Plant Aquaporins in Infection by and Immunity Against Pathogens - A Critical Review

Abstract

Plant aquaporins (AQPs) of the plasma membrane intrinsic protein (PIP) family face constant risk of hijack by pathogens aiming to infect plants. PIPs can also be involved in plant immunity against infection. This review will utilize two case studies to discuss biochemical and structural mechanisms that govern the functions of PIPs in the regulation of plant infection and immunity. The first example concerns the interaction between rice Oryza sativa and the bacterial blight pathogen Xanthomonas oryzae pv. oryzae (Xoo). To infect rice, Xoo uses the type III (T3) secretion system to secrete the proteic translocator Hpa1, and Hpa1 subsequently mediates the translocation of T3 effectors secreted by this system. Once shifted from bacteria into rice cells, effectors exert virulent or avirulent effects depending on the susceptibility of the rice varieties. The translocator function of Hpa1 requires cooperation with OsPIP1;3, the rice interactor of Hpa1. This role of OsPIP1;3 is related to regulatory models of effector translocation. The regulatory models have been proposed as, translocon-dependent delivery, translocon-independent pore formation, and effector endocytosis with membrane protein/lipid trafficking. The second case study includes the interaction of Hpa1 with the H₂O₂ transport channel AtPIP1;4, and the associated consequence for H₂O₂ signal transduction of immunity pathways in Arabidopsis thaliana, a non-host of Xoo. H₂O₂ is generated in the apoplast upon induction by a pathogen or microbial pattern. H₂O₂ from this source translocates quickly into Arabidopsis cells, where it interacts with pathways of intracellular immunity to confer plant resistance against diseases. To expedite H₂O₂ transport, AtPIP1;4 must adopt a specific conformation in a number of ways, including channel width extension through amino acid interactions and selectivity for H₂O₂ through amino acid protonation and tautomeric reactions. Both topics will reference relevant studies, conducted on other organisms and AQPs, to highlight possible mechanisms of T3 effector translocation currently under debate, and highlight the structural basis of AtPIP1;4 in H₂O₂ transport facilitated by gating and trafficking regulation.

Keywords: H2O2 transport; aquaporin; immunity signaling; plasma membrane intrinsic protein; translocon; type III effectors.

FIGURE 1

Hypothetic routes of T3ET using Xoo as an example. Effector translocation may use the left black route (Rüter et al., 2010; Scheibner et al., 2017) or the right red pathway (Lu et al., 2007; Santi-Rocca and Blanchard, 2017) according to recently proposed models. In a previously proposed model, T3ET occurs via the translocon (the middle purple route) hypothetically assembled by interactions between translocators, and their receptors in eukaryotic PMs (Büttner, 2012; Ji and Dong, 2015b). Three translocators have been identified in animal-pathogenic bacteria, but the number of T3 translocator remains unknown in plant-pathogenic bacteria including Xoo. In Xoo, the hydrophilic protein Hpa1 (Wang X. et al., 2018) and the hydrophobic protein HrpF (Büttner et al., 2002; Li et al., 2011) were determined to function as T3 translocators, but whether the third translocator exists is unclear (question marks). Regarding molecular interactions during the translocon assembly, OsPIP1;3 has been verified to interact with Hpa1 at rice PMs to expedite the translocation of TAL effectors AvrXa10 and PthXo1 (Zhang et al., 2018; Li et al., 2019). In the cartoon, numbers 1 through 5 refer to the order of the translocator in self oligomerization to form the homogenous complex, which is assumed to be consisting of 5 or 8 monomers (Mueller et al., 2008). In vitro assays indicated HrpF binding to lipids (Büttner et al., 2002; Li et al., 2011), such as PI4P, but no evidence was available to demonstrate the lipid binding at plant PMs and the subsequent effect on T3 effector translocation.

FIGURE 2

Diagram of hypothesized PM protein and lipid trafficking that is going through ER (Obacz et al., 2017) or vesicles (Wudick et al., 2015) and drives T3 effector endocytosis (Domingues et al., 2016). The OsPIP1;3-dependent and/or PI4P-involved PthXo1 and AvrXa10 translocation is used as a study paradigm. The protein and lipid trafficking pathways are annotated as a motivation for both effectors to be internalized and then both effectors execute the transcriptional regulation on their target genes. Both pathways may involve unannotated response, that is the recognition of Hpa1 by OsPIP1;3 and HrpF by PI4P (Büttner et al., 2002, Büttner, 2012; Ji and Dong, 2015b). The protein and lipid trafficking may be concurrent, cooperative or independent, making responses on PMs more intricate than the regular remodeling in the absence of bacterial proteins (Hubber and Roy, 2010; Piscatelli et al., 2016; Santi-Rocca and Blanchard, 2017).

FIGURE 3

Crosstalk of AtPIP1;4-mediated H₂O₂ transport with the intracellular immunity pathways and predicted mechanisms by which AtPIP1;4 fulfills the substrate transport function. (A) Plant sensing of a pathogen or microbial pattern not only is an essential step of apoplastic generation and cytoplasmic import of H₂O₂, but also induces damages to the PM integrity (Guignot and Tran Van Nhieu, 2016). Impairment of the PM integrity is likely to provide an abnormal channel, which is wider than the normal conduit, and capable of accommodating substrates larger than H₂O. (B) Hypothetic determinants of AtPIP1;4 conformation for H₂O₂ transport include amino acid compositions and locations in the NPA and SF regions. (C) Gating and trafficking regulation of the AtPIP1;4 channel for H₂O₂ transport across plant PMs (left) may be subject to the annotated factors (right). The 3D-structure of AtPIP1;4 was predicted by using the PHYRE2 (Protein Homology/analogy Recognition Engine V 2.0) program (http://www.sbg.bio.ic.ac.uk/phyre2/html/page.cgi?id~=~index). The diagrammatic transport of H₂O₂ over H₂O is a surmise, predicted to occur by the combined mechanisms indicated on right.