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).