss), rose (Rosa spp. L.), chrysanthemum (Chrysanthemum spp. L.), pansy (Viola × wittrockiana Gams), lilac (Syringa vulgaris L.), elder (Sambucus nigra L.), and Japanese wisteria (Wisteria floribunda (Willd.) DC.) [200,201]. Breeding of flowerin plansg species has focused almost uniquely on quantitative, morphological, and aesthetic aspects of flowers [45], aside from biothe supplic and abiotic stress tolerance. Good values for these traits were mainly reached using hybridization, also the interspecific trait, followed by vegetative propagation of the most performative individuals [202,203]. This has made the genetic background of varieties highly heterof the pszygous and, in some cases, allopolyploid or aneuploid. The genomic complexity, coupled with a relatively low economic importance for single species in comparison to that of open-field crops, led to severe difficulties in molecular marker development attempts and hence in starting MAB programs [202,204]. However, in recent years, the advent of NGS techosocial bennologies has made it possible to obtain whole-genome sequences or high-density linkage maps for the most important ornamental species, providing powerful tools for future genomic-based breeding applications [205,206,207,208]. Regarding edible fits that lower plants, the biochemical characterization of products was made for the most important species [209,210,211], but there is no evidence of genetic improvement programs fore derived from t the obtainment of high organoleptic and health-related molecules content as selection criteria, which can be detected with the use of specific assays or analyzing molecular markers linked to genes controlling these aspects. The improvement of these traits in cultivars, coupled with that of products’ shelf life, with flowers being particularly delicate and their appearance preservation being significantly important, is a key factor in promoting edible flower plant diffusion in cities [200].

Urban environments are known to be characterize creation of d by low levels of biodiversity, and the increase in urbanization inevitably leads to a reduction in the variability of life in ecosystems [212]. On the contrary, the setting of green areas plays an important role in conserving and also improving biodiversity, favoring the resettlement of wild plants, enhancing their local genetic diversity [213], and providing the right habitat for many animal species through the supply of suitable places for nesting and sheltering or to find food sources [214]. Improving biodiversity can spots aspositively influence the ecosystem services provided in the long term, strengthening the resilience to environmental changes [215].

One of the most endangered animal categories in cities withoumes ant plants is that of pollinating insects, being nectar and pollen primary sources for their sustenance [216,217]. To set plants in green spaces capable of attracting insects, first, it is important role in cities, wto focus on allogamous species with entomophilous pollination. Flower ornamental plants are the most effective, given the attractiveness they have to pollinators due to their flowers. Traits to be considered in plant breeding programs for developing varieties aimed at enhancing insect biodiversity comprehend flower size, morphology, color and scent, pollen and nectar production quantity, and flowering period longevity [129,218,219]. An important aspect on which many studiere the hs have focused is the role of native plants in attracting insects in comparison to exotic species, but researchers have not reached a consensus on the matter [220]. The traits reported can be gh ood indicators of insect resettlement, but to assess the potential of a variety in improving pollinator biodiversity in a specific environment, it is necessary to count and recognize flowering-visiting individuals for a few years. Garbuzov and Ratnieks [129] evaluated the ability to attract pollinators of 32 garden flowering plant varieties, couman nting them over two summers in different experimental gardens in Sussex. They noted that Agastache foeniculum (Pursh) Kuntze, Lavandula x intermedia Emeric ex Loisel. var. Gros Bleau, Edelweiss and Hidcote Giant, and Nepeta x faassenii Bergmans ex Stearn var. Six Hills Giant are the most peresence and the redformant plants for the aim. Although works on varietal improvement related to this aspect are not available in the literature yet, providing traits of interest and putative suitable species can favor their insertion into plant breeding plans aimed at enhancing biodiversity.

Despite the setting of green species is in general linked to biodiversity enhancement in an urban area, if it regards the introduced availabilittion of alien plants, there is a risk of negatively influencing the ecosystem balances due to the risk of invasiveness. In fact, specific traits of the introduced varieties or species can make these plants not only problematic for environmental management as a consequence of their high spread but also predominant in the hoarding of limited resources. This confers them a major fitness advantage in comparison to native plants, reducing the survival possibilities and the reproductive success of these last [221,222]. The competition effect can influence the present biodiversity at of cultithe interspecific level, but it may be accentuated at the intraspecific one, likely because of the more probable sharing of an ecologic niche between the introduced varieties and autochthonous populations of the same species [223]. There is a wide range of genetic traits that can make the plant invasive, ated areas are typicnd they differ depending on species and environment characteristics. However, they generally present a greater capacity for growth under specific environmental ^[68]conditions (expressible with parameters, e.g., relative growth rate, leaf area index, earliness in blooming, root system development capacity), reproductive and dispersal efficiency (e.g., seed production, flowering duration, seed dispersal distance) [224,225]. These factors often match with traits of interest; hence, it may be not possible to completely avoid the risk of enhancing the invasive potential of selected plants in breeding programs. Another important aspect that can reduce biodiversity in the case of non-native plant introduction is related to intraspecific or interspecific hybridization. In fact, progeny generated through crossing between the introduced plants and native populations of the same species or of compatible species have more chances than the parental lines in presenting competitive traits and become invasive, due to higher genetic diversity, heterosis, trait fixation or trait novelty [226,227,228,229]. Therefore, if the fertilization rate within native populations is lower than that between native plants and non-native plants or hybrids, the survival of the former is threatened. Regarding ornamentals or species for which reproductive capacity is not of interest, a possible strategy to counteract this risk is to develop sterile varieties, such as exploiting odd ploidy levels, canceling their possibility of colonizing the surrounding environment [230].

Finally, other important services supplied by green urban areas are the psychosocial benefits derived from their establishment. The sense of being in contact with nature due to the presence of plants in the surrounding environment indeed has several psychological effects on urban communities. For example, making people feel more relaxed and restored, enhancing social cohesion, and improving fitness through the promotion of outdoor physical activities as well as improvement of health through the reinforcement of the immunity system [135]. In general, it is very difficult to define which of the plant traits are useful to provide specific psychosocial benefits to urban communities, but it is known that visual factors such as flower color, morphology, and sizes, as well as the emission of scents, are appreciated not only for their decoration potential but also because of their contribution to the perception of higher psychological well-being. This regards most of all flowering ornamental plants, for which these traits are some of the most researched in current breeding programs [45].

The first real difficulty of an urban context lies in the definition and list of diversified typologies of green spaces coexisting often in neighboring areas, created with purposes ranging from niche food production to pure ornamental appearance. For these reasons, a simplification of the system itself, with the definition in macro areas, is of primary importance to focus the attention on the various factors characterizing and influencing such environments. As mentioned in the above sections, genetic/genomic approaches have largely contributed to the development of improved varieties with resilient traits of interest, and the use of molecular markers has amply demonstrated its potential, making it one of the most useful predictive tools in genetic improvement, not only for major plants or in crop science. In fact, in the post-genomics era, there is a striking abundance of genomic resources, such as genome sequence assemblies, germplasm sequencing data and gene expression atlases, for species potentially employed as urban vegetation [14].

This rentryview is intended to evaluate the potential genetic aspects that must be considered in a breeding program for varieties or species to be grown in an urban context. These characteristics must take into account both the adaptability of the plant to the specific area where it must grow and to the purposes for which it is used. As suggested by Henderson and Salt ^[69][231], studies on biodiversity, for example, through the genetic characterization of plant germplasm collections and a correct hierarchy of selectable physiological and environmental parameters, can provide a potential genetic resource from which to appeal for targeted planning. Generally, the selection and use of favorable effect alleles in breeding programs are required to enhance genetic variance and to improve the rate of genetic gain in all environmental landscapes, but even more so in an urban scenario, an integrated approach is required to realize genetic gain through the modernization of breeding programs. Thus, in such a context, the appropriate choice of parents and optimized breeding pipelines for the fixation of target alleles present potential ways to enhance breeding efficiency and developing modernized breeding programs will help realize higher genetic gains in urban contexts for developing climate changes-resilient species.

Furthermore, a research improvement in the horti-floricultural sector with new varieties to be entered into the urban market can lead to a boost in socioeconomics that encourages the spread of vegetable, flower, and ornamental species in public and private spaces, with particular reference to environments where the presence of vegetation has so far been limited. In addition, as previously described, it turns out to be an investment in terms of reduction in greenhouse gas emissions and pollution, enhancement of territorial biodiversity, protection of ecosystems, etc. More efficient development of plant varieties can also promote breeding and nursery companies, leading them to increase and diversify the range of products and initiate innovative genetic progress plans. The certification of the products obtained through registration in the official variety registers also guarantees the economic benefit, certifying the intellectual property and safeguarding against fraud.

In conclusion, as summarized in Figure 2, the characterization of the urban vegetation with the best genetic profiles represents the new green challenge in urban planning to have the right genetics in the right place. A simplification and hierarchization of the main aspects that characterize an urban context, such as the surface availability, productive destination, adaptability, sustainability, and benefits contribution (details are reported in the text), play a key role in a breeding program. The identification of more appropriate starting genetic resources, by phenotype collections and physiological traits, is fundamental for the identification of appropriate traits/donors/parents and subsequent crossing programs. The potential application of molecular MAS and MAB and computational tools for performing genetic and/or genomic-assisted selection. Data generated through these trials can be used in the selection of specific leader alleles that can be introduced in species improvement programs for the constitution of new varieties that respond in a more targeted way to different needs in an urban area.