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 .