Melatonin is a hormone that is found across all three domains of life – Bacteria, Archaea and Eukaryota. It is most well known as the hormone that helps regulate the sleep cycle of humans, however, melatonin has different functions in plants. Primarily, it has been reported to activate various signalling events during plant responses to abiotic and biotic stress conditions, helping to safeguard them under stressful conditions. Scientific studies have found that melatonin can induce tolerance to various abiotic stress conditions, including heavy metals, high temperature and salinity.
During stress, melatonin enhances multiple adaptive responses in plants. It can boost stomatal conductance, photosynthesis, and transpiration, increase nutrient uptake and promote sugar metabolism. It also upregulates processes that prevent oxidative damage in cells, including synthesis of antioxidants and scavenging of reactive oxygen species. Yet, the underlying mechanisms of melatonin in alleviating drought stress have rarely been investigated in crops. Specifically, little is known about whether foliar or rhizospheric application of melatonin improves stress tolerance or not.
In their new study published in AoBP, Imran et al. investigate the roles of exogenous melatonin application (foliar or root zone) in improving drought stress tolerance of soybean (Glycine max) seedlings. Their results showed that pre-treatment of soybean seedlings with melatonin was found to significantly mitigate the negative effects of drought stress on plant growth-related parameters and chlorophyll content. The beneficial impacts against drought were more pronounced by melatonin application in the rhizosphere than in foliar treatments. The melatonin-induced enhanced tolerance could be attributed to improved photosynthetic activity, reduction of abscisic acid and drought-induced oxidative damage by lowering the accumulation of reactive oxygen species and malondialdehyde.
This study demonstrated that melatonin-induced improvement in drought stress tolerance in soybean plants was associated with enhanced functioning of the antioxidant defence machinery and the scavenging of hydrogen peroxide, which alleviated the oxidative damage caused by drought stress. The root zone application of melatonin resulted in significantly higher physiological and phytohormonal regulation than foliar application. This could be an essential factor determining the feasibility of melatonin application at large-scale field levels. However, the findings Imran et al. provide good evidence for the physiological role of melatonin and serve as a platform for possible applications in agricultural or related fields of research.
The interpretation of the effects of application of melatonin as inducing “drought tolerance” is not justified as the experiment fails to test the water balance correctly. Nor is the nature of the “drought resistance” considered . So the conclusions are not justified. May I suggest reading
J Exp Bot
. 2013 Jan;64(1):83-108. doi: 10.1093/jxb/ers326. Epub 2012 Nov 16.
Genetic engineering to improve plant performance under drought: physiological evaluation of achievements, limitations, and possibilities
David W Lawlor 1
Fully drought-resistant crop plants would be beneficial, but selection breeding has not produced them. Genetic modification of species by introduction of very many genes is claimed, predominantly, to have given drought resistance. This review analyses the physiological responses of genetically modified (GM) plants to water deficits, the mechanisms, and the consequences. The GM literature neglects physiology and is unspecific in definitions, which are considered here, together with methods of assessment and the type of drought resistance resulting. Experiments in soil with cessation of watering demonstrate drought resistance in GM plants as later stress development than in wild-type (WT) plants. This is caused by slower total water loss from the GM plants which have (or may have-morphology is often poorly defined) smaller total leaf area (LA) and/or decreased stomatal conductance (g (s)), associated with thicker laminae (denser mesophyll and smaller cells). Non-linear soil water characteristics result in extreme stress symptoms in WT before GM plants. Then, WT and GM plants are rewatered: faster and better recovery of GM plants is taken to show their greater drought resistance. Mechanisms targeted in genetic modification are then, incorrectly, considered responsible for the drought resistance. However, this is not valid as the initial conditions in WT and GM plants are not comparable. GM plants exhibit a form of ‘drought resistance’ for which the term ‘delayed stress onset’ is introduced. Claims that specific alterations to metabolism give drought resistance [for which the term ‘constitutive metabolic dehydration tolerance’ (CMDT) is suggested] are not critically demonstrated, and experimental tests are suggested. Small LA and g (s) may not decrease productivity in well-watered plants under laboratory conditions but may in the field. Optimization of GM traits to environment has not been analysed critically and is required in field trials, for example of recently released oilseed rape and maize which show ‘drought resistance’, probably due to delayed stress onset. Current evidence is that GM plants may not be better able to cope with drought than selection-bred cultivars.
Hope this will advance the science.
David W Lawlor