A root developmental gradient in vacuolar sequestration
Sodium uptake, transport, and sequestration across the three examined
roots zones are closely linked to their developmental stages. Thus, we
have proposed a model of sodium tolerance based on the developmental
gradient of pistachio roots (Fig. 6). Sodium uptake and transport at the
meristematic region are relatively unhindered by apoplastic barriers,
and it is accompanied by vacuole sequestration to minimize the impact of
the salt ions. In the second developmental zone (zone 1), the peak of
Na+ sequestration in parenchyma cells likely reflects
their higher vacuole storage capacity and maturation status compared to
the meristematic region. This corroborates the pattern of
Na+ vacuolar sequestration in diverse species
(Bojórquez-Quintal et al., 2014; Wu et al., 2015; Wu et al., 2019). It
is possible that more ionic uptake occurs in this zone, due to the
increase in the number of mature vessel elements compared to the
younger, more meristematic zone 0 (Baum et al., 2002; Wachsman et al.,
2015), and the higher rate of sodium entry becomes reflected in the
vacuolar sequestration. Apoplastic entry of sodium to the third
developmental zone (zone 2) may be more tightly controlled due to the
extensive suberization of apoplastic barriers, resulting in enhanced
blockage of apoplastic transport and absorption (Serra et al., 2009;
Ranathunge et al., 2011; Barberon, 2017). This may lead to a reduction
in the rate of sodium entry into root at zone 2 and reduced vacuolar
sequestration compared to the younger developmental zones.
Together, our results suggest that vacuolar sequestration is coordinated
across different developmental zones. The responses of other subcellular
compartments, such as the different endosomes/vesicles beyond the
vacuole, and cell type specificity (Oh et al., 2015) are added layers of
complexity that can be considered within the root developmental gradient
in future studies.