The role of aquaporins in plant responses to drought

Abstract

Aquaporins are a family of integral membrane proteins that facilitate transport of water and other small molecules across membranes. Regulation of their expression and/or activity has significant influence on transcellular water flow through changes in membrane permeability. Therefore, they can regulate water flow through plants and control the balance between water uptake from the soil and water loss via transpiration. The aim of this research was to understand aquaporin responses to drought and their relationship to changes in plant physiological parameters. Special attention was given to the role of the TIP2 sub-group of tonoplast localised aquaporins. In Arabidopsis thaliana, drought had significant effects on aquaporin gene expression in leaves, while watering with abscisic acid (ABA), a plant hormone involved in drought signalling, had a different effect. Gene expression of most aquaporin genes was down-regulated during drought, but some isoforms (AtPIP1;4, AtPIP2;4, and AtPIP2;5) were up-regulated, similar to genes involved in abiotic stress responses through ABA. Changes in expression of down-regulated genes AtPIP1;1, AtPIP1;2, AtPIP2;2, and AtTIP2;2 were observed concomitantly with changes in stomatal conductance in response to soil drying, but earlier than ABA induction. TIP2 isoforms AtTIP2;1, AtTIP2;2, and AtTIP2;3 that were expressed with a C-terminal GFP-tag under the control of a UBQ10 constitutive promoter in Arabidopsis showed tonoplast localisation. Preliminary results indicated significant higher leaf area and a divergent drought response of stomata for an overexpressing line of AtTIP2;1. Promoter-GUS lines demonstrated that promoters of AtTIP2;1, AtTIP2;2, and AtTIP2;3 were mainly active around vascular tissue and potentially in stomata. Genome editing was successfully used to create knockout lines for the TIP2 isoforms AtTIP2;1, AtTIP2;2, and AtTIP2;3. One base insertions in the coding sequence of these genes were induced by CRISPR-Cas, which cause frame-shifts that should disrupt protein function. Preliminary observations found a conditional short root phenotype for knockout lines of AtTIP2;3. In the perennial model plant Vitis vinifera, differences in hydraulic control during mild water deficit and rehydration were found between the isohydric cultivar Grenache and the anisohydric cultivar Syrah mediated by aquaporins. Grenache showed stronger adjustment of leaf, plant, and root hydraulic conductance to changes in transpiration compared to Syrah. This was associated with stronger correlations between gene expression of some aquaporin isoforms in leaves and roots and plant hydraulic parameters in Grenache. While Grenache responded more readily to changes in soil water availability, Syrah still responded to VPD during mild water deficit. The results demonstrated that significant relationships exist between aquaporin expression, plant hydraulic parameters, and leaf gas exchange both in the annual model species Arabidopsis thaliana and the perennial model species Vitis vinifera; even between cultivars difference were reflected by aquaporin expression. Changes in gene expression during drought could not be explained through regulation by ABA, but may be rather controlled by hydraulic signals. Aquaporin isoforms that were induced during drought could be involved in stress signalling. Overexpression of AtTIP2;1 was associated with a different stomatal response to drought. The uncharacterised isoform AtTIP2;3 may be involved in root growth.