Auxin inhibits the outgrowth of tiller buds in rice (Oryza sativa L.) by downregulating OsIPT expression and cytokinin biosynthesis in nodes

Auxin and cytokinin (CTK) play important roles in regulating the growth of rice tiller buds. Auxins inhibit bud growth and CTK, which are regulated by auxins, promote growth. However, little is known about the underlying molecular mechanisms. Here, we studied auxin/CTK regulation in Nanjing 44 rice, a japonica cultivar. After full heading, we cut off the roots and removed the panicles. The tiller buds grew rapidly after panicle removal; significantly elevated CTK levels were found in tiller buds and nodes preceding bud growth. However, external application of the auxin indole-3-acetic acid (IAA) inhibited the growth of tiller buds and reversed the increase of CTK levels in tiller buds and nodes induced by panicle removal. According to previous studies, the rice gene adenosine phosphate isopentenyltransferase (OsIPT) encodes a key enzyme in CTK biosynthesis. To determine the site of CTK synthesis involved in bud growth, we examined the expression pattern of OsIPT in tiller buds and nodes. Our results demonstrated that tiller bud growth is not regulated by CTK derived from roots; rather, CTK is biosynthesised mainly in the nodes and subsequently delivered to tiller buds. Auxin negatively regulates the expression of OsIPT and the biosynthesis of CTK in nodes, thus inhibiting the growth of tiller buds in rice.