**1. Introduction**

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A Comprehensive Survey of International Soybean Research - Genetics, Physiology, Agronomy and Nitrogen

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In many regions of the world, agriculture is the primary consumer of water. As the world population increases, and arid regions become more abundant, water will become an in‐ creasingly scarce resource [1]. In 2011, the world's soybean crops produced 263.7 million tons from an area of 103,5 million hectares [2]. This global production required an input of 0,2 to 0,25 inch of water per acre per day during peak demand, which represents a major problem for the producer countries [3]. In Brazil, the second largest soybean producer in the world, there was a 7% reduction in soybean production in 2011/2012 compared to the previ‐ ous season. This yield loss can be attributed to drought in the soybean-growing regions of the country, which in turn resulted in increased use of irrigation water in an attempt to min‐ imize yield losses [4].

Understanding the molecular consequences of drought on soybean plants can accelerate breeding programs aimed at increasing productivity and decreasing the negative impacts of climate change on this important crop. Several classical physiology reviews from recent dec‐ ades consolidated knowledge of the relationship between leaf structure and function during drought stress [5,6], the morphology of the root during stress tolerance [5,6] and other as‐ pects of the effects of drought on plant morphology. Understanding the physiological re‐ sponses of plants undergoing drought stress is essential to understanding their ability to survive the water shortage.

In recent years, due to advancements in plant molecular biology methodologies, molecular aspects of drought tolerance have received special attention from researchers [7]. To date, hundreds of genes that are induced by drought stress have been identified and a range of genetic, biochemical and molecular assays (gene expression profiles, transgenic plants, and various functional assays), are being used to elucidate the roles of these genes in response to

properly cited.

© 2013 Siqueira Alves and Gomes Fietto; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is © 2013 Siqueira Alves and Gomes Fietto; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

drought. However, the complexity of the plant response to drought stress makes it difficult to identify genes that are responsible for drought tolerance [7]. In physiological terms, drought stress is characterized by reduction in plant water content, decrease in water poten‐ tial, loss of leaf turgor, stomata closure and reduction in cell growth [8]. Conditions of severe and prolonged drought result in cessation of photosynthesis, metabolic disorder, and finally plant death [8].

Many abiotic stresses such as high salt levels and low temperature have similar physiologi‐ cal consequences to drought, and therefore similar signaling pathways are induced [7]. The similarity of the cold and drought stress response is illustrated by the observation that plants subjected to drought stress display an increase in frost tolerance [9]. An increase in osmotic pressure is common to these abiotic stresses [10]. The increased osmolarity induces transcription of genes encoding proteins involved in synthesis of osmo-protective com‐ pounds, lipid desaturases and transcription factors [11]. Several of these genes have been frequent targets of genetic engineering in breeding programs aimed at producing cultivars with increased tolerance to these adverse conditions [11]. These genes are also induced by other environmental factors such as high salinity and chemical signals such as abscisic acid (ABA), the main phytohormone related to abiotic stress responses in plants.

ABA serves as an endogenous messenger in response to biotic and abiotic stress in plants. Drought results in production of high levels of ABA, accompanied by a major shift in global gene expression in plant cells and, consequently, an adaptive physiological response to the stress [12]. In addition to stress, ABA also controls other important and finely regulated processes such as growth and development, structure and regulation of stomatal function and seed dormancy [13]. During regulation of plant development, ABA also acts in intricate cross-communication with other important phytohormones, such as gibberellic acid, ethyl‐ ene, auxin and brassinosteroids [13].

How and what environmental stimuli are perceived and result in changes in physiological levels of ABA is still a difficult issue. Drought stress provides an immediate hydraulic signal to the plant, which activates ABA biosynthesis over a great distance [14]. High humidity ac‐ tivates cytochrome P450 enzymes that catalyze ABA synthesis minutes after perception of the stress [15]. Recent studies have shown the importance of the transport driven by absorp‐ tion and export of ABA. Upon perception of the stress signal, ABA synthesis is primarily in‐ duced in vascular tissues, and ABA is exported from the site of biosynthesis to other cells. The absorption is stimulated by ATP-dependent ABC-family transporters. This mechanism allows rapid distribution of ABA to the surrounding tissues [16,17].

Although expression of many genes is induced by ABA-dependent responses to drought, cold and salinity stresses, upregulated genes can be sub-grouped according to the stress they were found to respond to and also by the timing of induction post stress. Genes includ‐ ed in the RD group (responsive to dehydration) include the drought-induced gene RD26, which encodes a NAC (NAM/ATAF/CUC plant protein domains)-family transcription factor [18], ERD (early responsive to dehydration), which includes a gene that encodes a Clp pro‐ tease [19]. The COR (cold regulated), LTI (low-temperature induced) groups of genes in‐ clude LOS2, which encodes a bifunctional enolase [20]. The KIN (cold inducible) group of

genes includes SCOF-1, which encodes a protein with a zinc finger domain [21]. The KIN group also contains groups of genes, which also respond to osmotic stress [22-23, 7]. The products of many of these genes are most likely the main components of the first line of plant defense against potential structural damage, or they may be components of signaling pathways such as transcription factors or protein kinases. An example is induction of the gene COR15a; the Arabidopsis homolog ERD1 prevents the injury to the chloroplast mem‐ brane [24]. Another gene, GmERD15, from the ERD15 gene family in soybean, acts as a tran‐ scription factor, which regulates gene transcription related to programmed cell death [25].

drought. However, the complexity of the plant response to drought stress makes it difficult to identify genes that are responsible for drought tolerance [7]. In physiological terms, drought stress is characterized by reduction in plant water content, decrease in water poten‐ tial, loss of leaf turgor, stomata closure and reduction in cell growth [8]. Conditions of severe and prolonged drought result in cessation of photosynthesis, metabolic disorder, and finally

A Comprehensive Survey of International Soybean Research - Genetics, Physiology, Agronomy and Nitrogen

Many abiotic stresses such as high salt levels and low temperature have similar physiologi‐ cal consequences to drought, and therefore similar signaling pathways are induced [7]. The similarity of the cold and drought stress response is illustrated by the observation that plants subjected to drought stress display an increase in frost tolerance [9]. An increase in osmotic pressure is common to these abiotic stresses [10]. The increased osmolarity induces transcription of genes encoding proteins involved in synthesis of osmo-protective com‐ pounds, lipid desaturases and transcription factors [11]. Several of these genes have been frequent targets of genetic engineering in breeding programs aimed at producing cultivars with increased tolerance to these adverse conditions [11]. These genes are also induced by other environmental factors such as high salinity and chemical signals such as abscisic acid

ABA serves as an endogenous messenger in response to biotic and abiotic stress in plants. Drought results in production of high levels of ABA, accompanied by a major shift in global gene expression in plant cells and, consequently, an adaptive physiological response to the stress [12]. In addition to stress, ABA also controls other important and finely regulated processes such as growth and development, structure and regulation of stomatal function and seed dormancy [13]. During regulation of plant development, ABA also acts in intricate cross-communication with other important phytohormones, such as gibberellic acid, ethyl‐

How and what environmental stimuli are perceived and result in changes in physiological levels of ABA is still a difficult issue. Drought stress provides an immediate hydraulic signal to the plant, which activates ABA biosynthesis over a great distance [14]. High humidity ac‐ tivates cytochrome P450 enzymes that catalyze ABA synthesis minutes after perception of the stress [15]. Recent studies have shown the importance of the transport driven by absorp‐ tion and export of ABA. Upon perception of the stress signal, ABA synthesis is primarily in‐ duced in vascular tissues, and ABA is exported from the site of biosynthesis to other cells. The absorption is stimulated by ATP-dependent ABC-family transporters. This mechanism

Although expression of many genes is induced by ABA-dependent responses to drought, cold and salinity stresses, upregulated genes can be sub-grouped according to the stress they were found to respond to and also by the timing of induction post stress. Genes includ‐ ed in the RD group (responsive to dehydration) include the drought-induced gene RD26, which encodes a NAC (NAM/ATAF/CUC plant protein domains)-family transcription factor [18], ERD (early responsive to dehydration), which includes a gene that encodes a Clp pro‐ tease [19]. The COR (cold regulated), LTI (low-temperature induced) groups of genes in‐ clude LOS2, which encodes a bifunctional enolase [20]. The KIN (cold inducible) group of

(ABA), the main phytohormone related to abiotic stress responses in plants.

allows rapid distribution of ABA to the surrounding tissues [16,17].

plant death [8].

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476

ene, auxin and brassinosteroids [13].
