Dealing with Environmental Stresses: Role of Polyamines in Stress Responses

Extreme environmental conditions including drought, high- and low-temperatures, high salinity, mineral deficiency and heavy metal toxicity severely affect crop loss worldwide. Improvement of plants for enhanced resistance to adverse climatic conditions is a key issue in sustainable crop production, strengthening the global food safety. Understanding stress tolerance mechanisms of plants are a prime importance in crop improvement. Among the array of components involved, plant polyamines (PAs) are identified as one such group of components that play an important role in diverse environmental stress responses. PAs are small organic cations containing two or more amino groups. These are growth regulators present widely in all living organisms with varying quantities ranging from micromolar to milimolar. In plants, the most abundantly found PAs are di-amine putrescine, tri-amine spermidine and tetra-amine spermine. Accumulation of long chain and conjugated forms of PA occur under some environmental and growth conditions. Biosynthesis, transport, degradation and conjugation determine the level of PAs and vary throughout a plant life cycle. Catabolism of PAs by amine oxidases is trivial in the regulation of cellular levels of PAs. Apart from the essential functions in growth and development, PAs play a key role in environmental stress responses such as drought, chilling, salinity, mineral deficiencies such as potassium, nitrogen and magnesium deficiency, heavy metal toxicity, mechanical injuries and defence signalling against pathogens. Differential transcriptional regulation of several stress-related genes in PA-overexpressed transgenic plants suggests potential signalling function of PAs in stress responses. Genetic manipulation of crop plants for altered regulation of PA biosynthesis/catabolism may lead to improved stress tolerance potential. This article summarizes the recent findings on the involvement of PAs in abiotic stress responses in plants and possible means of manipulating PAs in the crop plants for enhanced stress tolerance.