Aquaporins (AQPs) are membrane channel proteins that are primarily associated with water transport across cell membranes [
1]. Water transportation is extremely important for all living cells to maintain cellular functions and normal vital activities under various conditions. Less than 30 years after the discovery in human red blood cell membranes, AQPs are now known to exist in nearly all living organisms, suggesting their essential role in physiological functions [
1,
2,
3]. In addition to water, some AQPs can also transport small solutes (including urea, ammonium, arsenite, lactic acid, boric acid, and glycerol), micronutrients (including silicon and boron), other small molecules (reactive oxygen species, ROS), and even gas molecules (including CO
2, O
2 and NO), some of which may function as crucial signaling molecules during various cellular responses under stress conditions [
4,
5,
6,
7,
8,
9,
10]. In contrast, non-transporting functions of some AQPs include cell-cell adhesion, membrane polarization, and regulation of interacting proteins, such as ion channels [
1]. Compared to the functions of AQPs in symbiotic plant-microbe interaction, it has become increasingly clear that AQPs also play an important role in host-pathogen interaction. The present review focuses on the roles of AQPs in plant immunity, pathogen pathogenicity, and communications during pathogenic plant-microbe interaction.
The numbers of
AQP genes vary significantly among different species [
11]. Recent genomic sequencing projects have shown that AQPs are more abundant in eukaryotes as compared to prokaryotes [
12]. To date, at least 35, 36, 33, 70, 47 aquaporin genes of higher plants have been identified in
Arabidopsis, maize, rice, cotton, and tomato, respectively [
13,
14,
15,
16,
17]. The first reported AQP in the plant was AtTIP1;1, a tonoplast intrinsic protein from
Arabidopsis. Its functions have been further analyzed through expression studies in
Xenopus oocytes and cell-swelling experiments in hypoosmotic media [
18]. Since their discovery in the 1990′s, numerous AQPs have been identified and investigated in plants. Based on their sequence similarity and specific subcellular localization, AQPs in the plant are divided into five major subgroups, including the plasma membrane (PM) intrinsic proteins (PIPs), X intrinsic proteins (XIPs), and nodulin 26 like intrinsic proteins (NIPs) in the PM, tonoplast intrinsic proteins (TIPs) in tonoplast, and small basic intrinsic proteins (SIPs) in the endoplasmic reticulum [
19,
20]. Each subfamily can be further divided into different subgroups according to their specific locations and functions. For example, PIPs have been classified into two subgroups, namely, the PIP1 subfamily composed of PIP1;1 to PIP1;5 and the PIP2 subfamily composed of PIP2;1 to PIP2;8 [
21]. PIPs are mainly in charge of substrate transport between the exterior and interior of cells, whereas the others function in transport between organelles [
22,
23]. Gene knockout studies have revealed that AQPs participate in regulating the many physiological processes in plants, including water uptake, gas exchange, nutritional elements and heavy metal acquisition, seed formation and germination, calcium, and ROS-mediated signaling and biotic and abiotic stresses responses [
19,
21]. Some microbes, such as bacteria, show less aquaporin diversity, typically possessing only one or two
AQP genes, and the absence of such genes has not revealed any definite phenotype [
1]. Moreover, AQP-deletion mutants have also been studied in
Botrytis cinerea and
Fusarium graminearum, respectively, suggesting that AQPs also have important roles in growth, development, secondary metabolism, and pathogenicity of fungal pathogens [
5,
24].
AQPs are tightly controlled through multiple mechanisms, mostly including transcriptional control of their expression and post-translational modifications to control their abundance and transport activity [
25,
26]. Many reports suggest that AQPs are upregulated or downregulated in plants in response to environmental cues [
27,
28,
29]. Nevertheless, the post-translational regulation (such as phosphorylation, methylation, deamidation, and acetylation) that regulates PM delivery and the activity of PIPs is still unexplored [
19,
30]. These regulation mechanisms can influence the conformation of AQP monomers, their stability in PMs, and their trafficking or subcellular localization [
26,
31]. Phosphorylation is a common mode of post-translational modification that acts as a molecular ‘switch’ to regulate protein activity in response to various stresses. It has been proven that phosphorylation of AtPIP2;1 at multiple sites in the C-terminal occurs under salt stress conditions, leading to the switch of AtPIP2;1 from PMs to intracellular regions, reducing the hydraulic conductivity of
Arabidopsis [
32].