Salinity-induced alteration in expression of ion transporter genes
In the present study, Pongamia exhibited tissue specific expression of salt-responsive transporter genes including SOS1, NHXs, PM-H+-ATPases, V-H+-ATPases, CNGCs, and other transporter genes. The expression patterns of SOS1 and SOS1-like genes correlated with Na+ fluoresence andNa+ion content,indicating that SOS1 genes are responsible for low Na+ levels in the leaves of Pongamia by increasing Na+ loading into the xylem/ apoplastic region. Further, the expression levels of SOS1 and SOS1-like genes increased significantly in salt-treated roots, which may contribute to apoplastic Na+ depostion under high salinity. The differential expression of SOS2 and SOS3in the both leaves and roots suggest their crucial role in salt tolerance of Pongamia through SOS pathway (Marriboina et al., 2017). In this study, we observed that the expression of NHX1 was unchaged upon 300 mM salt stress in both leaves and roots of Pongamia, while there was significant induction in 500 mM NaCl stress in both leaves and roots, which correlates with the low levels of Na+ fluoresence intensity and Na+content data in leaves of 300 mM NaCl treated plants. At high salt concentration (500 mM NaCl), Pongmia might induce NHX1 experssion to sequester Na+ ions rapidly into the vacuole to mitigate the Na+ toxicity. Similarly, in roots, induced NHX1 expression may indicate the vacuolar Na+ sequestration. In contrast, NHX1 levels wereunaffected at intial impostion of stress, suggesting involvement of other NHXs isofoms for vacuolar Na+ sequestration at intial stages of salt stress. The differential expresssion of other NHXs isoforms such as NHX3, NHX6 nd NHX6-like in both leaves and roots suggests their possible roles in ion homeostasis, plant growth and development under salt stress (Bassil et al., 2018; Dragwidge et al., 2018).
In this study,expression of HKT1:1 followed similar trend in both leaves and roots of salt stressed plants. Previous studies suggested that HKT family transporter proteins can mediate exclusion of Na+from leaves and roots by translocating shoot-to-rootand root-to-shoot Na+ delivery by withdrwaing Na+ from xylem stream into phloem (Munns et al., 2012; Hill et al., 2013; An et al., 2017; Zhang et al., 2018). Induction of HKT1:1 expression upon initial imposition of salt stress in both leaves and roots may promote Na+ exclusion tolimit Na+ toxcity in the respective tissues(Zhang et al., 2018). However, with increasing salt stress treatment time, the marginal expression of HKT1:1genemay regulate the retrevial of Na+ from the xylem (Davenport et al., 2007; Ali et al., 2016). The expression levels of CLC1 indicates that CLC1 may not be involved in the Cl- vacuolar sequestation (Wei et al., 2016).
PM-H+-ATPase family pumps playa crucial role in improving the salt tolerance by maintaing the intracellular pH balance, transmembrane potential and ion homeostasis under salt stress conditions (Olfatmiri et al., 2014;Falhof et al., 2016; Shabala et al., 2016). The diffrences in the expression levels of PM-H+-ATPase isoforms could be due toconfigurational and/or post-translational modifications of these isforms under salt stress, which could promote growth under salinity stress. The increased expression of V-H+-PPase and V-H+-ATPaseB and E subunit in leaves of salt treated plants may control the depolarization of vacuolar membrane potential, which is generated by excess depostion of Na+ ions in leaves of Pongamia upon prolonged salt exposure (Graus et al., 2018; Marriboina & Attipalli, 2020a).
An increased expression levels of V-CHX1 may involve in plant growth by improving cellular ion homeostasis, pH balance and osmoregulation under saline conditions (Guan et al., 2014; Qi et al., 2014; Liu et al., 2017). Enhanced experssion of CCX1 may involved in regulation of intracellular Ca2+ levels, which may help in vacuolar Na+ sequestration, ROS and ion homeostasis under salt stress (Yong et al., 2014; Li et al., 2016; Corso et al., 2018). Differential expression of CNGC5, CNGC17 and CNGC17-like may induce Ca2+ derived reponses to mitigate the negative effects of salt stress (Wang et al., 2013; Saand et al., 2015).