In this article we will discuss about the plant responses to drought stress.
With the exception of flooding, the major drought stresses result in water-deficit stress. The stressed plants exhibit striking responses by producing plant hormone abscisic acid (ABA), which plays an important role in tolerance against drought. Abscisic acid can induce the expression of stress relieving genes. More than half of the drought-inducible genes are also induced by abscisic acid.
However, dehydration responsive genes are not induced by external application of ABA. These studies clearly show the existence of different signal transduction pathways involved in the expression of stress relieving genes. The drought responsive genes expression involves existence of ABA-independent as well as ABA-dependent signal transduction pathways.
The drought inducible gene products are, enzymes involved in particular metabolic pathway, regulatory proteins or proteins with specific properties. These proteins are also involved in the regulation of other genes by signal transduction in the drought stress response. Kazuko (2002) classified this into two groups.
The gene products of the first group include:
(1) Functional proteins that directly protect macromolecules and membranes (LEA proteins, osmotin, antifreeze proteins, chaperones and mRNA binding proteins, etc.),
(2) The proteins maintain water movement through membranes (water-channel proteins and membrane transporters),
(3) The enzymes catalyzing the biosynthesis of various osmoregulators (proline, betaine and sugars, etc.),
(4) Detoxifying enzymes to maintain normal cellular functions (glutathione s-transferase, soluble epoxide hydrolase, catalase, superoxide dismutases and ascorbic peroxidases).
The gene product of the second group includes:
(1) Transcription factors (DREB, MYC, MYB, bZIP),
(2) Protein kinases (MAP kinases, CDP kinases, ribosomal-protein kinase and transcription-regulation protein kinase, etc.),
(3) Proteinases (phospholipase C and phosphoesterase).