Molecular responses to drought stress in plants

Drought is a severe environmental constraint to plant productivity. Being a multidimensional stress, it triggers a wide variety of plant responses ranging from physiological, biochemical to molecular levels. One of the inevitable consequences of drought stress is an increase in reactive oxygen species (ROS) production in different cellular compartments, namely the chloroplasts and mitochondria. This enhanced ROS production is, however, kept under tight control by a versatile and cooperative antioxidant system that modulates intracellular ROS content and sets the redoxstatus of the cell. Furthermore, ROS production under stresses functions as an alarm signal that triggers defence or acclimation. Specific signal transduction pathways involve, e.g., H2O2 as a secondary messenger. ROS signalling under drought is linked to abscisic acid (ABA) and Ca2+ fluxes. At molecular levels, several drought-responsive genes, transcription factors, aquaporins, late embryogenesis abundant proteins, heat shock proteins, and dehydrins have been identified. This review discusses recent understanding on molecular responses and protective mechanisms of drought stress.