The role of cytokinin in ovule development in Arabidopsis

The life cycle of higher plants alternates between the haploid gametophyte and diploid sporophyte. The female gametophyte (FG), surrounded by the sporophyte, develops within the ovule and orients along the chalazal/micropylar axis. This polarity is important in cell specification and development for both the ovule and FG. Previously, cytokinin was shown to act in the sporophytic tissue to regulate FG development. 1,2 In the highlighted study, 3 we further showed that enriched cytokinin signaling in chalaza, the central domain of the ovule, is required for the specification of the functional megaspore, which usually occurs in the chalazal-most megaspore after meiosis. The restricted cytokinin signaling in the chalaza is achieved by localized cytokinin biosynthesis and perception. Here, we discuss the implications of this and other studies for the understanding of the role of two-component signaling in FG development and the genetic and cellular interactions between gametophytic and sporophytic cells. Further, we show that cytokinin-deficient mutants display distorted cell morphology in the inner integument and elevated mitotic activity in the maternal sporophyte. These results suggest that cytokinin negatively regulates cell proliferation in the sporophytic tissues surrounding the developing FG, consistent with previous results indicating that cytokinin deficiency causes an increase in the number of cells in the embryos and consequently an enlarged seed size.