Both vacuolar sequestration and suberin deposition are
implicated in high salt tolerance
In the light of climate change based drought conditions which have led
to increased soil salinity, pistachio provides an attractive model to
dissect mechanisms of salinity tolerance and abiotic stress response
(Ahmad and Prasad, 2012; Jazi et al., 2016; Bailey-Serres et al., 2019).
While most of the studies so far have focused on the overall
physiological plant responses (Picchioni et al., 1990; Ferguson et al.,
2002; Karimi et al., 2009; Godfrey et al., 2019), there has been no
analysis at the cellular and structural level in pistachio roots. In
order to determine salinity tolerance mechanisms in pistachio, we
analyzed Na+ sequestration and apoplastic barrier
differentiation across a root developmental gradient using fluorescence
microscopy. Our study demonstrated a significant increase of vacuolar
Na+ sequestration and levels of suberin in the
endo/exodermis in the more salinity tolerant UCB1 compared to P.
integerrima, after salt treatment, suggesting that both mechanisms are
utilized in tandem for salinity tolerance in pistachios.
Salinity tolerance is a complex trait that includes coordination of
maximizing salt ion exclusion, regulation of salt ion transport, and ion
sequestration at the cellular, tissue, and organ level (Tester and
Davenport, 2003; Yang et al., 2009; Chen et al., 2011; Gupta and Huang,
2014; Chen et al., 2018). While salt ion extrusion at the plasma
membrane is important in many species (Yang et al., 2009; Ji et al.,
2013; Chen et al., 2018), it does not provide complete salinity
resistance, as significant amounts of Na+ still enter
the roots. Thus, additional mechanisms must be employed to limit salt
transport to the salinity sensitive leaves. Vacuolar sequestration of
Na+ is one such mechanism that reduces the amount of
salt reaching the shoot, therefore reducing salt toxicity (Zhang and
Blumwald, 2001; Gonzalez et al., 2012; Gupta and Huang, 2014; Guo et
al., 2020). The dominant role of vacuolar Na+sequestration over Na+ exclusion in salinity tolerance
was recently demonstrated in barley. Screening of 45 barley accessions
for salinity tolerance mechanisms, using a fluorescence microscopy
approach, demonstrated a positive correlation of vacuolar
Na+ sequestration with the salinity tolerant varieties
(Wu et al., 2019).
UCB1 plants showed a higher vacuolar Na+ sequestration
capacity compared to P. integerrima (Fig 3). It is likely that
UCB1 is more efficient at vacuolar sequestration due to the increased
activity of salt ion antiporters such as the NHX transporter
family (Gupta and Huang, 2014; Bassil et al., 2019; Guo et al., 2020).
Overexpression of NHX1 increases salt tolerance in diverse species such
as wheat, rice, tomato and mung bean (Zhang and Blumwald, 2001; Moghaieb
et al., 2014; Kumar et al., 2017; Zeng et al., 2018) and likely
contributes to vacuolar sequestration in pistachio. Retention of
Na+ in the vacuole is also regulated by the control of
Na+‐permeable vacuolar channels, which mediate the
back‐leak of Na+ into the cytosol (Isayenkov et al.,
2010; Munns et al., 2016; Munns et al., 2020). Thus, control of
expression or activity of these channels can also contribute to the
increased vacuolar Na+ in UCB1.
Another plausible scenario for lower P. integerrima vacuolar
sequestration is that P. integerrima may be less efficient at
minimizing salt ion entry, which leads to Na+ levels
exceeding its tolerance threshold, causing cell damage and loss of
vacuolar sequestration capacity. Analyses of expression and activity of
salt ion antiporters such as NHX, AKT1, and SOS1, along with other
Na+‐permeable channels and biotic stress response
markers can help dissect the prominent pathway in UCB1 (Yang et al.,
2009; Gupta and Huang, 2014; van Zelm et al., 2020).