Development of Cowpea (Vigna unguiculata) Mutant Lines for Dissecting Resilience to Drought Through Physiological and Molecular Crosstalk Analysis

Cowpea (Vigna unguiculata) is a legume crop widely grown in tropical and subtropical regions for human consumption and animal feeding. Despite this importance, drought stress is a major constraint on cowpea production, affecting plant growth, seed quality, and yield. Thus, this study aimed to understand the mechanisms controlling drought tolerance in cowpea. To achieve this goal, two cowpea varieties (Ndout violet pods and Bambey 21) and one mutant line (Me51M4-39M9) were mutagenized using gamma rays. At M6, the seeds of the most productive and vigorous plants were identified, leading to the selection of 33 genotypes. They were sown and grown in the field to develop M7; then, the plants were subjected to drought stress for 38 days at an average daily temperature of 34.5 °C. The analyses focused on physiological parameters such as chlorophyll a and b and total contents and relative water content. In addition, malondialdehyde and proline contents; catalase, ascorbate peroxidase, and superoxide dismutase activities; and the expression of the VunP5CS and VubZip09 genes were quantified. A wide range of variability of relative water content was observed among the mutant lines subjected to drought stress. Chlorophyll a and b and total contents varied under drought but increased in the stay-green genotype (4), while proline content increased significantly in some genotypes such as line 10 but decreased in the other mutant lines under drought. The amount of malondialdehyde decreased in some mutant lines and increased in others under drought stress in comparison to the unstressed control. The activities of catalase and ascorbate peroxidase increased under drought stress in mutant lines 4 (stay green) and 15 (early flowering). Based on the RT-qPCR analysis, proline and the bZIP transcription factor genes were highly expressed under drought in tolerant genotypes 4 and 15. This study revealed that cowpea responds to drought stress through complex processes involving multiple physiological, biochemical, and transcriptomic changes that need to be explored in depth to enhance our understanding of the genetic basis controlling drought tolerance.