Silent S-Type Anion Channel Subunit SLAH1 Gates SLAH3 Open for Chloride Root-to-Shoot Translocation

Abstract

Higher plants take up nutrients via the roots and load them into xylem vessels for translocation to the shoot. After uptake, anions have to be channeled toward the root xylem vessels. Thereby, xylem parenchyma and pericycle cells control the anion composition of the root-shoot xylem sap [1-6]. The fact that salt-tolerant genotypes possess lower xylem-sap Cl(-) contents compared to salt-sensitive genotypes [7-10] indicates that membrane transport proteins at the sites of xylem loading contribute to plant salinity tolerance via selective chloride exclusion. However, the molecular mechanism of xylem loading that lies behind the balance between NO3(-) and Cl(-) loading remains largely unknown. Here we identify two root anion channels in Arabidopsis, SLAH1 and SLAH3, that control the shoot NO3(-)/Cl(-) ratio. The AtSLAH1 gene is expressed in the root xylem-pole pericycle, where it co-localizes with AtSLAH3. Under high soil salinity, AtSLAH1 expression markedly declined and the chloride content of the xylem sap in AtSLAH1 loss-of-function mutants was half of the wild-type level only. SLAH3 anion channels are not active per se but require extracellular nitrate and phosphorylation by calcium-dependent kinases (CPKs) [11-13]. When co-expressed in Xenopus oocytes, however, the electrically silent SLAH1 subunit gates SLAH3 open even in the absence of nitrate- and calcium-dependent kinases. Apparently, SLAH1/SLAH3 heteromerization facilitates SLAH3-mediated chloride efflux from pericycle cells into the root xylem vessels. Our results indicate that under salt stress, plants adjust the distribution of NO3(-) and Cl(-) between root and shoot via differential expression and assembly of SLAH1/SLAH3 anion channel subunits.

Biology of SLAC1‐type anion channels – from nutrient uptake to stomatal closure

“…a). Interestingly, both single and double SLAH1/SLAH3 loss‐of‐function mutants conducted only half the chloride content in their xylem sap compared with the corresponding wild‐type plants, while the xylem sap nitrate content appeared to be unchanged (Cubero‐Font et al ., ; Qiu et al ., ). Subsequent coexpression studies between SLAH3 and SLAH1 in Xenopus oocytes revealed that these SLAH‐subunits specifically interact with each other, resulting in the activation of SLAH3 even in the absence of an activating kinase and nitrate (Fig.…”

“…When coexpressed with AtSLAH1, this kinase‐independent SLAH3 isoform gains a major chloride conductance. This behaviour suggests that SLAH1–SLAH3 heteromerization represents a general species‐independent SLAH3 nitrate–chloride switch (Cubero‐Font et al ., ; Jaborsky et al ., ).…”

“…Recently, Cubero‐Font et al . () colocalized SLAH3 and SLAH1 in the xylem pole pericycle cells adjacent to the water and nutrient conducting xylem vessels (Fig. a).…”

AtNPF2.5 Modulates Chloride (Cl−) Efflux from Roots of Arabidopsis thaliana

“…However, the finding that AtCCC and Vitis vinifera CCC (VviCCC) are localized to the Golgi and trans -Golgi network (Henderson et al, 2015), and that Orysa sativa CCC (OsCCC) is involved in osmoregulation and tissue growth suggests that the role of CCC in long distance Cl − transport is likely to be indirect. More recently, AtNPF2.4 (Li et al, 2016) and AtSLAH1 ( Slow Anion Channel Associated 1 Homolog1 ) (Cubero-Font et al, 2016; Qiu et al, 2016) have been identified as genes encoding plasma membrane-localized anion transporters in the root stele that facilitate transfer of Cl − into the root xylem. As one member of the NRT1/PTRs in Arabidopsis, NPF2.4 belongs to the NAXT (for Nitrate Excretion Transporter) subfamily (a seven-gene clade) that was named after NAXT1 ( NPF2.7 ), a plasma membrane transporter involved in the efflux of ${\text{NO}}_3^{-}$ from the root surface (Segonzac et al, 2007; Tsay et al, 2007; Léran et al, 2014).…”

Investigation of the regulation mechanism of Arabidopsis thaliana anion channel SLAH2

“…SLAH3 also plays a role in nitrate-dependent alleviation of ammonium toxicity in Arabidopsis roots (Zheng et al, 2015). Moreover, SLAH1 and SLAH3 are involved in chloride (Cl -) translocation from root to shoot (Cubero-Font et al, 2016;Qiu et al, 2016). SLAH4 is also expressed in roots (Zheng et al, 2015), but its functional characterization has not been reported yet.…”