INTRODUCTION
Soil salinization is a major environmental constraint to limit plant growth and production which is closely associated with arable land degradation (Shahid et al., 2018; Marriboina & Attipalli, 2020a). Approximately 1.5 million hectares of cultivable land are becoming saline marginal lands by every year because of high salinity levels and nearly 50% of arable lands will be lost by year 2050 (Hossain, 2019). Global climate change, increasing population and excessive irrigation are further limiting the availability of cultivable land for crop production (Raza et al., 2019). To date, attempts are being made to extend the crop productivity on saline lands. However, progress of these attempts is greatly hampered by the genetic complexity of salt tolerance, which largely depends on physiological and genetic diversity of the plant and spatio-temporal heterogeneity of soil salinity (Morton et al., 2019). To address this issue, several plant species were introduced to rehabilitate the saline lands and certain economically important nitrogen fixing biofuel tree species are of immense importance not only for sustenance to saline marginal lands but also economic gain towards saline lands (Samuel et al., 2013; Hanin et al., 2016; Marriboina & Attipalli, 2020a).
A comprehensive understanding of physiological, hormonal and molecular adaptive mechanisms is crucial to cultivate these tree species on saline marginal lands (Quinn et al., 2015). Plants growing in saline soils prevent the excess Na+ ion disposition in the leaves in order to protect the photosynthetic machinery from salt-induced damage. The decrease in net CO2 assimilation rate and optimum quantum yield of PSII (Fv/Fm) might substantiate the leaf performance under salt induced drought stress. Calcium ion (Ca2+) is known as an intracellular second messenger and plays an important role in plant growth and development. It also plays an essential role in amelioration of sodium toxicity through activating several Ca2+ responsive genes and channels (Thor, 2019). In response to salt stress, plant produces several phytohormones such as ABA, JA, MeJA, zeatin, IAA, IBA and SA, which plays crucial role in sustaining its growth under extreme saline conditions. ABA is well-known stress induced phytohormone, critical for plants growth and regulating numerous downstream signalling responses (Tuteja, 2007). ABA causes stomatal closure to prevent excess water evaporation and regulate root growth under salinity stress (Zelm et al., 2020). Auxins and cytokinins are growth promoting phytohormones interacts to regulate various growth and developmental process such as cell division, elongation and differentiation. Salt induced endogenous accumulation of cytokinin improves the salt tolerance in crop species by delaying leaf senescence and marinating photosynthetic capacity (Liu et al., 2012; Gloan et al., 2017). Upon salt stress, the raise in endogenous SA levels can cause a significant reduction in the ROS and Na+ accumulation across the plant, whereas SA deficient plant produced an elevated levels of superoxide and H2O2 (Yang et al., 2004). According to Sahoo et al., (2014), the perfect harmony among phytohormones played a significant role in improving the salt tolerance in rice. However, the synergistic and antagonistic interactions between phytohormones are mostly depending on plant species and type of stress imposed on plants, but their interactions are still not clearly understood (Gupta et al., 2017). To combat against salinity induced ROS damage, plants adapted jasmonates directed anthocyanin accumulation to mitigate its negative effects (Ali & Baek, 2020). Further, JAs can positively regulate the endogenous ABA level, together regulate the guard cell movement during salt stress (Siddiqi & Husen, 2019). JA and ABA together regulates antioxidant status of the cell to enhance the survivability of plant towards osmotic stress. Additionally, JA and SA positively regulate the several protein coding genes which are responsible for plant salt tolerance (Wang et al., 2020). Plant pre-treated with SA alleviates salinity stress by decreasing Na+ transport and by increasing H+-ATPase activity (Jayakannan et al., 2013; Gharb et al., 2018). Excessive deposition of salts in the cell and celluar compartments causes membrane depolarization. Counteract to the salt-induced membrane depolarization ABA regulates expression of numerous vacuolar and plasma membrane transporters such as vacuolar H+-inorganic pyrophosphatase, vacuolar H+-ATPase, NHX1, V-PPase, and PM-H+-ATPase pumps (Fukuda and Tanka, 2006). According to Shahzad et al., (2015), exogenous application of JAs on maize improved salt tolerance by regulating Na+ ion uptake at the root level. Proton pumps and cation channels such as H+-ATPase pumps, CHXs and CCXs were involved in maintaining the membrane potential under salt stress conditions (Falhof et al., 2016; Li et al., 2016; Liu et al., 2017). Importantly, plant induces the expression of several isoforms of NHXs namely SOS1, NHX1, NHX2, NHX3 and NHX6 under salt stress to regulate Na+fluxes in and out of the cell (Dragwidge et al., 2018). Upon salt stress, plants activate a complex antioxidant defense mechanism to minimize oxidative stress damage by ROS under salt stress conditions (Xie et al., 2019).
Population and industrialization pressure has increased the demand for land and fossil fuel resources. In addition, there is increasing demand for renewable energy resources due to the fast depletion of fossil fuel resources. For the first time, we report here the mechanisms of salinity tolerance in Pongamia at molecular level with the help of hormonal, metabolomics, gene expression and computational approaches. Further, the assessment of tissue specific-phytohormone profiling elucidates the key role of specific hormones in conferring the tissue-specific associated mechanisms of salt tolerance in Pongamia. In addition, the correlation studies between the phytohormones enable to identify the crosstalk between phytohormones, which may regulating the growth and development in Pongamia under salt stress (Maury et al., 2019). Time-course metabolic profiling and correlations under salt stress in Pongamia would certainly contribute to understand the biochemical changes involving metabolic pathways, which is crucial in plant adaptation to salinity stress conditions. The interaction studies between hormones and metabolites should certainly create new opportunities for the discovery of hormone-metabolite associates, which are very crucial to understand stress tolerant mechanisms (Cao et al., 2017). The present study provides an evidence for the hormone-metabolite interactions as well as novel hormone-metabolite associated signalling pathways to understand high salinity tolerance mechanisms in Pongamia pinnata .