Effect of waterlogging on carbohydrate metabolism in pigeon pea (Cajanus cajan L.): Upregulation of sucrose synthase and alcohol dehydrogenase

The objective of this study was to examine the role of root carbohydrate levels and metabolism in the waterlogging tolerance of contrasting pigeon pea genotypes. An experiment was conducted with 4 pigeon pea (Cajanus cajan) genotypes, 2 tolerant (ICPL 84023 and ICP 301) and 2 susceptible (ICP 7035 and Pusa 207) to waterlogging stress. Waterlogging treatment was given by placing pots with 25 days old plants in plastic troughs filled with water. Waterlogging resulted in decrease in leaf area, dry matter, relative water content and chlorophyll content in leaves, and membrane stability index in root and leaf tissues. The decline was greater in ICP 7035 and Pusa 207, which also suffered almost 100% mortality during recovery of 6 days waterlogged plants, than ICPL 84023 and ICP 301. Tolerant genotypes ICPL 84023 and ICP 301 showed higher total, reducing and non-reducing sugars content than ICP 7035 and Pusa 207. Waterlogging resulted in decline in total and non-reducing sugars in all the genotypes and reducing sugars in ICP 7035 and Pusa 207, while the content of reducing sugars increased in ICPL 84023 and ICP 301. The pattern of variation in reducing sugar content in the 4 genotypes was parallel to sucrose synthase activity. ICPL 84023 and ICP 301 also showed fewer declines in total and non-reducing sugars and greater increase in reducing sugar and sucrose synthase (SuSy) activity than ICP 7035 and Pusa 207. Waterlogging resulted in increase in alcohol dehydrogenase (ADH) activity, and increase was higher in ICPL 84023 and ICP 301 than ICP 7035 and Pusa 207. Gene expression studies done by RT-PCR under 24h waterlogging showed enhanced expression of ADH and SuSy in the roots of ICPL 84023, while in ICP 7035 there was no change in expression level in control or treated plants. The susceptible genotype ICP 7035 showed mutation in the CAAT box region of the ADH promoter, which could be the possible reason of lower ADH gene expression, activity and sensitivity to waterlogging. The results suggest that waterlogging tolerance of pigeon pea genotypes ICPL 84023 and ICP 301 depends on the availability of sufficient sugar reserve in the roots, activity of sucrose synthase to provide reducing sugars for glycolytic activity and ADH for the recycling of NADH for the continuation of glycolysis, the major source of energy under hypoxia. This was reflected in better RWC and Chl content in leaves, and membrane stability of leaf and root tissue in ICPL 84023 and ICP 301.