Year after year, we find several parts of India hit by drought, food-grain production affected and farmers suffering greatly. During the recent decades, this climate change-induced effect has affected not only India but many lands across the globe. How do plants react and adjust to drought mediated stress? This is an area of considerable interest and activity and we have come to understand same aspects of it.
Every school child knows that plants collect energy from sunlight, absorb carbon dioxide from the atmosphere, pick up water from the soil, and using theses, make food for us. This seemingly simple chemical reaction called photosynthesis generates not only carbohydrates but produces oxygen as well, letting us breathe and use it to help our metabolism and gain energy. The key needs for the plants are thus simple - sunlight, carbon dioxide (CO2) from the air and water. If there be a shortage of any of these three, plant productivity falls.
Water crisis
Fortunately, sunlight is regular and abundant during day time. Carbon dioxide is also available in plenty (indeed it happens to be in excess, and increasing every year, thanks to the burning of fossil fuels like coal, petrol and natural gas) but it is the water shortage that has reached famine proportions in many parts of the world. How do plants react to drought conditions, what built-in mechanisms do they have, and how do they cope with drought stress—this is an area of intense activity among plant biologists.
Two recent papers throw light on these aspects of how plants adapt to drought stress. The first one comes from the group of Dr Andy Pereira of the University of Arkansas, Fayetteville, Arkansas, in the US, who used rice as the crop plant to study (see their paper: Basu S, Ramegowda V, Kumar A, Pereira A. ‘Plant adaptation to drought stress’: F1000Res. 2016 Jun 30;5. pii: F1000 Faculty Rev-1554. doi: 10.12688/f1000research.7678.1. eCollection 2016., free on the web)
The paper describes the various strategies that plants adapt. Drought resistance (DR) is one aspect in which enables plants to escape, avoid and tolerate drought stress. Drought escape (DE) is where a plant attempts to complete its life cycle before the onset of drought; this would involve the plant capturing a signal for the onset of drought conditions and preparing ahead of time— ‘smart’! Drought avoidance (DA) involves the ability of plants to maintain relatively higher tissue water content, despite the water the water scarcity in the soil (saving for a deficit day) and drought tolerance (DT) where in the plant endures low water content on its tissues through various adaptive traits.
How does a plant display all these traits under conditions of drought stress? The authors point out that there are at least five different ways used by plants. The first is to reduce the level of photosynthesis (recall it uses water) by decreasing the leaf area (close and expose less) and slowing down the rate of photosynthesis.
The second is by regulating the action of the hormones present in the plant, in particular, one called abscisic acid (or ABA). During drought stress, ABA moves from the roots to the leaves, helping them close the very small openings (called stomata) in them, which allow for the entry and exit of gases (CO2, oxygen, water vapour), and reduce plant growth. Other signalling molecules called cytokinins in the plant cells also act up, delaying premature leaf ageing and death. The third is to control transpiration (water release from the plant to the air) by closing the stomata, reducing water loss and reducing CO2 uptake. The fourth way is to change the growth, size, shape and branching out of the roots, and the fifth is through what is termed osmotic adjustment. Here the pressure exerted by the contents of the cell against the cell wall or membrane is maintained sufficiently tense for stiffness (and no collapse or breakdown). Botanists call this turgor (from the Latin for swelling).
Clearly, these five processes must be controlled and triggered by genes that express proteins and other molecules that carry out the stress response. How this process is controlled has been the study of another group, led by Dr. Yanhai Yin of Iowa State University at Ames, Iowa, USA. Their paper has appeared three weeks ago (Ye H et al., Nature Communications 2017 Feb 24;8:14573. doi: 10.1038/ncomms14573). They discuss the roles of two molecules called BES1 and RD26 which play key roles in regulating plant growth under drought conditions. These two belong to the class called transcription factors, which are molecules that regulate (allow or stop) the expression of chosen genes into making the relevant protein molecules.
Frenemies
These two molecules thus look like they are working at cross-purposes, yet the pathways that these two regulate are highly interconnected. Dr. Yin has described them as ‘frenemies’, when the science writer Rashmi Shivni interviewed him. (Frenemies are individuals who combine in them characteristics of friends as well as enemies). “We found that these pathways are kind of like frenemies that stay together but ‘antagonise’ each other most of the time. They both bind to the same site on the DNA, but only one pathway is active, depending on the environmental conditions”. BES1 is involved in the process by which certain plant steroids regulate plant growth. RD26 is active only when the plant experiences drought stress. Greater understanding of the ‘frenemical’ action would thus lead us to help increase crop yields when drought strikes.
dbala@lvpei.org
