Altered cell wall hydroxycinnamate composition impacts leaf and canopy-level CO2-uptake and water-use in rice

Cell wall properties can play a major role in determining photosynthetic carbon-uptake and water-use through impacts on mesophyll conductance (CO2 diffusion from substomatal cavities into photosynthetic mesophyll cells) and leaf hydraulic conductance (water movement from xylem, through leaf tissue to stomata). Consequently, modification of cell wall properties is proposed as a major path for improving photosynthesis and crop water-use efficiency. We tested this using two independent transgenic rice lines that overexpress the rice OsAT10 gene (a “BAHD” CoA acyltransferase) which altered cell wall hydroxycinnamic acid content (greater para -coumaric acid and lower ferulic acid). Plants were grown under high and low water-levels and traits related to leaf anatomy, cell wall composition, gas exchange and hydraulics, plant biomass, and canopy-level water-use were measured. Alteration of hydroxycinnamic acid content led to significant decreases in mesophyll cell wall thickness (−14%), and increased mesophyll conductance (+120%) and photosynthesis (+22%). However, concomitant increases in stomatal conductance negated increased photosynthesis, resulting in no change in intrinsic water-use efficiency (ratio of photosynthesis/stomatal conductance). The leaf hydraulic conductance was also unchanged; however, the transgenics showed small, but significant increase in above-ground biomass (+12.5%), and canopy-level water-use efficiency (+8.8%; ratio of above-ground biomass/ water-used) and performed better under low water-level. Our results demonstrate that changes in cell wall composition, specifically hydroxycinnamic acid content, can increase mesophyll conductance and photosynthesis in C3 cereal crops like rice. However, attempts to improve photosynthetic water-use efficiency will need to enhance mesophyll conductance and photosynthesis whilst maintaining or decreasing stomatal conductance.