Drought Tolerance in Maize and Breeding Strategies

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Maize is the most world grown crop in the America, Asia etc.Hybrid maize, because of its high grain yield as a result of heterosis (hybrid) is preferred by farmers over conventional varieties. When a region notes a deficiency in its water supply then a drought condition occurs. This occurs when a region receives consistently below average precipitation. It can have a substantial impact on the ecosystem and agriculture of affected region. Drought is important due to instability of national maize grain yields and of food supply and economy of small-scale maize based farming systems in the tropics. Water shortage affect maize yields throughout the crop cycle, but most severely at flowering and to a lesser degree at establishment. There are some techniques which are used to improve the maize in drought tolerance conditions, such as;

1)-Improvement of drought tolerance through Conventional Breeding:

(a)-Population Improvement for Drought Tolerance in Tropical Maize.

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(b)-Hybrid Improvement for Drought Tolerance in Temperate Maize.

2)-Molecular Breeding Approaches:

(a)-Marker-Assisted Back Cross (MABC) Approaches.

(b)-The Marker-Assisted Recurrent Selection (MARS) Approaches.

Introduction of Maize:

Importance:

Maize the American word for corn, means literally ''that which sustains life''.Maize is the most world grown crop in the America and Asia. It is after wheat and rice, the most important cereal grain in the world, providing nutrients for human and animals and serving as a basic raw material for the production of starch, oil and protein, alcoholic beverages, food sweeteners and more recently fuel. Special crops grown primarily for food include sweet corn and popcorn, although starchy or floury and flint maize is also widely used as food. After harvest of the grian, the dried leaves and the upper part, including the flowers are still used today to provide relatively good forage for ruminant animals owned by many small farmers in developing countries. Some varieties of maize stalk are very strong, which is used as long-lasting fences and walls.

Biology of Maize crop:

Maize (zea mays) belongs to the grass family (Gramineae) and is a tall annual plant with an extensive fibrous root system. It is a cross pollinating species, with the female (ear) and male (tassel) flowers in separate places on the plant. Maize stems resemble bamboo canes .The ears are female inflorescences, tightly covered over by several layers of leaves. The apex of the stem ends in the tassel, an inflorescence of male flowers. When the tassel is mature and conditions are suitably warm and dry, anthers on tassel release pollen. Maize pollen is dispersed by wind, most pollen falls within a few meters of the tassel. The kernel of maize has a pericarp of the fruit fused with the seed coat, typical of the grasses. It is close to a multiple fruit in structure, except that the individual fruits never fuse into a single mass. The grains are about the size of peas, and adhere in regular rows. The grain develops in the ears or cobs often one on each stalk; each ear has about 300 to 1000 kernels, weighing between 190 and 300 g per 1000 kernels, in a variable number of rows (12 to 16).Weight depends on genetic, environmental and cultural practices. Grain makes up about 42 % of the dry weight of the plant. The kernels are often white or yellow in colour, although black, red and a mixture of colours is also found. There are a number of grain types, distinguished by differences in the chemical compounds deposited or stored in the kernel.18

Development of Maize:

The development of the maize plant divides into two physiological stages. In the first (vegetative stage) different tissues develop and differentiate until the flower structure appears. The vegetative stage is made up of two cycles. In the first cycle the first leaves are formed and development is upward. Dry matter production in this cycle is slow. It ends with the tissue differentiation of the reproductive organs. In the second cycle the leaves and reproductive organs develop, begins with the fertilization of the female structures, which will develop into ears and grains. The initial phase of this stage is characterized by an increase in the weight of leaves and other flower parts. During the second phase, the weight of the kernels rapidly increases (Tanaka and Yamaguchi, 1972).

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The plant develops morphological characteristics and differences in the vegetative and reproductive stages as evolutionary consequences of natural selection and domestication. Some genotypes have adapted to specific ecological zones and so have developed such barriers as day-length sensitivity and temperature sensitivity, which limit their adaptability to specific areas of latitude and altitude. Thus improvement programmes must be conducted within the areas where the improved varieties are to be grown. This does not mean, however, that specific genetic characteristics can be attained by backcrossing. The morphology of the plant has also suffered evolutionary pressures that have resulted in great variability in the number, length and width of leaves, plant height, and positions of ears, number of ears per plant, maturation cycles, grain types and number of rows of grain, among many other characteristics. This variability is of great value in improving the productivity of the plant and specific organic components of the grain. The main yield components include the number and weight of grains. These yield components are determined by quantitative genetic effects that can be selected relatively easily. The number of grains depends on the ear and is determined by the number of rows and the number of kernels per row. The size and shape of the kernel determine its weight in the presence of other constant factors such as grain texture and grain density. (Barber, 1979).

Drought Tolerance:

Limitation of water caused a major problem permanently affecting 28% of the world's soil. (Dual, R.1976). When a region notes a deficiency in its water supply then there is drought condition occurs. This occurs when a region receives consistently below average precipitation. In agriculture, drought stress refers to situation where the amount of soil moisture does not meet the needs of a particular crop. The severity of the stress depends upon the degree and duration of moisture deficiency. It can have a substantial impact on the ecosystem and agriculture of affected region. Drought is mostly to reduce the crop yields and growth (Arus et al.,2002).So morphological, physiological and metabolic changes occurs, because there is loss of water and nutrients uptake into the tolerant plants that will directly effect on the plant performance due to this status (Tester & Bacic.,2005).

Physiological Changes:

If we talks about the physiological changes due to drought stress, there is a change in membrane fluidity, composition and turgor loss. This changes occurs during plant response to water stress and have been correlated with rapid translocated of abscisic acid (ABA) in the transpiration and therefore the plant concentration of ABA is also increased in the plant organs (Alvarez et al., 2008).So during the drought stress, the hormone concentration will be increase due to xylem vessels (including ABA) give up contents to the leaf apoplast. (apoplast is the free diffusion outside the plasma membrane). It reaches the stomatal guard cells to the epidermis which contain ABA receptors with external loci in their plasma membrane (Wasilewska et al., 2008).

Morphological Changes:

If we talk about the morphological changes, there is changes in the cell elongation, stimulation of cell division and alternation in cell differentiation status (Potters et al., 2007).So therefore there is negative effect on the plant growth and development through the arrest of the cell cycle machinery (Peres et al., 2007).In plant tissues, water potential and content are maintained to increasing uptake or limiting loss, so they are in balance. So these balance are achieved by the morphological traits and their development, that is parallel to decrease the photosynthesis rate (Lawlor, 2002).So therefore decreasing the CO2 and water loss from the leaves will affect the mesophyll metabolism (Parry et al.,2002).But if we look on long term, there is also root and shoot growth effects which leads to increased growth, tissue water storage capacity effect and therefore there is change in root growth to maximize water uptake are most crucial for crop plants (Verslues et al.,2006).

Metabolic Changes:

Metabolic change, associated to drought stress include the modification of the interaction of protein-protein and protein-lipid (Valliyodan & Nguyen, 2006).There are different multiple transgenic compounds such as dimethylsulfoniopropinate,glycine betaine (Chen and Murata,2002),sorbitol (Abebe et al.,2003).,sugars such as sucrose,trehalose (Garg et al.,2002),galactinol (Taji et al.,2002),ononitol (Sheveleva et al.,1997) and fructan (Pilon-Smits et al.,1995);or amino acids such as proline (Kishor et al.,1995) and ectoine that serve as osmolytes(these are organic compounds that are affect on osmosis) and osmoprotectants(these are small molecules that help organism to survive in extreme osmotic stress and act as osmolytes)( Lang F ;October 2007).

Genetic Basis of Drought Tolerance in Maize:

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Numerous QTL involved in the determination of morphological traits, yield components, flowering traits and plant height of drought tolerance traits have been carried out in maize over the last decade.(Agrama & Moussa,1996;Austin & Lee,1998,Veldboom & Lee,1996a,b;Frova et al.,1999;Ribaut et al.,1996,1997;Sari-Gorla et al.,1999;Tuberosa et al., 2002a,b,2005;Li et al.,2003;Xiao et al.,2004).QTL of major effect in maize (all QTLs in the same spot) are in fact the clusters of genes(e.g,homeotic genes and other genes encoding for transcription factors), regulating development and that many plant reactions to abiotic stresses on such gene clusters(Khavkin E.,Coe E.H.(1997).

Genetic gains:

1-Improvement of Drought Tolerance through Conventional Breeding:

(a)-Population improvement for Drought Tolerance in Tropical Maize:

Improvement in drought tolerance maize on flowering stage in CIMMYT (International Maize & Wheat Improvement Center) are using recurrent selection to improve under drought tolerance condition. So in this way, the grain yield of maize increased between 3.8% to 12.6%(Edmeades et al., 1999; Bolanos and Edmeadas, 1993a).Due to this, there will be increases the EPP(Ear per plant) and HI(Harvest Index) and reduce the ASI(Anthesis-silking interval), leaf senescence, plant height, stem biomass, time to anthesis and tassel primary branch number, and a small but significant increase in grain yield,EPP(Ear per plant),kernel weight per fertile ear and individual kernel weight was also be achieved under WW(well-water) conditions.(Westgate,1997).Increase the yield of grain in drought tolerance has improved in the specific conditions and low fertility conditions, two common adaptive stress involved(Bänziger et al.,1999).A common genetic basis between drought tolerance and better performance under low N conditions has been confirmed through QTL analysis, and QTL common for both abiotic stresses were identified for ASI(Anthesis-silking interval) and EPP(Ear per plant) (Ribaut et al.,2007).

(b)-Hybrid Improvement for Drought Tolerance in Temperate Maize:

To improve the hybrid seed for drought tolerance, the seed industry has succeeded in efficient testing to improve mechanization and imposing high selection intensities (Coors, 1999).These hybrid evaluation are designed as such to maximize the adaptation and stability under different environment cultural practices such as planting density, planting date, drought stress, fertilizer input, tillage and crop rotation etc; climatically and edaphically. Most of recent improvement in hybrid performance is due to a greater tolerance to abiotic stress, particularly in situations where high planting densities are used (Duvick et al., 2004; Tollenaar and Wu, 1999).

2-Molecular Breeding Approaches:

A few examples of new technologies are given below;

(a)-Marker-Assisted Back-Cross (MABC) Approaches:

Ribaut & Ragot (2007), they describe a successful experiment, in which favorable alleles were expressed in five genomic regions in plants under limited water conditions. The plants are selected from MB (Molecular Breeding) BC2F3 families and crossed with two testers and evaluated phenotypically.Under severe condition of WS, the yield would be decreased 60-80% as compared to WW conditions. On observation under mild stress condition, less than 50% yield is reduced, no difference was observed between MABC & control hybrids. No yield was observed under WW conditions. So under this demonstrated that genetic gains can be achieved by introgressing drought QTL.

(b)-The Marker-Assisted Recurrent Selection (MARS) Approaches:

The advantage of MARS is higher as compared to phenotypic selection. Although the use of MARS has enjoyed only in public sector, only limited success in public sector (Ragot et al., 2000; Johnson, 2004; Crosbie et al., 2006). For example, (Ragot et al., 2000) he identify QTL in bi parental maize population and then applied a genetic index where is involving agronomic performance (grain yield, and moisture at harvest) & adapted to abiotic stress (early vigor under cold conditions).Similarly other scientist (Eathington et al., 2005) he demonstrated that the rate of genetic gain achieved through MARS was about twice that possible using in the phenotypic selection. So there are several accounts in which at least one of parental lines of commercial maize hybrids was derived via MARS.

Conclusion:

Conventional breeding has improved the drought tolerance of temperate maize hybrids and the use of managed drought environments, accurate phenotyping and the identification and deployment of secondary traits has been effective in improving the drought tolerance of tropical maize populations and hybrids as well. The contribution of molecular biology will be identify key genes involved in metabolic pathways related to stress response.e.g;the factors involved in kernel development. Armed with better understanding of the physiological mechanisms and the genetic basis of the response of maize to drought, it should become increasingly feasible to identify, transfer and select key genes and alleles to build genotype with much improved tolerance to drought.