Abstract: Plants can coordinate the growth direction of their various organs upon the gravity stimulus. In the process of plant gravitropism, gravity sensing and gravity signal transduction have always been the focus of attention in the field of plants. The classical “starch-statolith” hypothesis proposes that plants sense gravity through the sedimentation of amyloplasts that contain starch granules. In addition, previous studies have shown that LAZY proteins regulate plant gravitropism by mediating the asymmetric distribution of auxin. However, the molecular mechanism underlying how sedimentation of amyloplasts triggers gravity signal transduction and its coordination with LAZY proteins remains unclear. Recently, Professor Haodong Chen’s team from Tsinghua University reveals that gravistimulation induces the phosphorylation of LAZY proteins via MKK5-MPK3 kinase pathway in Arabidopsis, which modulates the phosphorylation of LAZY proteins. The phosphorylated LAZY proteins can enhance their interaction with the TOC proteins on the surface of amyloplasts, leading to the enrichment of LAZY proteins on the surface of amyloplasts and the polarity relocation on the new bottom of plasma membrane. This study illustrates the molecular mechanism underlying gravity signal transduction in plants and establishes the molecular connection between gravity sensing and LAZY mediated auxin asymmetric distribution, which is a major breakthrough in the field of plant gravitropism.

Key words: gravitropism, amyloplast, polar localization, phosphorylation, “starch-statolith” hypothesis, LAZY, TOC, MPK