Auxin and cytokinin have opposite effects on amyloplast development and the expression of starch synthesis genes in cultured Bright Yellow-2 tobacco cells

In cultured Bright Yellow-2 (BY-2) tobacco (Nicotiana tabacum) cells, the d epletion of auxin (2,4-dichlorophenoxyacetic acid) in the culture medium in duces the accumulation of starch. This is accelerated by the addition of cy tokinin (benzyladenine). Light and electron microscopic observations reveal ed that this amyloplast formation involves drastic changes in plastid morph ology. The effects of auxin and cytokinin on amyloplast development were in vestigated by adding auxin or cytokinin to cells grown in a hormone-free cu lture. Auxin repressed amyloplast development, whereas cytokinin accelerate d starch accumulation regardless of the timing of hormone addition. RNA gel -blot analysis revealed that the accumulation of the ADP-glucose pyrophosph orylase small subunit gene (AgpS), granule-bound starch synthase, and starc h branching enzyme transcripts were also affected by hormonal conditions. H igh levels of AgpS, granule-bound starch synthase, and starch branching enz yme transcripts accumulated in amyloplast-developing cells grown in auxin-d epleted conditions. Furthermore, the addition of auxin to the cells culture d in hormone-free medium reduced the level of AgpS transcripts, whereas the addition of cytokinin increased it, irrespective of the timing of hormone addition. These results suggest that auxin and cytokinin exert opposite eff ects on amyloplast development by regulating the expression of the genes re quired for starch biosynthesis.