Developmental gradient in suberization
An analysis of the developmental zones showed that the increase in
endodermis suberization after salt treatment in UCB1 can be mainly
attributed to zone 0 and zone1 (Fig. 5d). Furthermore, in both
apoplastic barriers, the greatest difference in suberization between the
two genotypes, when under salinity stress, was observed in the youngest
zone (Fig. 5f). The development of the apoplastic barriers is
highly responsive to abiotic stress (Enstone et al., 2003; Barberon,
2017; Byrt et al., 2018; Chen et al., 2018; Kreszies et al., 2019; Wang
et al., 2020), and contributes in salinity tolerance (Chen et al., 2018;
Wang et al., 2020). The root meristematic zone, in particular,
represents a region with the highest plasticity (Rost, 2011) that
differentiates earlier under abiotic stress (Ji et al., 2014;
Cajero-Sanchez et al., 2019). Our result suggests that both the timing
of apoplastic barrier differentiation and the increase of suberin
deposition contributed to the increase in suberization under salt
stress. The development of the apoplastic barrier closer to the root tip
in response to salt and other abiotic stresses in maize, barley, and
rice support this hypothesis (Shen et al., 2015; Kreszies et al., 2019;
Vishal et al., 2019). However, the increase in suberin deposition,
rather than earlier differentiation of the apoplastic barrier, appears
to be the main contributor to salinity tolerance in UCB1 when compared
to P. integerrima . Salinity stress induces modifications in the
overall cell wall, including suberin, lignin, and polysaccharide
deposition (Byrt et al., 2018; Rui and Dinneny, 2020; Wang et al.,
2020). It remains to be seen if the overall regulation of polysaccharide
biosynthesis and deposition also changes in pistachio to reinforce
minimization of salt ion entry and to limit toxic ion transport to
leaves (Byrt et al., 2018).