SCARECROW gene function is required for photosynthetic development in maize

C4 photosynthesis in grasses relies on a specialized leaf anatomy. In maize, this Kranz leaf anatomy is patterned in part by the duplicated SCARECROW (SCR) genes ZmSCR1 and ZmSCR1h. Here we show that in addition to patterning defects, chlorophyll content and levels of transcripts encoding Golden2-like regulators of chloroplast development are significantly lower in Zmscr1;Zmscr1h mutants than in wild-type. These perturbations are not associated with changes in chloroplast number, size or ultrastructure. However, the maximum rates of carboxylation by ribulose bisphosphate carboxylase/oxygenase (RuBisCO, Vcmax) and phosphoenolpyruvate carboxylase (PEPC, Vpmax) are both reduced, leading to perturbed plant growth. The CO2 compensation point and 13C‰ of Zmscr1;Zmscr1h plants are both normal, indicating that a canonical C4 cycle is operating, albeit at reduced overall capacity. Taken together, our results reveal that the maize SCR genes, either directly or indirectly, play a role in photosynthetic development.