Fruit Development in Sweet Cherry: Comparison
Please note this is a comparison between Version 1 by Andrew John Simkin and Version 3 by Sirius Huang.

Fruits are an important source of vitamins, minerals and nutrients in the human diet. They also contain several compounds of nutraceutical importance that have significant antioxidant and anti-inflammatory roles. Cherries contain high concentrations of bioactive compounds and minerals, including calcium, phosphorous, potassium and magnesium, and it is, therefore, unsurprising that cherry consumption has a positive impact on health. The sweet cherry fruit is a drupe—an indehiscent fruit of 1–2 cm in diameter (in some cultivars the diameter can be larger) that has an attractive appearance due to its color (bright red to dark purple depending on the cultivar) and desirable, intense flavor.

  • tree fruit
  • fruit ripening
  • rootstock
  • Prunus avium

1. Introduction

Cherry (Prunus avium) is believed to be native to Europe and southern Asia [1] and to a small isolated region in the western Himalayas. Commonly called sweet cherry, it is a deciduous tree, 15–32 m in height and with a trunk up to 1.5 m in circumference. It belongs to the Rosaceae family, which includes many plants, such as rose, apple (Malus x domestica), peach (Prunus persica) and strawberry (Fragaria vesca and F. ananassa) that are very important for the human economy. The sweet cherry genome is diploid (2n = 16), simple, compact and about 350 Mb in size. The genome structure is predicted to be similar to that of the peach, although the sequences have diverged [2][3][4][2,3,4] See (http://www.rosaceae.org accessed on 1 June 2022) [5].
Young cherry trees show strong apical dominance with a straight trunk and symmetrical conical crown that becomes rounded to irregular for old trees. All parts of the plant, except for the ripe fruit, are slightly toxic, due to the presence of cyanogenic glycosides. The species has also become naturalized in North America and Australia since it is largely cultivated in these regions.

2. Cherry from Flower to Fruit

2.1. Cherry Flower Pollination

The sweet cherry flower is hermaphroditic; possessing both female and male reproductive organs. Each flower is approximately 2.5 cm in diameter, with five petals surrounded by five green sepals, a single upright pistil with an ovary, two ovules, and 30 stamens. The stamens are the male reproductive organs consisting of anthers, where the pollen develops; it sits on top of the stalk-like filaments, which allows water and nutrients to reach the anthers from the mother plant and facilitate pollen dispersal [6][7][6,7]. The pistil or gynoecium is the female reproductive organ, and it occupies the centre of the flower. The flowers open for between three and five days and the stigma is receptive to pollination at this time. The anthers begin to open shortly after the flower and continue into the second day [8]. In sweet cherry, it takes two to three days for the pollen tube to grow from the stigma to the base of the style, whilst fertilization occurs six to eight days after pollination [9][10][9,10]. Stösser and Anvari [11] determined that effective pollination takes between 4 and 5 days; however, under some conditions, effective pollination can last up to 13 days [12]. There are a number of varieties of sweet cherry that exhibit self-fertility, the first and most notable being ‘Stella’, which was first identified in Canada [13], and is highly sought-after and considered a cultivar of great importance [14][15][16][14,15,16]. However, most sweet cherry cultivars exhibit self-incompatibility (SI), a characteristic that involves inhibition of pollen tube growth, and is genetically controlled by multiple allelic S-loci [17][18][19][20][17,18,19,20]. SI sweet cherry varieties still produce a small number of fruit through self-pollination; however, different varieties are considered to have different levels of self-sterility, with important commercial varieties, such as ‘Kordia’ and ‘Regina’ being highly incompatible and setting virtually no fruit in the absence of cross-pollination [21]. The nectar of sweet cherry is rich in sugars (from 28% to 55% sugar), the most abundant of which are fructose, glucose and sucrose, which are highly attractive to pollinating insects, including bees [22]. It is well known that insect-mediated pollination in sweet cherry is important for the production of a viable crop and besides the environmental conditions, pollinating insects are the most important factor governing yield [23]. Work by Holzschuch et al. [24] showed that bagged flowers produced only 3% of the number of fruits produced by unbagged flowers and that the rates of pollination and fruit set were related to wild bee visitation. Wild pollinators, including solitary bees, have been described as being instrumental in achieving adequate sweet cherry yields [25]. These authors also showed that pollination by wild bees surpassed pollination by honeybees. However, even with this knowledge, less than 17% of growers provide trap nests for solitary bees in their orchards and rely on commercial domesticated honeybees (Apis mellifera) to carry out this function at a cost up to 1000 Euro per hectare—a considerable investment for commercial cherry producers. A recent review has outlined grower knowledge of the role of insects on sweet cherry crops [23]. Air temperature is also known to influence flowering and fruit sets and has no influence on the level of SI; however, the air temperature may influence pollen tube growth [26]. The optimal temperature for flowering has been reported to be around 20 °C, with temperatures as low as 15 °C slowing the disappearance of the embryo sac, as compared to 25 °C [27]. Temperatures exceeding 30 °C are considered too high for successful flowering [26][28][29][26,28,29]. Furthermore, it has been reported that a sudden fall in temperature at the end of the flowering period can result in a total loss of the crop [21].

2.2. The ABCDE and the Floral Quartet Models

Flower organs are arranged in whorls, with sepals that are located in the most external whorl and carpels in the inner whorl, in the form of petals and stamens. Floral homeotic mutants, i.e., mutants with an altered floral organ identity, such as in Arabidopsis thaliana and Antirrhinum majus (snapdragon), have been studied and used to propose the ABC model of flower formation [30][31][30,31]. ABC genes encode three different classes of MADS-domain transcription factors involved in the flower organ identity determination. These factors interact to give the organ identity to sepal, petals, stamens and carpel. Further studies showed the existence of two other classes: the D class, which is responsible for the ovule identity, and the E-class, which has been proposed as another class of redundant floral organ identity genes [32]. This enhanced ABCDE model postulates that sepals are specified by A + E, petals by A + B + E, stamens by B + C + E, carpels by C + E and ovules by C + D + E (Figure 12) [32][33][34][35][32,33,34,35].
Figure 12. Floral quartet model (FQM) integrated into the ABCDE model proposed to explain the flower whorls’ identity determination in Arabidopsis thaliana. This enhanced ABCDE model postulates that sepals are specified by A + E, petals by A + B + E, stamens by B + C + E, carpels by C + E and ovules by C + D + E. Class-A protein: APETALA1 (AP1); Class-B proteins: PISTILLATA (PI) and APETALA3 (AP3); Class-C protein: AGAMOUS (AG); Class-D proteins: SEEDSTICK (STK) and SHATTERPROOF (SHP); Class-E protein: SEPALLATA (SEP).
In addition, the floral quartet model (FQM, Figure 12) has been suggested to integrate the ABCDE model [32]. According to the FQM, the factors form tetrameric complexes to carry out their functions, binding the DNA to activate or repress the expression of their target genes [32]. In particular, two MIKC-type MADS-box transcription factors belonging to the same class form a dimer, which can interact with another dimer of the same class or of a different class, to promote the development of a specific floral organ [35]. The MIKC-type MADS-box transcription factors are composed of specific domains:
  • MADS (M) domain: a DNA-binding domain, but it is also involved in dimerization and in nuclear localization. It is the most conserved domain among the MADS-box transcription factors [36].
  • ].
  • MADS (M) domain: a DNA-binding domain, but it is also involved in dimerization and in nuclear localization. It is the most conserved domain among the MADS-box transcription factors [36].
  • Intervening (I) domain: takes part in the selective formation of DNA-binding dimers. It shows only limited conservation
  • [37].
  • Keratin-like (K) domain: an essential element for dimerization and multimeric complex formation. This domain has a particular structural organization (amphipathic helices) in which the hydrophobic and charged residues are conserved and regularly spaced [38
  • Intervening (I) domain: takes part in the selective formation of DNA-binding dimers. It shows only limited conservation [37].
  • ]
  • Keratin-like (K) domain: an essential element for dimerization and multimeric complex formation. This domain has a particular structural organization (amphipathic helices) in which the hydrophobic and charged residues are conserved and regularly spaced [38,
  • C-terminal (C) domain: is somewhat variable. In some cases, it takes part in the transcriptional activation of the target genes or in the formation of multimeric complexes [37].
According to the FQM, MADS-box transcription factors form dimers, which bind the DNA where the CArG-boxes are present. The two dimers that are present in the tetramer recognize and bind the two different CArG-boxes—which can be far away in the DNA sequence—by looping the DNA and making them spatially close [32][35][40][32,35,40].

3. Cherry Fruit Development

3.1. Cherry Fruit

The sweet cherry fruit is a drupe—an indehiscent fruit of 1–2 cm in diameter (in some cultivars the diameter can be larger) that has an attractive appearance due to its color (bright red to dark purple depending on the cultivar) and desirable, intense flavor. Three parts can be identified in a drupe: the outer exocarp or skin; the mesocarp, which is the fleshy part of the fruit; and a single central stone, which is the lignified endocarp that surrounds the seed. Drupe development in cherry fruit is consistent with the reported stages of growth in other drupes, as described below. Cherry varieties can be divided into early, mid and late ripening types (Table 1) based on the ripening of the reference variety at the European level, Burlat, a widespread cultivated cherry across Europe.
Table 1. Some of the most commonly commercially cultivated varieties of sweet cherry. Early, mid and late ripening types based on the ripening of the reference variety, Burlat, a widespread cultivated cherry across Europe. Comparisons to the variety Bing are based on interviews with growers in Stanislaus County, California and on published data. NR—not reported.
  • C-terminal (C) domain: is somewhat variable. In some cases, it takes part in the transcriptional activation of the target genes or in the formation of multimeric complexes
  • [
  • 37
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