Plant breeders modify crop traits using multiple tools, including polyploidization, to satisfy market demand. This technique creates intense phenotypes and high vigor, making it one of the most potent crop enhancement methods [38]. In addition, some plants have a specific demand for their specific traits, such as seedless fruits in grapevine and banana, which can be achieved through polyploidization [39,40]. Polyploidy results in higher heterozygosity and genome redundancy that are considered advantageous for improving crop plants over conventional plant breeding tools [41,42].

Interspecific hybridization helps to increase the diversity of crops and helps them adapt to new environments [43]. For example, Allopolyploid Triticale is a manufactured crop developed by crossing hexaploid bread wheat and rye to achieve specific goals (e.g., high yield, grain quality, less disease, and stress tolerance) [44,45]. In addition, bridge hybridization is done to transfer genes from one ploidy stage to another if a direct crossover is not feasible. Creation of diversity is one of the most important tasks to develop a crop variety where polyploidy has the efficacy to enhance crop diversity [46,47].

Polyploidy is common in newly domesticated crops [48]. In many cultivated crops, polyploidy has been observed in the speciation process and is now commonly used to create new species selected for features [3]. Unreduced gametes result in plant polyploidy and are used in crop breeding. Polyploidy increases the chromosome number, which helps plants tolerate the mutation by allelic modifications [49,50]. Chromosome deletion-related polyploidy breeding and substitution can produce targeted traits. We have seen many examples of successful cultivation influenced by polyploidy breeding. For example, seedless triploid watermelons, tetraploid red clovers, ryegrass, rye, and many ornamental plants have been developed or improved using polyploid breeding [17,51].

In summary, the main benefits of polyploidy are related to improving the use of heterozygosity. It buffers the effect of gene redundancy in mutations and, in some cases, facilitates reproduction by self-fertilization or asexual means [52]. It has a significant influence on farmers and food security issues.

The recurrence and frequency of polyploidization in plant species make polyploidization an influential research area [53] in which a major step is to select traits in plants [54]. The occurrence of polyploidy in plants was discovered about a century ago. Because of the widespread occurrence of polyploids in wild and cultivated plants, it is important for plant breeders and evolutionary biologists. In the past, antimitotic reagents-induced polyploids have not directly contributed to crop improvement. On the other hand, sexual polyploids (unreduced 2n gametes) are more relevant for crop improvement in many cases. Two pathways cause polyploids: mitotic polyploidization and meiotic polyploidization [55].

Mitotic polyploidization depends on doubling somatic tissue where homoeologous chromosome recombination occurs [56]. The first mitotic polyploidization was introduced in 1930 [57]. This activation polyploidization was tested on plants in vitro [55]. Colchicine, oryzalin, trifluralin, amiprophos-methyl, N₂O gas treatment, and caffeine have recently been used as antimitotic reagents [17]. Colchicine is an alkaloid from wild meadow saffron and was the most used as an antimitotic reagent. Oryzalin is a potent herbicide from the Dow AgroScience, USA toluidine chemical band [17,58]. Wetting roots or auxiliary buds or shoots with a colchicine solution of a specific concentration and duration resulted in the successful development of polyploids in many crop species [57,59], as shown in Figure 1. Previous studies successfully applied in vitro chromosomes doubling of colchicine and oryzalin for starch, fodder beet, ryegrass, oriental melon, watermelon, and red clover [17,60].

In vitro polyploidization showed better performance than the success rate of in vivo polyploidization in sugar and fodder beet, ryegrass, and red clover [55,59]. Table 1 provides a list of crops, vegetables, and ornamental and medicinal plants treated with chromosome antimitotic agents for chromosome duplication using different methods and protocols.

Meiotic polyploidization produces 2n gametes due to the incomplete division of chromosomes [86]. Polyploids that originate through the functioning of 2n gametes are called sexual polyploids, and their usefulness for crop improvement has been demonstrated in potato, alfalfa, and red clover. Introgression can be accomplished by recombination due to genetic crossing-over between alien chromosomes as well as the addition of alien chromosomes in the case of sexual polyploidization in allopolyploids, which is exceedingly difficult or unlikely in the case of colchicine or oryzalin induced allopolyploids. This deviation can occur in plants with normal chromosome pairing as well as in those with disturbed chromosome pairing such as homoeologous recombination of meiotic replication that was seen in Alstroemeria [87], Lilium [88] and Gasteria lutzii × Aloe aristate [89]. The process leading to the formation of 2n gamete is called meiotic nuclear restitution during micro- or megasporogenesis. Depending on the meiotic stage at which nuclear restitution occurs, different restitution mechanisms have been recognized, such as first division restitution (FDR), second division restitution (SDR) [90], and novel intermediate meiosis restitution [88]. In FDR, the non-sister chromatids are heterozygous from the centromere to the first convergence point, while preserving heterozygosity in both parents [91]. In SDR, the two sister chromatids are homozygous between the centromere and the first crossover point, and the resulting gametes have lowered heterozygosity levels compared to the parents [92]. In some cases, 2n gametes restitution cannot be classified as FDR or SDR; the word “indeterminate meiotic restitution” (IMR) has been coined to describe it [88]. Furthermore, IMR might be a widespread occurrence in allotriploids, where both bivalents and univalents are most produced.

3. Effect of Polyploidization at the Morphological and Molecular Level