The consequence of salt-induced phytohormonal disturbance in
leaves and roots of Pongamia.
Developmental plasticity under stress conditions largely depends upon
the interactions between hormones, which regulate stress-adaptation
responses and developmental processes. In this study, both leaves and
roots of salt treated plants showed a significant diversity in hormone
profile and their correlation patterns at all-time points. A significant
correlation was observed among all hormones due to initial exposure of
300 mM NaCl stress in leaves. The rise in all hormones and correlation
may be beneficial for the plant to maintain the growth under salt stress
conditions (Sahoo et al., 2014; Fahad et al., 2015). Moreover, increased
levels of zeatin in both leaves ad roots of salt treated plants improved
RRWC and stress-induced growth under salt stress conditions (Ghanem et
al., 2011; Nishiyama et al., 2011; Wu et al., 2014; Melo et al., 2016).
A strong correlation was also observed between zeatin and JAs in leaves
and roots of 300 mM NaCl treated plants. The interactive influence of
cytokinin may substantiate JAs negative impact to promote plant survival
under extreme saline conditions. The synergistic interaction between
zeatin and JAs may also enhance the salt induced-vasculature in roots to
enhance water uptake, which is also supported by the well maintained
RRWC in roots of treated plants (Supplementary Figure 2D) (Ueda& Kato,
1982; Nitschke et al., 2016; Jang et al., 2017).
IAA levels were significantly increased in leaves of salt treated plants
at all-time points, indicating that IAA might play an important role in
Pongamia salt tolerance. Transgenic poplar plants, overexpressingAtYUCCA6 gene associated with increased levels of auxin, showed
delayed chlorophyll degradation and leaf senescence (Kim et al., 2012).
The increased levels of IAA and IBA may due to tissue damage or cell
lysis. However, our fluorescence studies clearly suggest the viable
status of cells and tissue integrity. Therefore, increased IAA levels in
Pongamia might help in maintaining “stay-green” trait and steady
levels of chlorophyll pigments under salt stress (Kim et al., 2012).
Likewise, IBA levels also showed significant increase in both leaves and
roots of salt treated plants, suggesting that enhanced IBA levels may
also play a role in acquiring the stress-induced protective
architectural changes in Pongamia (Tognetti et al., 2010). In addition,
correlation studies revealed that IBA showed good interaction with IAA
in leaves of 300 mM NaCl treated plants. It was evident that the plants
deficient in both IAA and IBA genes showed defective plant growth and
development (Spiess et al., 2014). Auxins also showed a strong
association with JAs (JA and MeJA) in leaves and roots of salt treated
plants, which might involve in promoting salt-induced growth and tissue
integrity during salt stress (Cai et al., 2014; Fattorini et al., 2018;
Ishimaru et al., 2018).
A significant increase in JAs was detected in both leaves of salt
treated plants. However, in roots, JA levels were maintained low till
4DAS, and restored to control levels at 8DAS. Conversely, MeJA levels
were maintained high till 4DAS, while these levels returned to control
levels at 8DAS. The results indicates that the two different forms of
JAs are presumably interchangeable and might share common signal
transduction pathway in Pongamia during salt stress (Diallo et al.,
2014; Mitra & Baldwin, 2014; Cao et al., 2016; Li et al., 2017). An
exogenous application of JAs reduced shoot growth, enhanced water uptake
and cell wall synthesis in certain crop species (Kang et al., 2005;
Uddin et al., 2013; Shahzad et al., 2015; Tavallali & Karimi, 2019).
Previous studies suggest that the increased JAs level alleviate the
toxic effects of salt stress by lowering the Na+ and
Cl- ions accumulation across the plant (Shahzad et
al., 2015). Correlation studies revealed that JAs showed a strong
correlation with ABA in leaves of treated plants. The interactions
between JAs and ABA may induce stomatal closure by triggering the
stress-induced signalling pathways in guard cells, preventing water loss
from the leaves (Munemasa et al., 2011; Wang et al., 2016; Yang et al.,
2018) (Figure 7).
ABA levels were significantly increased in leaves of salt treated plants
which might limit the stomatal conductance, water content, transpiration
rate and CER by closing the stomatal apparatus (Skorupa et al., 2019).
The observed reduction in ABA accumulation in roots might be due to ABA
transfer from root to shoot or ABA exudation from the roots (Shi et al.,
2015). The prolonged maintenance of higher ABA levels negatively impacts
the plant growth, while transient increase helps in mitigation of salt
stress by enhancing the stress responsive genes (Shi et al., 2015).
Further, reduced ABA content may favour in maintaining RWC by regulating
aquaporin proteins (Shi et al., 2015). The correlation studies revealed
that ABA showed a strong interaction with SA in roots of salt treated
plants, which improves plant growth under saline conditions, albeit the
signalling mechanism is still unclear (Devinar et al., 2013) (Figure 7).
SA levels in roots were maintained little low at 1DAS and maintained
high at 4DAS, while these levels returned to control levels at 8DAS.
Exogenous application of SA on plants showed an improvement in LRWC and
ROS homeostasis under salt stress (Jayakannan et al., 2013; Husen et
al., 2018). The rise in SA levels might protect the leaves from salt
injury by inhibiting necrosis signalling pathways and also regulate the
leaf turgor by accumulating carbohydrate polyols (mannitol, pinitol, and
myo-inositol) (Husen et al., 2018). The observed reduction in SA levels
in roots may be due to the transportation of SA from root to shoot under
salt stress conditions (Xu et al., 2017). Correlation studies revealed a
positive correlation between SA and IAA in leaves of salt treated plants
might help in maintaining the leaf cell extensibility to promote better
plant growth under salt stress (Formentin et al., 2018; Shaki et al.,
2019).