The α subunit of the heterotrimeric G protein regulates mesophyll CO₂ conductance and drought tolerance in rice

Mesophyll conductance g_m determines CO₂ diffusion rates from mesophyll intercellular air spaces to the chloroplasts and is an important factor limiting photosynthesis. Increasing g_m in cultivated plants is a potential strategy to increase photosynthesis and intrinsic water use efficiency (WUE_i). The anatomy of the leaf and metabolic factors such as aquaporins and carbonic anhydrases have been identified as important determinants of g_m. However, genes involved in the regulation and modulation of g_m remain largely unknown. In this work, we investigated the role of heterotrimeric G proteins in g_m and drought tolerance in rice d1 mutants, which harbor a null mutation in the Gα subunit gene, RGA1. d1 mutants in both cv Nipponbare and cv Taichung 65 exhibited increased g_m, fostering improvement in photosynthesis, WUE_i, and drought tolerance compared with wild-type. The increased surface area of mesophyll cells and chloroplasts exposed to intercellular airspaces and the reduced cell wall and chloroplast thickness in the d1 mutant are evident contributors to the increase in g_m. Our results indicate that manipulation of heterotrimeric G protein signaling has the potential to improve crop WUE_i and productivity under drought.