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.