Controversial Regulation of Gene Expression and Protein Transduction of Aquaporins under Drought and Salinity Stress

Abstract

Enhancement of the passage of water through membranes is one of the main mechanisms via which cells can maintain their homeostasis under stress conditions, and aquaporins are the main participants in this process. However, in the last few years, a number of studies have reported discrepancies between aquaporin messenger RNA (mRNA) expression and the number of aquaporin proteins synthesised in response to abiotic stress. These observations suggest the existence of post-transcriptional mechanisms which regulate plasma membrane intrinsic protein (PIP) trafficking to the plasma membrane. This indicates that the mRNA synthesis of some aquaporins could be modulated by the accumulation of the corresponding encoded protein, in relation to the turnover of the membranes. This aspect is discussed in terms of the results obtained: on the one hand, with isolated vesicles, in which the level of proteins present provides the membranes with important characteristics such as resistance and stability and, on the other, with isolated proteins reconstituted in artificial liposomes as an in vitro method to address the in vivo physiology of the entire plant.

Keywords: abiotic stress; aquaporins; gene expression; plasma membrane; post-translational modifications; trafficking; turnover.

Figure 1

Graphical summary of the response of the model plant Arabidopsis thaliana to drought and salinity stress at the gene expression and protein levels. The box colour indicates the response of the plant compared to control conditions: the gene expression or protein amount does not change (blue), decreases (red), or increases (green); or unknown data (grey). Numbers in brackets refer to the related bibliography.

Figure 2

Graphical summary of the hypothetical regulation at cellular level of plant plasma membrane intrinsic proteins (PIPs) under short- (A) and long-term (B) salinity or drought stress. Most PIP genes decrease their expression under both conditions (1). Under short-term stress (drought/salinity), the functionality of the PIPs already in plasma membrane (PM) is repressed through gating or modifying their activity by post-translational modification (2), and the trafficking of PIPs in route to PM is blocked (3). Turnover of PIPs in PM would be fast under short-term stress (4) and slow with long-term stress (5). Under long-term stress (drought/salinity), PIPs could inhibit transcription of PIP genes (6) to promote a decrease in the amount of PIP.