In botany, drought tolerance is the ability by which a

plant Plants are the eukaryotes that form the Kingdom (biology), kingdom Plantae; they are predominantly Photosynthesis, photosynthetic. This means that they obtain their energy from sunlight, using chloroplasts derived from endosymbiosis with c ...

maintains its biomass production during arid or

drought A drought is a period of drier-than-normal conditions.Douville, H., K. Raghavan, J. Renwick, R.P. Allan, P.A. Arias, M. Barlow, R. Cerezo-Mota, A. Cherchi, T.Y. Gan, J. Gergis, D. Jiang, A. Khan, W. Pokam Mba, D. Rosenfeld, J. Tierney, ...

conditions. Some plants are naturally adapted to dry conditions'','' surviving with protection mechanisms such as desiccation tolerance,

detoxification Detoxification or detoxication (detox for short) is the physiological or medicinal removal of toxic substances from a living organism, including the human body, which is mainly carried out by the liver. Additionally, it can refer to the period o ...

, or repair of

xylem Xylem is one of the two types of transport tissue (biology), tissue in vascular plants, the other being phloem; both of these are part of the vascular bundle. The basic function of the xylem is to transport water upward from the roots to parts o ...

embolism. Other plants, specifically crops like corn,

wheat Wheat is a group of wild and crop domestication, domesticated Poaceae, grasses of the genus ''Triticum'' (). They are Agriculture, cultivated for their cereal grains, which are staple foods around the world. Well-known Taxonomy of wheat, whe ...

, and

rice Rice is a cereal grain and in its Domestication, domesticated form is the staple food of over half of the world's population, particularly in Asia and Africa. Rice is the seed of the grass species ''Oryza sativa'' (Asian rice)—or, much l ...

, have become increasingly tolerant to drought with new varieties created via

genetic engineering Genetic engineering, also called genetic modification or genetic manipulation, is the modification and manipulation of an organism's genes using technology. It is a set of Genetic engineering techniques, technologies used to change the genet ...

. 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 (such as

sucrose Sucrose, a disaccharide, is a sugar composed of glucose and fructose subunits. It is produced naturally in plants and is the main constituent of white sugar. It has the molecular formula . For human consumption, sucrose is extracted and refined ...

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. Glycine is one of the proteinogenic amino acids. It is encoded by all the codons starting with GG (G ...

, 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 (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,

flower Flowers, also known as blooms and blossoms, are the reproductive structures of flowering plants ( angiosperms). Typically, they are structured in four circular levels, called whorls, around the end of a stalk. These whorls include: calyx, m ...

ing 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,

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

tal closure, decreased

carbon Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...

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 Chitin (carbon, C8hydrogen, H13oxygen, O5nitrogen, N)n ( ) is a long-chain polymer of N-Acetylglucosamine, ''N''-acetylglucosamine, an amide derivative of glucose. Chitin is the second most abundant polysaccharide in nature (behind only cell ...

, 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 Senescence () or biological aging is the gradual deterioration of Function (biology), functional characteristics in living organisms. Whole organism senescence involves an increase in mortality rate, death rates or a decrease in fecundity with ...

, 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 In botany, succulent plants, also known as succulents, are plants with parts that are thickened, fleshy, and engorged, usually to retain water in arid climates or soil conditions. The word ''succulent'' comes from the Latin word ''sucus'', meani ...

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

above-ground parts or water-filled

tuber Tubers are a type of enlarged structure that plants use as storage organs for nutrients, derived from stems or roots. Tubers help plants perennate (survive winter or dry months), provide energy and nutrients, and are a means of asexual reproduc ...

s. * Crassulacean acid metabolism (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 In vascular plants, the roots are the plant organ, organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant body, which allows plants to grow taller and faster. They are most often bel ...

system to increase water absorption. *

Trichome Trichomes (; ) are fine outgrowths or appendages on plants, algae, lichens, and certain protists. They are of diverse structure and function. Examples are hairs, glandular hairs, scales, and papillae. A covering of any kind of hair on a plant ...

s (small hairs) on the

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

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). 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 (ICARDA) in

Aleppo Aleppo is a city in Syria, which serves as the capital of the Aleppo Governorate, the most populous Governorates of Syria, governorate of Syria. With an estimated population of 2,098,000 residents it is Syria's largest city by urban area, and ...

Syria Syria, officially the Syrian Arab Republic, is a country in West Asia located in the Eastern Mediterranean and the Levant. It borders the Mediterranean Sea to the west, Turkey to Syria–Turkey border, the north, Iraq to Iraq–Syria border, t ...

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

Andhra Pradesh Andhra Pradesh (ISO 15919, ISO: , , AP) is a States and union territories of India, state on the East Coast of India, east coast of southern India. It is the List of states and union territories of India by area, seventh-largest state and th ...

India India, officially the Republic of India, is a country in South Asia. It is the List of countries and dependencies by area, seventh-largest country by area; the List of countries by population (United Nations), most populous country since ...

; the International Rice Research Institute (IRRI) in Los Baños,

Philippines The Philippines, officially the Republic of the Philippines, is an Archipelagic state, archipelagic country in Southeast Asia. Located in the western Pacific Ocean, it consists of List of islands of the Philippines, 7,641 islands, with a tot ...

.; 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 plant Ornamental plants or ''garden plants'' are plants that are primarily grown for their beauty but also for qualities such as scent or how they shape physical space. Many flowering plants and garden varieties tend to be specially bred cultivars th ...

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

Drought-tolerant plants

This is a list of selected plant families, species and/or genus that tolerate drought: * Acacia * Adenium * Aeonium * Agapanthus * Agave *

Aloe ''Aloe'' (; also written ''Aloë'') is a genus containing over 650 species of flowering plant, flowering succulent plant, succulent plants.WFO (2022): Aloe L. Published on the Internet;http://www.worldfloraonline.org/taxon/wfo-4000001341. Acc ...

* Angelonia *'' Anredera cordifolia'' *'' Araujia sericifera'' *'' Arctotheca calendula'' * Asphodelus *'' Asparagus aethiopicus'' * Banksia * Begonia * Bougainvillea *'' Buddleja davidii'' * Bulbine * Cactus * Calibrachoa * Carpobrotus *'' Cestrum parqui'' *'' Cistus albidus'' *

Cosmos The cosmos (, ; ) is an alternative name for the universe or its nature or order. Usage of the word ''cosmos'' implies viewing the universe as a complex and orderly system or entity. The cosmos is studied in cosmologya broad discipline covering ...

* Crassula * Curio * Drosanthemum * Dudleya * Echeveria * Echinacea * Eriogonum *

Eucalyptus ''Eucalyptus'' () is a genus of more than 700 species of flowering plants in the family Myrtaceae. Most species of ''Eucalyptus'' are trees, often Mallee (habit), mallees, and a few are shrubs. Along with several other genera in the tribe Eucalyp ...

*'' Euphorbia milii'' * Gaillardia *'' Gazania rigens'' * Gonialoe * Graptopetalum * Grevillea * Haberlea *'' Haloxylon ammodendron'' * Haworthia *

Helianthus ''Helianthus'' () is a genus comprising around 70 species of annual and perennial flowering plants in the daisy family Asteraceae commonly known as sunflowers. Except for three South American species, the species of ''Helianthus'' are native to ...

*'' Impatiens hawkeri'' *'' Ipomoea cairica'' *''

Lantana camara ''Lantana camara'' (common lantana) is a species of flowering plant in the verbena family (Verbenaceae), native to the American tropics. It is a very adaptable species, which can inhabit a wide variety of ecosystems; once it has been introduced i ...

'' *

Melaleuca ''Melaleuca'' () is a genus of nearly 300 species of plants in the myrtle family, Myrtaceae, commonly known as paperbarks, honey-myrtles, bottlebrushes or tea-trees (although the last name is also applied to species of '' Leptospermum''). They ...

* Mesembryanthemum *'' Nandina domestica'' *

Nerium oleander ''Nerium oleander'' ( ), commonly known as oleander or rosebay, is a shrub or small tree cultivated worldwide in temperate and subtropical areas as an ornamental and landscaping plant. It is the Monotypic taxon, only species currently classifi ...

* Olea europaea *

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

Petunia ''Petunia'' is a genus of 20 species of flowering plants of South American origin. The popular flower of the same name derived its epithet from the French, which took the word , 'tobacco', from a Tupi–Guarani language. A tender perennial plan ...

* Portulaca *'' Portulacaria afra'' *'' Ramonda serbica'' *'' Ricinus communis'' * Rosemary *

Salvia ''Salvia'' () is the largest genus of plants in the sage family Lamiaceae, with just under 1,000 species of shrubs, Herbaceous plant, herbaceous Perennial plant, perennials, and Annual plant, annuals. Within the Lamiaceae, ''Salvia'' is part o ...

* Santolina * Sedum *'' Senecio angulatus'' *'' Senecio elegans'' *'' Tetradenia riparia'' *'' Thunbergia alata'' * Vinca *''

Vitis vinifera ''Vitis vinifera'', the common grape vine, is a species of flowering plant, native to the Mediterranean Basin, Mediterranean region, Central Europe, and southwestern Asia, from Morocco and Portugal north to southern Germany and east to northern ...

'' * Yucca

References

{{Authority control Agricultural economics Plant physiology Water conservation