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 (WUEi). 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 G alpha subunit gene, RGA1. d1 mutants in both cv Nipponbare and cv Taichung 65 exhibited increased g(m), fostering improvement in photosynthesis, WUEi, 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 WUEi and productivity under drought.