Photosynthesis and Photoprotection Capacity in Sorghum Bicolor: Discovery of Genetic Mechanisms Using Natural Variation.

Sorghum (Sorghum bicolor L. Moench) is an important C4 species produced worldwide as a grain, forage and biofuel crop. Its tropical origin and consequent natural adaptation to warm temperatures has haltered its cultivation in northern latitudes with frequent cold events in early spring. Photosynthesis is the basis of crop productivity and requires light energy but any environmental stress, such as a cold event, will limit the amount of fixed CO₂ and trigger photoprotection mechanisms to dissipate or reduce absorption of excessive energy that might cause cell damage. Considering the importance of photosynthesis as a determinant of yield and the importance of northern adaptation of sorghum, we have explored the natural variation in photosynthetic capacity under non-stress and cold stress conditions and we used that variation to discover markers/genomic regions that control these traits. A genome-wide association study (GWAS) was conducted for eleven photosynthesis and chlorophyll fluorescence parameters under three consecutive temperature treatments: control (28°C/24°C), cold stress (15°C/15°C) and recovery (28°C/24°C). In general, cold stress drastically reduced the photosynthetic performance of sorghum lines and significant phenotypic variation was observed for all traits. We discovered a total of 226 regions associated with at least one of the traits in a particular temperature period, or when traits were integrated over treatments. Several candidate genes were identified in those regions with predicted functions related to carotenoids, cyclic electron flow around PSI (CEF), components of PSI, phytohormones, thioredoxin and antioxidants. Eight regions on chromosomes 6, 8, 9 and 10 are the most important discoveries for future validation studies and with potential application for the improvement of crop productivity under non-stress and cold stress conditions.