Auxin-regulated OsRGP1 and OsSuS are involved in gravitropic bending of rice shoot bases.

Gravitropic bending of horizontally-oriented rice shoots results from the differential elongation of cells in the upper and lower halves of the shoot bases. In this study, genes encoding a reversibly glycosylated polypeptide (OsRGP1) and a sucrose synthase (OsSuS) related to sugar metabolism were identified by suppressive subtractive hybridization (SSH) as being differentially expressed in gravibending lower and upper halves of rice shoot bases. RT-PCR was used to monitor OsRGP1 and OsSuS gene expression. The two genes were differentially induced in lower and upper halves of the shoot bases during gravitropism and their expression was regulated by auxin. Gene promoter sequence analysis demonstrated the existence of elements related to auxin. Treatment with the auxin transport inhibitor TIBA inhibited the asymmetric expression of OsRGP1 and OsSuS. In addition, an increase in hexose sugars was detected in the lower half of the shoot bases during gravitropism. Our data suggest that asymmetric redistribution of auxin following gravistimulation results in differentially localized OsRGP1 and OsSuS expression. While asymmetric expression of OsSuS may result in a disproportionate distribution of hexose, asymmetric expression of OsRGP1 could induce cell wall polysaccharide synthesis in the lower half of shoot bases. Therefore hexose and cell wall polysaccharide accumulation in the lower half of rice shoot bases might contribute to cell expansion and subsequent gravitropic bending.