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Hexokinase1 (HXK1) is an Arabidopsis glucose sensor that has a variety of roles during plant growth and devlopment, including during germination, flowering, and senescence. HXK1 also acts as a positive regulator of plant immune responses. Previous research suggested that HXK1 might influence plant immune responses via responses to glucose. Plant immune responses are governed by two main pathways: PAMP-triggered immunity (PTI) and effector-triggered immunity (ETI). PTI involves the recognition of Pathogen-Associated Molecular Patterns (PAMPs) and leads to increased callose formation and accumulation of pathogenesis response (PR) proteins. ETI acts in response to effectors secreted by Gram-negative bacteria. During ETI, the membrane-localized protein RPM1-interacting protein 4 (RIN4) becomes phosphorylated in reponse to interactions with effectors and mediates the downstream response. In this study, the effects of glucose on plant immune responses against infection with Pseudomonas syringae pv. tomato DC3000 and other P. syringae strains were investigated in the presence and absence of HXK1. Infiltration of leaves with glucose prior to infection led to decreases in bacterial populations and reductions in disease symptoms in wild-type Arabidopsis plants, indicating that glucose plays a role in plant immunity. Both PTI and ETI responses were affected. However, these effects were not observed in a hxk1 mutant, indicating that the effects of glucose on plant immune responses were mediated by HXK1-related pathways.

Most secretory and membrane proteins are properly folded in the endoplasmic reticulum (ER) before being transferred to their functional destinations. Physiological and pathological stresses induce unfolded and misfolded protein accumulation in the ER, termed as ER stress. Under ER stress, cells initiate a protective response to maintain cellular homeostasis, which is referred as unfolded protein responses. Although protein processing in the ER has been known to regulate cell lifespan and disease, few evidences that prove the role of ER stress in plant immunity have been reported. We investigated the interaction between ER stress and pathogenicity in Arabidopsis by utilizing the N-glycosylation inhibitor, tunicamycin (TM) as an ER stress inducer. TM induced the accumulation of PR1 (pathogenesis-related protein 1) and callose in plant leaves, which are markers for PAMP-triggered immunity (PTI) activation. However, TM pre-treatment increased susceptibility of Arabidopsis to all bacterial pathogens tested. Moreover, TM resulted in cell death of plant leaves with an additive effect to hypersensitive response by bacterial effector proteins, suggesting TM-induced cell death is independent of the effector-triggered immunity. These results imply that TM-induced ER stress weakens overall immune system of plant not a specific immune pathway, probably via disruption of post-translational modification of immune-related proteins in the ER and subsequent cell death by apoptosis or autophagy. This study provides proves for the distinct suppressive effect of ER stress on the plant immune system.

The endoplasmic reticulum (ER) is a well-characterized protein folding mechanism in eukaryotic organisms. Many secretory and membrane proteins are folded in the ER before they are translocated to their functional destination. Various conditions, such as biotic, abiotic, or physiological stresses, lead to the accumulation of unfolded and misfolded proteins in the ER, resulting in ER stress. In response to ER stress, cells initiate a protective response called the unfolded protein response (UPR) to maintain cellular homeostasis. Previous studies suggest that inositol-requiring kinase 1 (IRE1) is a universal ER stress sensor in yeast, mammals, and plants. IRE1-mediated splicing of UPR transducers, such as HAC1, XBP1, and bZIP60, triggers the UPR in yeast, mammals, and plants, respectively. In mammals, activated transcription factor 6 and double stranded RNA-activated protein kinase-like ER kinases are involved in the UPR. In plants, the additional UPR transducers bZIP28 and bZIP17 are activated by Golgi-localized S1P and S2P proteases. Subsequently, these UPR transducers are exported to the nucleus and upregulate the expression of UPR-responsive genes encoding BiP, calreticulin, calnexin, protein disulfide isomerase, and glucose-regulated protein 94 to decrease the amount of misfolded proteins and induce endoplasmic reticulum-associated degradation. In plants, the UPR signaling pathway plays an important role in ER homeostasis and normal biological processes; however, the molecular mechanisms of the UPR in plants remain poorly understood. This paper provides an overview of the regulatory and signaling mechanisms of the UPR across kingdoms. In addition, the emerging role of the UPR in plant physiology and defense response will be discussed.