An LRR receptor kinase controls ABC transporter substrate preferences during plant growth-defense decisions

The conserved AvrE family of bacterial effectors: functions and targets during pathogenesis

The AvrE family of type III secreted effectors are highly conserved among many agriculturally important phytopathogenic bacteria. Despite their critical roles in the pathogenesis of phytopathogenic bacteria, the molecular functions and virulence mechanisms of these effectors have been largely unknown. However, recent studies have identified host-interacting proteins and demonstrated that AvrE family effectors can form water-permeable channels in the plant plasma membrane (PM) to create a hydrated and nutrient-rich extracellular space (apoplast) required for disease establishment. Here, we summarize these recent discoveries and highlight open questions related to AvrE-targeted host proteins.

Are miRNAs applicable for balancing crop growth and defense trade-off?

Securing agricultural supplies for the increasing population without negative impacts on environment demands new crop varieties with higher yields, better quality, and stronger stress resilience. But breeding such super crop varieties is restrained by growth-defense (G-D) trade-off. MicroRNAs (miRNAs) are versatile regulators of plant growth and immune responses, with several being demonstrated to simultaneously regulate crop growth and defense against biotic stresses and to balance G-D trade-off. Increasing evidence also links miRNAs to the metabolism and signaling of phytohormones, another type of master regulator of plant growth and defense. Here, we synthesize the reported functions of miRNAs in crop growth, development, and responses to bio-stressors, summarize the regulatory scenarios of miRNAs based on their relationship with target(s), and discuss how miRNAs, particularly those involved in crosstalk with phytohormones, can be applied in balancing G-D trade-off in crops. We also propose several open questions to be addressed for adopting miRNAs in balancing crop G-D trade-off.

Wound-induced small-peptide-mediated signaling cascade, regulated by OsPSKR, dictates balance between growth and defense in rice

Wounding is a general stress in plants that results from various pest and pathogenic infections in addition to environment-induced mechanical damages. Plants have sophisticated molecular mechanisms to recognize and respond to wounding, with those of monocots being distinct from dicots. Here, we show the involvement of two distinct categories of temporally separated, endogenously derived peptides, namely, plant elicitor peptides (PEPs) and phytosulfokine (PSK), mediating wound responses in rice. These peptides trigger a dynamic signal relay in which a receptor kinase involved in PSK perception named OsPSKR plays a major role. Perturbation of OsPSKR expression in rice leads to compromised development and constitutive autoimmune phenotypes. OsPSKR regulates the transitioning of defense to growth signals upon wounding. OsPSKR displays mutual antagonism with the OsPEPR1 receptor involved in PEP perception. Collectively, our work indicates the presence of a stepwise peptide-mediated signal relay that regulates the transition from defense to growth upon wounding in monocots.

AGC kinases OXI1 and AGC2-2 regulate camalexin secretion and disease resistance by phosphorylating transporter PDR6

Plant transporters regulating the distribution of secondary metabolites play critical roles in defending against pathogens, insects, and interacting with beneficial microbes. The phosphorylation of these transporters can alter their activity, stability, and intracellular protein trafficking. However, the regulatory mechanism underlying this modification remains elusive. In this study, we discovered two orthologs of mammalian PKA, PKG, and PKC (AGC) kinases, oxidative signal-inducible 1 (OXI1) and its closest homologue, AGC subclass 2 member 2 (AGC2-2; 75% amino acid sequence identity with OXI1), associated with the extracellular secretion of camalexin and Arabidopsis (Arabidopsis thaliana) resistance to Pseudomonas syringae, and Botrytis cinerea. These kinases can undergo in vitro kinase reactions with three pleiotropic drug resistance (PDR) transporters: PDR6, PDR8, and PDR12. Moreover, our investigation confirmed PDR6 interaction with OXI1 and AGC2-2. By performing LC-MS/MS and parallel reaction monitoring, we identified the phosphorylation sites on PDR6 targeted by these kinases. Notably, chitin-induced PDR6 phosphorylation at specific residues, namely S31, S33, S827, and T832. Additional insights emerged by expressing dephosphorylated PDR6 variants in a pdr6 mutant background, revealing that the target residues S31, S33, and S827 promote PDR6 efflux activity, while T832 potentially contributes to PDR6 stability within the plasma membrane. The findings of this study elucidate partial mechanisms involved in the activity regulation of PDR-type transporters, providing valuable insights for their potential application in future plant breeding endeavors.

Exploring the role of the LkABCG36 transporter in lignin accumulation

Lignin is a complex biopolymer formed through the condensation of three monomeric precursors known as monolignols. However, the mechanism underlying lignin precursor transport remains elusive, with uncertainty over whether it occurs through passive diffusion or an active energized process. ATP-binding cassette 36 (ABCG36) plays important roles in abiotic stress resistance. In this study, we investigated the transport functions of LkABCG36 (Larix kaempferi) for lignin precursors and the potential effects of LkABCG36 overexpression in plants. LkABCG36 enhanced the ability of tobacco (Nicotiana tabacum) bright yellow-2 (BY-2) cells to resist monolignol alcohol stress. Furthermore, LkABCG36 overexpression promoted lignin deposition in tobacco plant stem tissue. To understand the underlying mechanism, we measured the BY-2 cell ability to export lignin monomers and the uptake of monolignol precursors in inside-out (inverted) plasma membrane vesicles. We found that the transport of coniferyl and sinapyl alcohols is an ATP-dependent process. Our data suggest that LkABCG36 contributes to lignin accumulation in tobacco stem tissues through a mechanism involving the active transport of lignin precursors to the cell wall. These findings shed light on the lignin biosynthesis process, with important implications for enhancing lignin deposition in plants, potentially leading to improved stress tolerance and biomass production.

Transceptor NRT1.1 and receptor-kinase QSK1 complex controls PM H+-ATPase activity under low nitrate

NRT1.1, a nitrate transceptor, plays an important role in nitrate binding, sensing, and nitrate-dependent lateral root (LR) morphology. However, little is known about NRT1.1-mediated nitrate signaling transduction through plasma membrane (PM)-localized proteins. Through in-depth phosphoproteome profiling using membranes of Arabidopsis roots, we identified receptor kinase QSK1 and plasma membrane H+-ATPase AHA2 as potential downstream components of NRT1.1 signaling in a mild low-nitrate (LN)-dependent manner. QSK1, as a functional kinase and molecular link, physically interacts with NRT1.1 and AHA2 at LN and specifically phosphorylates AHA2 at S899. Importantly, we found that LN, not high nitrate (HN), induces formation of the NRT1.1-QSK1-AHA2 complex in order to repress the proton efflux into the apoplast by increased phosphorylation of AHA2 at S899. Loss of either NRT1.1 or QSK1 thus results in a higher T947/S899 phosphorylation ratio on AHA2, leading to enhanced pump activity and longer LRs under LN. Our results uncover a regulatory mechanism in which NRT1.1, under LN conditions, promotes coreceptor QSK1 phosphorylation and enhances the NRT1.1-QSK1 complex formation to transduce LN sensing to the PM H+-ATPase AHA2, controlling the phosphorylation ratio of activating and inhibitory phosphorylation sites on AHA2. This then results in altered proton pump activity, apoplast acidification, and regulation of NRT1.1-mediated LR growth.

Structure and function of the Arabidopsis ABC transporter ABCB19 in brassinosteroid export

Brassinosteroids are steroidal phytohormones that regulate plant development and physiology, including adaptation to environmental stresses. Brassinosteroids are synthesized in the cell interior but bind receptors at the cell surface, necessitating a yet to be identified export mechanism. Here, we show that a member of the ATP-binding cassette (ABC) transporter superfamily, ABCB19, functions as a brassinosteroid exporter. We present its structure in both the substrate-unbound and the brassinosteroid-bound states. Bioactive brassinosteroids are potent activators of ABCB19 ATP hydrolysis activity, and transport assays showed that ABCB19 transports brassinosteroids. In Arabidopsis thaliana, ABCB19 and its close homolog, ABCB1, positively regulate brassinosteroid responses. Our results uncover an elusive export mechanism for bioactive brassinosteroids that is tightly coordinated with brassinosteroid signaling.

Protein Phosphorylation Orchestrates Acclimations of Arabidopsis Plants to Environmental pH

Environment pH (pHe) is a key parameter dictating a surfeit of conditions critical to plant survival and fitness. To elucidate the mechanisms that recalibrate cytoplasmic and apoplastic pH homeostasis, we conducted a comprehensive proteomic/phosphoproteomic inventory of plants subjected to transient exposure to acidic or alkaline pH, an approach that covered the majority of protein-coding genes of the reference plant Arabidopsis thaliana. Our survey revealed a large set-of so far undocumented pHe-dependent phospho-sites, indicative of extensive post-translational regulation of proteins involved in the acclimation to pHe. Changes in pHe altered both electrogenic H+ pumping via P-type ATPases and H+/anion co-transport processes, putatively leading to altered net trans-plasma membrane translocation of H+ ions. In pH 7.5 plants, the transport (but not the assimilation) of nitrogen via NRT2-type nitrate and AMT1-type ammonium transporters was induced, conceivably to increase the cytosolic H+ concentration. Exposure to both acidic and alkaline pH resulted in a marked repression of primary root elongation. No such cessation was observed in nrt2.1 mutants. Alkaline pH decreased the number of root hairs in the wild type but not in nrt2.1 plants, supporting a role of NRT2.1 in developmental signaling. Sequestration of iron into the vacuole via alterations in protein abundance of the vacuolar iron transporter VTL5 was inversely regulated in response to high and low pHe, presumptively in anticipation of associated changes in iron availability. A pH-dependent phospho-switch was also observed for the ABC transporter PDR7, suggesting changes in activity and, possibly, substrate specificity. Unexpectedly, the effect of pHe was not restricted to roots and provoked pronounced changes in the shoot proteome. In both roots and shoots, the plant-specific TPLATE complex components AtEH1 and AtEH2-essential for clathrin-mediated endocytosis-were differentially phosphorylated at multiple sites in response to pHe, indicating that the endocytic cargo protein trafficking is orchestrated by pHe.

Differential expansion and retention patterns of LRR-RLK genes across plant evolution

To maximize overall fitness, plants must accurately respond to a host of growth, developmental, and environmental signals throughout their life. Many of these internal and external signals are perceived by the leucine-rich repeat receptor-like kinases, which play roles in regulating growth, development, and immunity. This largest family of receptor kinases in plants can be divided into subfamilies based on the conservation of the kinase domain, which demonstrates that shared evolutionary history often indicates shared molecular function. Here we investigate the evolutionary history of this family across the evolution of 112 plant species. We identify lineage-specific expansions of the malectin-domain containing subfamily LRR subfamily I primarily in the Brassicales and bryophytes. Most other plant lineages instead show a large expansion in LRR subfamily XII, which in Arabidopsis is known to contain key receptors in pathogen perception. This striking asymmetric expansion may reveal a dichotomy in the evolutionary history and adaptation strategies employed by plants. A greater understanding of the evolutionary pressures and adaptation strategies acting on members of this receptor family offers a way to improve functional predictions for orphan receptors and simplify the identification of novel stress-related receptors.