Sorghum Mutagenesis: Unraveling the road towards gene discovery of C4 photosynthesis

Sorghum bicolor is a model C4 crop with high yield potential even in adverse growing conditions. C4 crops have higher nitrogen and water-use efficiency than C3 crops. The genes controlling C4 photosynthesis are largely still unknown. The goals of the study are to mutagenize sorghum cultivar BT x 623 using gamma ray irradiation and EMS, to create the M2 population for screening of loss-of-C4 function traits and subsequently, to determine the trait inheritance pattern on advance back-crossed populations. Screening of the M2 sorghum mutants revealed alterations in leaf anatomy and in photosynthetic capacity. Leaf anatomical analysis of mutants showed lower vein density (i.e. 5-6 veins/mm) compared to the wild-type (WT) which has 8-10 veins/mm. One of the cause of changes in vein spacing was an increase in the number of interveinal mesophyll cells (i.e. 4-5 cells) which are distinct characteristics of C3 leaf anatomy. On the other hand, gas exchange measurements of low-vein-density (LVD) mutants revealed a significant decrease in carboxylation efficiency (CE) as compared to WT. Likewise, the maximum photosynthetic (Pmax) rate was significantly low. These changes in CE and Pmax suggest a malfunction in the photosynthesis of LVD mutants. Inheritance of LVD trait was observed in the successive mutant generation. All BC1 F1 progenies had normal vein density like the wild types. The BC1 F2 progenies followed a mendelian segregation pattern of 1:3:: LVD:WT VD ratio indicating LVD phenotype controlled by a single recessive trait. The results of vein density screening, gas exchange analyses and trait inheritance show promising prospect for the determination of genes involved in controlling C4 photosynthesis. Currently, DNA sequencing of representative gamma and EMS mutant populations is on going to identity the factors involved.