Coralie Salesse-Smith, Ed Lochocki, Lynn Doran, Ben Haas, Sam Stutz, Steve Long

Mesophyll conductance (g_m) describes the ease with which CO₂ 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 [CO₂] 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 CO₂ 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 CO₂ uptake. Up-regulating CGR3 provides a new strategy for increasing g_m in dicotyledonous crops, leading to higher CO₂ assimilation and a potential means to sustainable crop yield improvement.