Partitioning Apomixis Components to Understand and Utilize Gametophytic Apomixis

Apomixis is a method of reproduction to generate clonal seeds and offers tremendous potential to fix heterozygosity and hybrid vigor. The process of apomictic seed development is complex and comprises three distinct components, viz., apomeiosis (leading to formation of unreduced egg cell), parthenogenesis (development of embryo without fertilization) and functional endosperm development. Recently, in many crops, these three components are reported to be uncoupled leading to their partitioning. This review provides insight into the recent status of our understanding surrounding partitioning apomixis components in gametophytic apomictic plants and research avenues that it offers to help understand the biology of apomixis. Possible consequences leading to diversity in seed developmental pathways, resources to understand apomixis, inheritance and identification of candidate gene(s) for partitioned components, as well as contribution towards creation of variability are all discussed. The potential of Panicum maximum, an aposporous crop, is also discussed as a model crop to study partitioning principle and effects. Modifications in cytogenetic status, as well as endosperm imprinting effects arising due to partitioning effects, opens up new opportunities to understand and utilize apomixis components, especially towards synthesizing apomixis in crops

Phenotype study of multifoliolate leaf formation in Trifolium alexandrinum L.

Background. The genus Trifolium is characterized by typical trifoliolate leaves. Alterations in leaf formats from trifoliolate to multifoliolate i.e., individual plants bearing trifoliolate, quadrifoliolate, pentafoliolate or more leaflets were reported previously among many species of the genus. The study is an attempt to develop pure pentafoliolate plants of T. alexandrinum and to understand its genetic control. Methods. The experimental material consisted of two populations of T. alexandrinum with multifoliolate leaf expression i.e., interspecific hybrid progenies of T. alexandrinum with T. apertum, and T. alexandrinum genotype Penta-1. Penetrance of the multifoliolate trait was observed among multifoliolate and trifoliolate plant progenies. In vitro culture and regeneration of plantlets from the axillary buds from different plant sources was also attempted. Results. The inheritance among a large number of plant progenies together with in vitro micro-propagation results did not establish a definite pattern. The multifoliolate leaf formation was of chimeric nature i.e., more than one leaf format appearing on individual branches. Reversal to normal trifoliolate from multifoliolate was also quite common. Penetrance and expression of multifoliolate leaf formation was higher among the plants raised from multifoliolate plants. Multifoliolate and pure pentafoliolate plants were observed in the progenies of pure trifoliolate plants and vice-versa. There was an apparent increase in the pentafoliolate leaf formation frequency over the years due to targeted selection. A few progenies of the complete pentafoliolate plants in the first year were true breeding in the second year. Frequency of plantlets with multifoliolate leaf formation was also higher in in vitro axillary bud multiplication when the explant bud was excised from the multifoliolate leaf node. Conclusion. Number of leaflets being a discrete variable, occurrence of multifoliolate leaves on individual branches, reversal of leaf formats on branches and developing true breeding pentafoliolates were the factors leading to a hypothesis beyond normal Mendelian inheritance. Transposable elements (TEs) involved in leaf development in combination with epigenetics were probably responsible for alterations in the expression of leaflet number. Putative TE’s movement owing to chromosomal rearrangements possibly resulted in homozygous pentafoliolate trait with evolutionary significance. The hypothesis provides a new insight into understanding the genetic control of this trait in T. alexandrinum and may also be useful in other Trifolium species where such observations are reported.Peer reviewe

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Not AvailableIn vitro response of interaction of various growth regulators and explants was studied in Trifolium apertum (genotype EC 401712), a diploid species domesticated in Russia. The species is considered as a progenitor of T. alexandrinum, a very important fodder legume cultivated in Indian sub-continent. The callus induction was found to be dependent on level of growth regulators, auxin and cytokinin ratio and the explants. Medium ‘A’ with low level of growth regulators failed to induce callus in 5 out of 6 explants. High hormone medium ‘D’ showed good callogenesis in hypocotyl, cotyledon and petiole explants. In collar, leaf and root explants the frequency of callus induction was very low. Friable green calli were observed in hypocotyl and petiole explants. The high auxin: cytokinin ratio favoured callus induction. NAA in high concentration was found to be better than Picloram as auxin source. The differences in callus induction frequency were found to be statistically significant for both explant and media as well as their interaction. The calli from petiole explant-‘D’ medium combination showed very good organogenetic and embryogenetic potential on sub-culture. 50% of sub-cultures showed shoot induction and a large number of them showed root induction in rooting media. A suitable protocol for regeneration and transfer of plantlets in vitro in this genotype of T. apertum has been developed for first time. The cytology of the plants revealed normal meiotic pairing with 8 bivalents.Not Availabl