instance, RNA-seq is being used for transcription start site mapping, strand-specific
measurements, gene fusion detection, small RNA characterization, and detection of
alternative splicing events (Ozsolak and Milos 2010). Transcriptome profiling
revealed through massively parallel RNA sequencing has offered new insights into
gene networks that respond to drought stress (Table 4.5), such as NAC, etc. (Xu et al.
2018). These efforts can be used to generate an expression atlas for soybean genes
related to drought tolerance which may serve as a useful genomic resource.
4.7
Molecular Events During Drought Stress in Soybean
Understanding the molecular mechanism of stress tolerance and developing stresstolerant cultivars is important to achieve optimal yield from soybean crop.
Modulations in gene expression are the earliest responses in plants, and a number
of stress-responsive genes have been noted to have important functions in drought
and salt resistance. Drought tolerance property of soybean involves complex network of genes and metabolites. Calcium channels, calcium binding proteins, receptor like protein kinases (RLKs), G-protein coupled receptors, histidine kinases are
proposed to act as potential osmosensors in plants. Expression of several transcription factors (TFs), receptor like kinases (RLKs), calcium signaling components were
upregulated in roots under drought stress (Tripathi et al. 2016a). It was shown that
the genes involved in hormone, carbohydrate, and cell wall metabolism were
differentially regulated in soybean roots under water stress. In another study, the
level of expression of two auxin-responsive factors (ARFs), GmARF3 and
GmARF50, was increased in roots and shoots under dehydration stress (Ha et al.
2015). Proteomic analysis of root suggested the involvement of osmoprotectants,
kinases, and transcription factors in drought response (Mohammadi et al. 2012).
4.7.1 Signal Transduction Under Drought Stress
Abscisic acid (ABA) biosynthesis and accumulation in response to drought is
reported in several plants (Sachdeva et al. 2020). ABA regulates the stomatal closure
and other metabolic pathways during abiotic stress. Ca
2+ and ROS are reported to
participate in ABA mediated signal transduction pathway. Increased cytosolic Ca
2+
level induces several Ca
2+ binding proteins, viz. calmodulins (CaMs), calmodulin
like (CML), calcium-dependent protein kinases (CDPKs), and calcineurin B like
proteins (CBLs). Ca
2+ application affects the nodulation process in soybean. Under
drought stress, ROS generation is enhanced leading to accumulation of H 2 O 2 , which
activates ROS scavenging mechanism. Mitogen activated protein (MAP) kinase
cascade is involved in signaling pathway of many TFs under both biotic and abiotic
stresses (Fujita et al. 2006). Degradation of proteins mediated by ubiquitination is
another pathway involved in abiotic stress tolerance (Lyzenga and Stone 2012).
Metabolic engineering using TFs may regulate several genes of the downstream
pathway leading to improved tolerance to abiotic stresses. To reduce the undesired
4 Breeding and Molecular Approaches for Evolving Drought-Tolerant Soybeans
103
measurements, gene fusion detection, small RNA characterization, and detection of
alternative splicing events (Ozsolak and Milos 2010). Transcriptome profiling
revealed through massively parallel RNA sequencing has offered new insights into
gene networks that respond to drought stress (Table 4.5), such as NAC, etc. (Xu et al.
2018). These efforts can be used to generate an expression atlas for soybean genes
related to drought tolerance which may serve as a useful genomic resource.
4.7
Molecular Events During Drought Stress in Soybean
Understanding the molecular mechanism of stress tolerance and developing stresstolerant cultivars is important to achieve optimal yield from soybean crop.
Modulations in gene expression are the earliest responses in plants, and a number
of stress-responsive genes have been noted to have important functions in drought
and salt resistance. Drought tolerance property of soybean involves complex network of genes and metabolites. Calcium channels, calcium binding proteins, receptor like protein kinases (RLKs), G-protein coupled receptors, histidine kinases are
proposed to act as potential osmosensors in plants. Expression of several transcription factors (TFs), receptor like kinases (RLKs), calcium signaling components were
upregulated in roots under drought stress (Tripathi et al. 2016a). It was shown that
the genes involved in hormone, carbohydrate, and cell wall metabolism were
differentially regulated in soybean roots under water stress. In another study, the
level of expression of two auxin-responsive factors (ARFs), GmARF3 and
GmARF50, was increased in roots and shoots under dehydration stress (Ha et al.
2015). Proteomic analysis of root suggested the involvement of osmoprotectants,
kinases, and transcription factors in drought response (Mohammadi et al. 2012).
4.7.1 Signal Transduction Under Drought Stress
Abscisic acid (ABA) biosynthesis and accumulation in response to drought is
reported in several plants (Sachdeva et al. 2020). ABA regulates the stomatal closure
and other metabolic pathways during abiotic stress. Ca
2+ and ROS are reported to
participate in ABA mediated signal transduction pathway. Increased cytosolic Ca
2+
level induces several Ca
2+ binding proteins, viz. calmodulins (CaMs), calmodulin
like (CML), calcium-dependent protein kinases (CDPKs), and calcineurin B like
proteins (CBLs). Ca
2+ application affects the nodulation process in soybean. Under
drought stress, ROS generation is enhanced leading to accumulation of H 2 O 2 , which
activates ROS scavenging mechanism. Mitogen activated protein (MAP) kinase
cascade is involved in signaling pathway of many TFs under both biotic and abiotic
stresses (Fujita et al. 2006). Degradation of proteins mediated by ubiquitination is
another pathway involved in abiotic stress tolerance (Lyzenga and Stone 2012).
Metabolic engineering using TFs may regulate several genes of the downstream
pathway leading to improved tolerance to abiotic stresses. To reduce the undesired
4 Breeding and Molecular Approaches for Evolving Drought-Tolerant Soybeans
103