Professor Academia Sinica Taipei, Taipei, Taiwan (Republic of China)
Besides the late embryogenesis abundant (LEA) proteins, two groups of intrinsically disordered proteins, i.e., Repetitive Proline-Rich Proteins (RePRP) and Rice Big Grain protein (RBG), have been discovered to be involved in conferring tolerance to abiotic stresses in rice. RePRP reduces water loss by decreasing stomata conductance in shoot. In addition, RePRP enhances the level of extracellular water barriers such as lignin and suberin, primarily in the root vascular bundle. Several groups of genes involved in lignin biosynthesis, especially the wall-bound peroxidase responsible for the final assembly of the lignin network, are induced by RePRP. Furthermore, overexpression of RePRP leads to lowered root osmotic potential, and the protein levels of two aquaporins important for drought stress tolerance are elevated. Hence, ABA/stress-induced RePRP expression leads to several beneficial traits of drought resistance, including lower water loss rate upon dehydration and higher root water use efficiency under drought conditions. Rice Big Grain 1 (RBG1) regulates grain and organ development, as well as abiotic stress tolerance. Ectopic expression of RBG1 leads to significant increases in the size not only of grains but also other major organs such as roots, shoots and panicles. Increased grain size is primarily due to elevated cell numbers rather than cell enlargement. RBG1 is preferentially expressed in meristematic and proliferating tissues. Ectopic expression of RBG1 also increases auxin accumulation and sensitivity, which facilitates root development, particularly crown roots. Ectopic expression of RBG1 regulated by a specific constitutive promoter, GOS2, enhances harvest index and grain yield in rice. It appears that RBG1 regulates two distinct and important traits in rice, namely grain yield and stress tolerance, via its effects on cell division, auxin and stress protein induction.