Drought tolerance is the ability to which a plant maintains its biomass production during

arid A region is arid when it severely lacks available water, to the extent of hindering or preventing the growth and development of plant and animal life. Regions with arid climates tend to lack vegetation and are called xeric or desertic. Most a ...

or drought conditions. Some plants are naturally adapted to dry conditions'','' surviving with protection mechanisms such as desiccation tolerance, detoxification, or repair of

xylem Xylem is one of the two types of transport tissue in vascular plants, the other being phloem. The basic function of xylem is to transport water from roots to stems and leaves, but it also transports nutrients. The word ''xylem'' is derived from ...

embolism. Other plants, specifically crops like

corn Maize ( ; ''Zea mays'' subsp. ''mays'', from es, maíz after tnq, mahiz), also known as corn (North American and Australian English), is a cereal grain first domesticated by indigenous peoples in southern Mexico about 10,000 years ago. The ...

, wheat, and rice, have become increasingly tolerant to drought with new varieties created via genetic engineering. 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 Stomatal conductance, usually measured in mmol m−2 s−1 by a porometer, estimates the rate of gas exchange (i.e., carbon dioxide uptake) and transpiration (i.e., water loss as water vapor) through the leaf stomata as determined by the degree of ...

carotenoid Carotenoids (), also called tetraterpenoids, are yellow, orange, and red organic pigments that are produced by plants and algae, as well as several bacteria, and fungi. Carotenoids give the characteristic color to pumpkins, carrots, parsnips, ...

degradation and

anthocyanin Anthocyanins (), also called anthocyans, are water-soluble vacuolar pigments that, depending on their pH, may appear red, purple, blue, or black. In 1835, the German pharmacist Ludwig Clamor Marquart gave the name Anthokyan to a chemical com ...

accumulation, the intervention of

osmoprotectant Osmoprotectants or compatible solutes are small organic molecules with neutral charge and low toxicity at high concentrations that act as osmolytes and help organisms survive extreme osmotic stress. Osmoprotectants can be placed in three chemical c ...

s (such as sucrose,

glycine Glycine (symbol Gly or G; ) is an amino acid that has a single hydrogen atom as its side chain. It is the simplest stable amino acid ( carbamic acid is unstable), with the chemical formula NH2‐ CH2‐ COOH. Glycine is one of the proteinogen ...

, and proline), 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 (DREB),

abscisic acid Abscisic acid (ABA) is a plant hormone. ABA functions in many plant developmental processes, including seed and bud dormancy, the control of organ size and stomatal closure. It is especially important for plants in the response to environmental s ...

(ABA)-responsive element-binding factor (AREB), and NAM (no apical meristem).

Physiology of drought tolerance

Plants can be subjected to slowly developing water shortages (ie, taking days, weeks, or months), or they may face short-term deficits of water (ie, hours to days). 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 Important structures in plant development are buds, shoots, roots, leaves, and flowers; plants produce these tissues and structures throughout their life from meristems located at the tips of organs, or between mature tissues. Thus, a living plant ...

. 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,

stoma In botany, a stoma (from Greek ''στόμα'', "mouth", plural "stomata"), also called a stomate (plural "stomates"), is a pore found in the epidermis of leaves, stems, and other organs, that controls the rate of gas exchange. The pore is bor ...

tal 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

Transcription factors related to drought tolerance and stress

In response to drought conditions, there is an alteration of gene expression, induced by or activated by transcription factors (TFs). These TFs bind to specific cis-elements to induce the expression of targeted stress-inducible genes, allowing for products to be transcribed that help with stress response and tolerance. Some of these include dehydration-responsive element-binding protein (DREB), ABA-responsive element-binding factor (AREB), no apical meristem (NAM), Arabidopsis transcription activation factor (ATAF), and cup-shaped cotyledon (CUC). Much of the molecular work to understand the regulation of drought tolerance has been done in ''Arabidopsis'', helping elucidate the basic processes below.

DREB TFs

DREB1/CBF TFs

DREB1A, DREB 1B, and DREB 1C are plant specific TFs which bind to drought responsive elements (DREs) 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 (At3g50260) 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 (ABREs) 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 (composed of NAM, ATAF, and CUC), 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 A perennial plant or simply perennial is a plant that lives more than two years. The term ('' per-'' + '' -ennial'', "through the years") is often used to differentiate a plant from shorter-lived annuals and biennials. The term is also wide ...

which restrict their growth only to periods of moisture availability. #Drought-enduring plants: also known as

xerophyte A xerophyte (from Greek ξηρός ''xeros'' 'dry' + φυτόν ''phuton'' 'plant') is a species of plant that has adaptations to survive in an environment with little liquid water, such as a desert such as the Sahara or places in the Alps or the ...

s, 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

Many adaptations for dry conditions are structural, including the following: * Adaptations of the

ta 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

tuber Tubers are a type of enlarged structure used as storage organs for nutrients in some plants. They are used for the plant's perennation (survival of the winter or dry months), to provide energy and nutrients for regrowth during the next growing ...

s. *

Crassulacean acid metabolism Crassulacean acid metabolism, also known as CAM photosynthesis, is a carbon fixation pathway that evolved in some plants as an adaptation to arid conditions that allows a plant to photosynthesize during the day, but only exchange gases at night. I ...

(CAM 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 The absorption of electromagnetic radiation by water depends on the state of the water. The absorption in the gas phase occurs in three regions of the spectrum. Rotational transitions are responsible for absorption in the microwave and far-in ...

. * Trichomes (small hairs) on the

leaves A leaf ( : leaves) is any of the principal appendages of a vascular plant stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", while the leaves, ste ...

to absorb atmospheric water.

Importance for agriculture

With the frequency and severity of droughts increasing in recent years, damage to crops has become more serious, lowering the crop yield, growth, and production. However, research into the molecular pathways involving stress tolerance have revealed that overexpression of such genes can enhance drought tolerance, leading to projects focused on the development of transgenic crop varieties. 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 (

CGIAR CGIAR (formerly the Consultative Group for International Agricultural Research) is a global partnership that unites international organizations engaged in research about food security. CGIAR research aims to reduce rural poverty, increase food ...

). 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 The Brazilian Agricultural Research Corporation (Embrapa - pt, Empresa Brasileira de Pesquisa Agropecuária) is a state-owned research corporation affiliated with the Brazilian Ministry of Agriculture. Since its inception on April 26, 1973, it ha ...

, RIKEN, and the

University of Tokyo , abbreviated as or UTokyo, is a public research university located in Bunkyō, Tokyo, Japan. Established in 1877, the university was the first Imperial University and is currently a Top Type university of the Top Global University Project by ...

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 (ICARDA) in

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, Syria; the International Crops Research Institute for the Semi-Arid Tropics (ICRISAT) in

Andhra Pradesh Andhra Pradesh (, abbr. AP) is a state in the south-eastern coastal region of India. It is the seventh-largest state by area covering an area of and tenth-most populous state with 49,386,799 inhabitants. It is bordered by Telangana to the ...

, India; the International Rice Research Institute (IRRI) in Los Baños, Philippines.; and the Heat and Drought Wheat Improvement Consortium (HeDWIC), a network that facilitates global coordination of wheat research to adapt to a future with more severe weather extremes.

Impediments to the agricultural commercialization of drought tolerant plants

The development of genetically modified crops includes multiple patents for genes and promoters, such as the marker genes in a vector, as well as transformation techniques. Therefore, freedom-to-operate (FTO) surveys should be implemented in collaborations for developing drought tolerant crops. Large amounts of money are also needed for the development of genetically modified groups. To bring a new genetically modified crop into the commercial market, it has been estimated to cost USD 136 million over 13 years. This poses a problem for development, as only a small number of companies can afford to develop drought-tolerant crops, and it is difficult for research institutions to sustain funding for this period of time. Therefore, a multinational framework with more collaboration among multiple disciples is needed to sustain projects of this size.

Importance in horticulture

Plant transformation has been used to develop multiple drought resistant crop varieties, but only limited varieties of ornamental plants. This significant lag in development is due to the fact that more transgenic ornamental plants are being developed for other reasons than drought tolerance. However, abiotic stress resistance is being explored in ornamental plants by Ornamental Biosciences. Transgenic Petunias,

Poinsettia The poinsettia ( or ) (''Euphorbia pulcherrima'') is a commercially important flowering plant species of the diverse spurge family Euphorbiaceae. Indigenous to Mexico and Central America, the poinsettia was first described by Europeans in 1834 ...

s, New Guinea Impatiens, and

Geraniums ''Pelargonium'' () is a genus of flowering plants that includes about 280 species of perennials, succulents, and shrubs, commonly called geraniums, pelargoniums, or storksbills. ''Geranium'' is also the botanical name and common name of a separa ...

are being evaluated for frost, drought, and disease resistance. This will allow for a wider range of environments in which these plants can grow.

References

{{Global warming Agricultural economics Plant physiology Water conservation