Physiological and genetic control of photosynthetic CO2 flux in maize leaves
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The carbon-concentrating mechanism of C4 photosynthesis requires a complex set of anatomical and biochemical adaptations. Although there is growing interest in using components of C4 photosynthesis for enhancing photosynthetic efficiency in non-C4 crops, questions remain about the genetic and physiological factors driving resource-use efficiency in C4 photosynthesis. Therefore, detailed genetic and physiological characterizations of the individual components of the C4 pathway and their relationship with water-use efficiency is necessary to advance such breeding efforts. This dissertation describes some of the genetic and physiological factors that control CO2 flux into the C4 pathway. First, a diverse group of maize lines was used to characterize biochemical and transcriptomic diversity as it relates to variation in carbon isotope composition (d13C). Although many factors may contribute to variation in d13C, we hypothesized that stomatal and mesophyll conductance (defined as the movement of CO2 from the intercellular air spaces to the site of carboxylation) likely drive the majority of intraspecific variation, supporting previous suggestions that d13C could be used to screen for water-use efficiency. Second, a subset of these diverse lines was used to characterize variation in mesophyll conductance and its transient responsiveness to CO2. No intraspecific variation was observed, but a significant response to CO2 was found using multiple methods of estimating mesophyll conductance. These results highlighted the importance of future work on mesophyll conductance in C4 plants, and demonstrated that larger mesophyll conductance could drive increased water-use efficiency. Finally, the function of carbonic anhydrase (CA) was explored using RNA-seq on maize CA knockout plants. We observed that CA mutants experience low carbon stress, but are not able to enhance photosynthesis through redundant CA genes or other photosynthetic genes. Furthermore, our results support a role for CA in the CO2 signaling pathway in maize guard cells. Overall, this dissertation advanced the understanding of carbon isotope composition, mesophyll conductance, and carbonic anhydrase in C4 plants.