Introduction
Soybean is an economically important legume-cum-oilseed crop being rich source of protein (40%) and edible oil (20%) (Chaudhary et al. , 2015). However, soybean production is affected by a range of environmental stresses (Sharmin et al. , 2020). In recent years, increased occurrence of drought events was reported due to global climate change (Bailey-Serres et al., 2012). Drought stress at germination stage is one of the major constraints that badly affects seed germination and seedling establishment. Accumulated evidence indicates that yield is affected by drought stress at early (germination and vegetative) and late (reproductive) growth stages (Zhao et al ., 2017; Wijewardana et al. , 2019). However, a little progress has been made to elucidate the genetic mechanisms of drought tolerance during germination stage in soybean. Hence, concerted efforts are needed to understand the genetic basis and genes associated with drought tolerance during germination stage.
During germination stage many essential physiological and biochemical processes are induced to initiate germination such as hydrolysis, subcellular structures, cell elongation biosynthesis of macromolecules and respiration (Bewley et al ., 2013). Many phytohormones i.e., ethylene (ETH), auxin (AUX), abscisic acid (ABA), gibberellic acid (GA), salicylic acid (SA) and jasmonic acid (JA) as well as some environmental conditions, including light, oxygen and temperature regulates seed germination (Miransari and Smith, 2014). Drought stress is an important environmental factor hampering seed germination and seedling vigor (Delachiave et al. , 2003; Ahmad et al. , 2009). It not only hinder the seed germination but also increases mean germination time (Valliyodan and Nguyen, 2006). At germination, soybean seed must absorb water (50% of its weight) to begin normal germination. Insufficient water availability especially during seed emergence stage lower the soybean yield (Wijewardana et al. , 2019; Kobraee et al. , 2011). Several researchers have reported the negative impact of drought on seed germination and seedling vigor in different crops such as mung bean (Rani et al. , 2018), barnyard grass (Wu et al. , 2019), sesame (Boureima et al . 2011), maize ( Ahmad et al. , 2018) and soybean (Vieira et al . 1991).
Response of crop plants to drought stress at molecular level involve perception, signal transduction, gene expression and ultimately metabolic changes resulting in stress tolerance (Huang et al. , 2012). Drought tolerance being a complex quantitative trait involves numerous regulatory and functional genes (Jiménez et al. , 2013). Hence, numerous genes are involved in stomatal movement, osmolyte metabolism, antioxidant activity and phytohormones signalling, under drought stress and re-watering (Shinozaki and Yamaguchi-Shinozaki, 2007; Laxa et al. , 2019). Besides, drought stress stimulates many adaptive signaling pathways to cope with adverse effects of stress, such as MAPK cascade (Sinha et al. , 2011) and Ca2+signaling (Wilkins et al. , 2016) and signaling of phytohormones (Tiwari et al. , 2017). These stress signaling pathways modify the expression of drought responsive genes and provide defensive mechanisms through activating downstream TFs, (Lan et al. , 2017) However, most of these gene function as well as regulatory networks involved in drought stress at germination stage have remained elusive. Hence, it is prerequisite to identify the genetic mechanism and networks involved in modulation of drought tolerance.
Advances in next-generation sequencing (NGS) platforms i.e., Illumina/Solexa has made it possible to understand the complexity and regulation of gene expression networks in different crop species under dehydration stress (Chen et al. , 2016). The NGS platforms provides wider and more robust transcriptome analyses (Severin et al. , 2010). Transcriptome studies (with Affymetrix/RNA sequencing technology) utilizing contrasting lines to understand drought stress response has been carried in different crops such as rice (Degenkolbeet al . 2009; Lenka et al. , 2011), but in case of soybean mostly single genotype was used (Le et al. 2012; Song et al. , 2016). Use of contrasting genotypes in transcriptome studies will provide comparative and detailed information on DEGs, that could possibly assist in identifying important genes regulating drought tolerance. Comparative transcriptomic analysis of various genotypes will be helpful to dissect biological pathways and mechanisms imparting tolerance to environmental stresses, as well as to explore genes under selection. Although, significant insights have been provided by fundamental research into the physiological and molecular responses of plants to drought stress, but the divergence in root transcriptome of soybean drought responsive genotypes during germination remains largely unexplored.
In the present study, the comparative transcriptomic analysis between drought-tolerant (PI342618B/DTP & A214/DTL) and drought-sensitive (NN86-4/DSP & A195/DSL) soybean genotypes were analyzed comprehensively to gain insights into soybean defense response under drought stress at germination stage. This study provided an in-depth information at the global transcriptome level, and identified pathways and candidate genes associated with drought tolerance at germination stage in soybean.