Greater mesophyll conductance and leaf photosynthesis in the field through modified cell wall porosity and thickness via AtCGR3 expression in tobacco

Mesophyll conductance (g m) describes the ease with which CO 2 passes from the sub-stomatal cavities of the leaf to the primary carboxylase of photosynthesis, Rubisco. Increasing g m is suggested as a means to engineer increases in photosynthesis by increasing [CO 2] at Rubisco, inhibiting oxygenation and accelerating carboxylation. Here, tobacco was transgenically up-regulated with Arabidopsis Cotton Golgi-related 3 (CGR3), a gene controlling methylesterification of pectin, as a strategy to increase CO 2 diffusion across the cell wall and thereby increase g m. Across three independent events in tobacco strongly expressing AtCGR3, mesophyll cell wall thickness was decreased by 7%-13%, wall porosity increased by 75% and g m measured by carbon isotope discrimination increased by 28%. Importantly, field-grown plants showed an average 8% increase in leaf photosynthetic CO 2 uptake. Up-regulating CGR3 provides a new strategy for increasing g m in dicotyledonous crops, leading to higher CO 2 assimilation and a potential means to sustainable crop yield improvement.