Leaf size is a major determinant of crop performance by influencing leaf physiological processes, such as light capture, transpiration, and gas exchange. Therefore, understanding the genetic basis of leaf size regulation is imperative for crop improvement. Natural variation in leaf size for a crop plant is a valuable genetic resource for a detailed understanding of leaf size regulation. We investigated the mechanism controlling the rice leaf length using cultivated and wild rice accessions that showed remarkable differences for the leaf features. Comparative transcriptomic profiling of the contrasting accessions suggested the involvement of Gibberellic Acid (GA), Growth Regulating Factor (GRF) transcription factors, and cell cycle in the rice leaf size regulation. Leaf kinematics studies showed that the increased domain of cell division activity along with a faster cell production rate drove the longer leaves in the wild rice Oryza australiensis compared to the cultivated varieties. Higher GA levels in the leaves of Oryza australiensis, and GA-induced increase in the rice leaf length via an increase in cell division zone emphasized the key role of GA in rice leaf length regulation. Zone-specific expression and silencing of the GA biosynthesis and signaling genes confirmed that OsGRF7 and OsGRF8 function downstream to GA for controlling cell cycle to determine the rice leaf length. The GA-GRF-cell cycle module for rice leaf length regulation might have contributed to optimizing leaf features during the domestication and could also be a way for plants to achieve leaf plasticity in response to the environment.
10The importance of increasing photosynthetic efficiency for sustainable crop yield 11 increases to feed the growing world population is well recognized. The natural genetic 12 variation for leaf photosynthesis in crop plants is an overlooked and untapped resource. The 13 genus Oryza, including cultivated rice and wild relatives, offers tremendous genetic 14 variability to explore photosynthetic differences, and underlying developmental, 15 photochemical, and biochemical basis. We quantified leaf photosynthesis and related 16 physiological parameters for ten cultivated and wild rice genotypes to identify 17 photosynthetically efficient wild rice species. Wild rice species with high leaf photosynthesis 18 per unit area had striking anatomical features, such as larger mesophyll cells with more 19 chloroplasts, larger and closer veins, and a smaller number of mesophyll cells between two 20 consecutive veins. In addition, photosynthetically efficient wild rice species showed an 21 efficient Photosystem II as well as higher carboxylation activity of Rubisco compared to less 22 efficient cultivated varieties, such as IR 64. Our results show the existence of desirable 23 variations in mesophyll and vein features, Photosystem II efficiency, and Rubisco activity in 24 the rice system itself that could possibly be targeted for higher leaf photosynthesis. Detailed 25 genetic and molecular understanding of these traits shall be instrumental in increasing 26 photosynthetic efficiency of cultivated rice. 27 28
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