Cl- homeostasis in includer and excluder citrus rootstocks: transport mechanisms and identification of candidate genes

To reveal specific Cl- transport activities in the symplastic pathway, uptake, long-distance transport and distribution of Cl- have been investigated in the citrus rootstocks Carrizo citrange (CC, Cl- includer) and Cleopatra mandarin (CM, Cl- excluder). Using an external concentration of 4.5 mm Cl-, both species actively transported Cl- to levels that exceeded the critical requirement concentration by one and two orders of magnitude in the excluder and the includer rootstocks, respectively. Both CC and CM modulated Cl- influx according to the availability of the nutrient as uptake capacity was induced by Cl- starvation, but inhibited after Cl- resupply. Net Cl- uptake was higher in the includer CC, an observation that correlated with a lower root-to-shoot transport capacity in the excluder CM. The patterns of tissue Cl- accumulation indicated that chloride exclusion in the salt-tolerant rootstock CM was caused by a reduced net Cl- loading into the root xylem. Genes CcCCC1, CcSLAH1 and CcICln1 putatively involved in the regulation of chloride transport were isolated and their expression analysed in response to both changes in the nutritional status of Cl- and salt stress. The previously uncharacterized ICln gene exhibited a strong repression to Cl- application in the excluder rootstock, suggesting a role in regulating Cl- homeostasis in plants.