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).