Mechanisms Underlying Root System Architecture and Gene Expression Pattern in Pearl Millet (Pennisetum glaucum)

Pearl millet could be considered a drought-tolerant crop, but the mechanisms modulating this tolerance are not extensively documented. Deciphering the physiological and molecular mechanisms underlying drought tolerance in pearl millet with regards to the root system will underpin precision in breeding. This study was conducted to unravel the mechanism of pearl millet adaptability to drought stress. A pot experiment was laid out in a 2 × 3 factorial arrangement in completely randomized design. The pearl millet variety IP14599 was exposed to three drought/control regimes: control and 25 and 50% relative water content (RWC). Rhizosheath, root system architecture (RSA) traits, gas exchange parameters, antioxidant enzymes, total soluble sugars, and gene expression profiling were evaluated at both seedling and vegetative growth stages after drought treatment. The results showed that at both growth stages, IP14599 plants had higher shoot/root dry weight, seminal roots, rhizosheath weight, and root hair length at 50% stress than those exposed to the 25% stress regime. Drought stress induced an increase in antioxidant enzymes, while the variety was better adapted at 50% drought stress and maintained higher photosynthetic efficiency compared to the control without drought stress. Four drought responsive genes (GBSS11a, FPS, ELIP, and RubSc) were identified and shown to play vital roles in the hardiness of pearl millet. The ability of the plant to adapt to drought stress conditions could be attributed to its superior rhizosheath and RSA during its growth stages. Therefore, the protocol used in this study could be reliable for evaluation of other cereal crops for their RSA characteristics and to validate the functions of drought-responsive genes in P. glaucum seedlings when subjected to drought stress.