Artificial polyploidy in improvement of horticultural crops

Polyploids are organisms that have more than two paired homologous sets of chromosomes. Polyploidy has played a significant role in the evolution and diversification of higher plants. Artificial polyploidization has been induced using a few antimitotic chemicals such as colchicine, trifluralin and oryzalin. The type of mitotic inhibitors and their concentration and duration time are variable and species dependent. This technique can be applied ex vitro or in vitro, which tissue culture is simpler and more efficient. Changes in nuclear DNA content, gene expression and developmental processes due to ploidy manipulation can lead to morphological, anatomical and physiological changes in polyploid plants. In general, polyploid plants exhibit larger organs, greater biomass, higher yield, superior tolerance to biotic and abiotic stresses as well as higher primary and secondary metabolites. Also, polyploidy often reduce the fertility and allows the production of seedless fruits. In ornamental plants, increasing the size of polyploid flowers is aesthetically and economically important. There are two direct and indirect methods for ploidy determination of plants. Indirect methods are simpler but more inaccurate, involving the relationship between ploidy level with morphological (i.e. plant height, leaf size, and pollen diameter) and anatomical (i.e. stomatal frequency and size as well as chloroplast number in guard cells) properties. In contrast, direct assay methods, such as chromosome counting in mitotic cells of root-tips and flow cytometry are accurate techniques for the determination of ploidy level in plants. Overall, polyploidy manipulation has long been used in the improving yields of many crop plants and can be considered as one of the most promising tools in plant breeding programs.