The relationship between nitrogen, auxin and cytokinin in the growth regulation of rice (Oryza sativa L.) tiller buds

Nitrogen (N), auxin and cytokinins (CTKs) play important roles in regulating the growth of rice tiller buds. However, little is known about the underlying mechanisms and the relationships between them in the regulation of tiller bud growth in rice. In the present study, N and auxin indole-3-acetic acid (IAA) were used to regulate tiller bud growth in Nanjing 44 rice, a japonica cultivar. The tiller buds grew rapidly after external application of 40 mg L N; significantly elevated CTK levels were found in tiller buds and nodes preceding bud growth. However, external application of IAA inhibited the growth of tiller buds and reversed the increase in CTK levels in tiller buds and nodes induced by N. According to previous studies, the rice adenosine phosphate isopentenyltransferase (OsIPT) genes encode a key enzyme in CTK biosynthesis. To determine the site of CTK synthesis involved in bud growth, we examined the expression patterns of the OsIPT genes in tiller buds, tiller nodes and roots. Our results indicate that the CTKs that promote tiller bud growth are biosynthesised mainly in the tiller nodes, and there is little or no contribution from the roots. N and IAA regulated the expression of the OsIPT genes and inhibited the biosynthesis of CTKs in the tiller nodes, thus regulating the growth of rice tiller buds.