Abstract
The accumulation of high concentrations of chloride (Cl−) in leaves can adversely affect plant growth. When comparing different varieties of the same Cl− sensitive plant species those that exclude relatively more Cl− from their shoots tend to perform better under saline conditions; however, the molecular mechanisms involved in maintaining low shoot Cl− remain largely undefined. Recently, it was shown that the NRT1/PTR Family 2.4 protein (NPF2.4) loads Cl− into the root xylem, which affects the accumulation of Cl− in Arabidopsis shoots. Here we characterize NPF2.5, which is the closest homolog to NPF2.4 sharing 83.2% identity at the amino acid level. NPF2.5 is predominantly expressed in root cortical cells and its transcription is induced by salt. Functional characterisation of NPF2.5 via its heterologous expression in yeast (Saccharomyces cerevisiae) and Xenopus laevis oocytes indicated that NPF2.5 is likely to encode a Cl− permeable transporter. Arabidopsis npf2.5 T-DNA knockout mutant plants exhibited a significantly lower Cl− efflux from roots, and a greater Cl− accumulation in shoots compared to salt-treated Col-0 wild-type plants. At the same time, content in the shoot remained unaffected. Accumulation of Cl− in the shoot increased following (1) amiRNA-induced knockdown of NPF2.5 transcript abundance in the root, and (2) constitutive over-expression of NPF2.5. We suggest that both these findings are consistent with a role for NPF2.5 in modulating Cl− transport. Based on these results, we propose that NPF2.5 functions as a pathway for Cl− efflux from the root, contributing to exclusion of Cl− from the shoot of Arabidopsis.
Highlights
It has been well documented that for plants such as grapevine (Vitis spp.) and citrus (Citrus spp.), when grown in saline soils, the accumulation of chloride ions (Cl−) in shoot tissues is more commonly associated with a reduction in plant growth and fruit yield than the accumulation of sodium ions (Na+) in the shoot (Walker et al, 1997; Storey and Walker, 1999; Munns and Tester, 2008; Teakle and Tyerman, 2010)
The Arabidopsis Nitrate Excretion Transporter (NAXT) sub-family members are clustered on chromosome 3 and have high sequence similarity to each other (Segonzac et al, 2007; Tsay et al, 2007)
NPF2.5 was selected as a putative Cl− transporter due to its sequence similarity to NPF2.4, which had previously been characterized to facilitate loading of Cl− from the root symplast to the xylem (Li et al, 2016)
Summary
It has been well documented that for plants such as grapevine (Vitis spp.) and citrus (Citrus spp.), when grown in saline soils, the accumulation of chloride ions (Cl−) in shoot tissues is more commonly associated with a reduction in plant growth and fruit yield than the accumulation of sodium ions (Na+) in the shoot (Walker et al, 1997; Storey and Walker, 1999; Munns and Tester, 2008; Teakle and Tyerman, 2010) This negative association between plant salinity tolerance and accumulation of Cl− in leaves has been shown for varieties of several species generally thought. The exclusion of Cl− has been found to be a multigenic trait (Gong et al, 2011; Long et al, 2013; Genc et al, 2014), and this is not surprising considering the many potential processes that can underpin Cl− transport to the shoot that are detailed above
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