Early characterization of the adaptation of millet roots to drought stress

Climate change is currently increasing the frequency and intensity of drought events, posing a major threat to global crop productivity and food security. Indeed, drought is the leading cause of yield losses for traditional crops like rice, wheat and maize, as it disrupts plant water balance, nutrient absorption and key physiological processes, ultimately resulting in reduced plant productivity. Seed germination and seedling establishment are among the stages most susceptible to drought stress. Water is needed to reactivate seed metabolism and nutrient transport, to allow radicle emergence and early embryo development. Facing drought at early growth stages requires plant’s root system to rapidly adapt to this stressful condition, because roots are the first organs to encounter water stress. Millets are a group of small-grain cereals, known for exceptional tolerance to a wide range of environmental stresses, including drought, and therefore considered promising climate-resilient alternatives to traditional crops to ensure nutritional security in challenging environments. They exhibit agronomic traits suitable for managing long-term droughts, such as short life cycles, dense root systems and C4 metabolism. However, stress responses are strongly related to the genotypic variability within species, and millets display high genetic diversity. Hence, some mechanisms associated with drought tolerance in millets have not been completely elucidated yet. This research aims to identify drought-tolerant and drought-sensitive genotypes of two key and underexplored millet species, namely Pennisetum glaucum and Panicum miliaceum, and to characterize their response in relation to drought stress at early stages of plant development. An in vitro germination screening was performed on six genotypes per species through PEG-6000 simulated drought stress treatments at four different concentrations (0%, 10%, 20% and 25%). Germination rate and root system morphology were evaluated to assess drought response and identify tolerant and sensitive genotypes. These analyses were complemented by a preliminary screening of reactive oxygen species production and localization in drought-stressed roots using fluorescence microscopy. The results show differences in response among the genotypes. Some genotypes showed lower sensitivity to drought stress, even under severe conditions, whereas others exhibited reduction in germination rate and root system development with increasing stress levels compared with unstressed controls. The root systems of the tested genotypes are being currently analyzed using optical and confocal microscopy to further elucidate their anatomical adaptation differences to drought stress. These first results highlight the heterogeneity of responses to drought stress in millets and provide an initial basis for the identification of early markers of stress resilience.