Drought tolerance
In botany, drought tolerance is the ability by which a plant maintains its biomass production during arid or drought conditions. Some plants are naturally adapted to dry conditions, surviving with protection mechanisms such as desiccation tolerance, detoxification, or repair of xylem embolism. Other plants, specifically crops like corn, wheat, and rice, have become increasingly tolerant to drought with new varieties created via genetic engineering. From an evolutionary perspective, the type of mycorrhizal associations formed in the roots of plants can determine how fast plants can adapt to drought.
The plants behind drought tolerance are complex and involve many pathways which allows plants to respond to specific sets of conditions at any given time. Some of these interactions include stomatal conductance, carotenoid degradation and anthocyanin accumulation, the intervention of osmoprotectants, ROS-scavenging enzymes. The molecular control of drought tolerance is also very complex and is influenced other factors such as environment and the developmental stage of the plant. This control consists mainly of transcriptional factors, such as dehydration-responsive element-binding protein, abscisic acid -responsive element-binding factor, and NAM.
Physiology of drought tolerance
Plants can be subjected to slowly developing water shortages, or they may face short-term deficits of water. In these situations, plants adapt by responding accordingly, minimizing water loss and maximizing water uptake. Plants are more susceptible to drought stress during the reproductive stages of growth, flowering and seed development. Therefore, the combination of short-term plus long-term responses allow for plants to produce a few viable seeds. Some examples of short-term and long-term physiological responses include:Short-term responses
- In the leaf: root-signal recognition, stomatal closure, decreased carbon assimilation
- In the stem: inhibition of growth, hydraulic changes, signal transport, assimilation of transport
- In the root: cell-drought signalling, osmotic adjustment
Long-term responses
- In the above-ground portion of the plant: inhibition of shoot growth, reduced transpiration area, grain abortion, senescence, metabolic acclimation, osmotic adjustment, anthocyanin accumulation, carotenoid degradation, intervention of osmoprotectants, ROS-scavenging enzymes
- In the below-ground portion of the plant: turgor maintenance, sustained root growth, increased root/shoot, increased absorption area
Regulatory network of drought tolerance
DREB TFs
DREB1/CBF TFs
DREB1A, DREB 1B, and DREB 1C are plant specific TFs which bind to drought responsive elements in promoters responsive to drought, high salinity and low temperature in Arabidopsis. Overexpression of these genes enhance the tolerance of drought, high salinity, and low temperature in transgenic lines from Arabidopsis, rice, and tobacco.DEAR1/DREB and EAR motif protein 1
is a TF with an entirely different purpose nothing to do with drought stress. Tsutsui et al 2009 found Arabidopsis DEAR1 to respond to pathogen infection, chitin, and oligomers of chitin.DREB2 TFs
DREB proteins are involved in a variety of functions related to drought tolerance. For example, DREB proteins including DREB2A cooperate with AREB/ABF proteins in gene expression, specifically in the DREB2A gene under osmotic stress conditions. DREB2 also induces the expression of heat-related genes, such as heat shock protein. Overexpression of DREB2Aca enhances drought and heat stress tolerance levels in Arabidopsis.AREB/ABF TFs
AREB/ABFs are ABA-responsive bZIP-type TFs which bind to ABA-responsive elements in stress-responsive promoters and activate gene expression. AREB1, AREB2, ABF3, and ABF1 have important roles in ABA signalling in the vegetative stage, as ABA controls the expression of genes associated with drought response and tolerance. The native form of AREB1 cannot target drought stress genes like RD29B in Arabidopsis, so modification is necessary for transcriptional activation. AREB/ABFs are positively regulated by SnRK2s, controlling the activity of target proteins via phosphorylation. This regulation also functions in the control of drought tolerance in the vegetative stage as well as the seed maturation and germination.Other TFs
TFs such as NAC, are also related to drought response in Arabidopsis and rice. Overexpression in the aforementioned plants improves stress and drought tolerance. They also may be related to root growth and senescence, two physiological traits related to drought tolerance.Natural drought tolerance adaptations
Plants in naturally arid conditions retain large amounts of biomass due to drought tolerance and can be classified into 4 categories of adaptation:- Drought-escaping plants: annuals that germinate and grow only during times of sufficient times of moisture to complete their life cycle.
- Drought-evading plants: non-succulent perennials which restrict their growth only to periods of moisture availability.
- Drought-enduring plants: also known as xerophytes, these evergreen shrubs have extensive root systems along with morphological and physiological adaptations which enable them to maintain growth even in times of extreme drought conditions.
- Drought-resisting plants: also known as succulent perennials, they have water stored in their leaves and stems for sparing uses.
Structural adaptations
- Adaptations of the stomata to reduce water loss, such as reduced numbers, sunken pits, waxy surfaces....
- Reduced number of leaves and their surface area.
- Water storage in succulent above-ground parts or water-filled tubers.
- Crassulacean acid metabolism allows plants to get carbon dioxide at night and store malic acid during the day, allowing photosynthesis to take place with minimized water loss.
- Adaptations in the root system to increase water absorption.
- Trichomes on the leaves to absorb atmospheric water.
Importance for agriculture
Drought-tolerant plants which are developed through biotechnology enable farmers to protect their harvest and reduces losses in times of intense drought by using water more efficiently.
Collaborations to improve drought tolerance in crop-variety plants
International research projects to improve drought tolerance have been introduced, such as the Consultative Group on International Agricultural Research. One such project from CGIAR involves introducing genes such as DREB1 into lowland rice, upland rice, and wheat to evaluate drought tolerance in fields. This project aims to select at least 10 lines for agricultural use. Another similar project in collaboration with CGIAR, Embrapa, RIKEN, and the University of Tokyo have introduced AREB and DREB stress-tolerant genes into soybeans, finding several transgenic soybean lines with drought tolerance. Both projects have found improved grain yield and will be used to help develop future varieties that can be used commercially.Other examples of collaborations to improve drought tolerance in crop-variety plants include the International Center for Agricultural Research in Dry Areas in Aleppo, Syria; the International Crops Research Institute for the Semi-Arid Tropics in Andhra Pradesh, India; the International Rice Research Institute in Los Baños, Philippines.; and the Heat and Drought Wheat Improvement Consortium, a network that facilitates global coordination of wheat research to adapt to a future with more severe weather extremes.