The T-loop extension of the tomato protein kinase AvrPto-dependent Pto-interacting protein 3 (Adi3) directs nuclear localization for suppression of plant cell death

Regulatory role of AGC genes in heat stress adaptation in maize (Zea mays)

Heat stress represents a significant environmental challenge that restricts maize (Zea mays ) growth and yield on a global scale. Within the plant kingdom, the AGC gene family, encoding a group of protein kinases, has emerged as crucial players in various stress responses. Nevertheless, a comprehensive understanding of AGC genes in Z. mays under heat-stress conditions remains elusive. A genome-wide analysis was done using bioinformatics techniques to identify 39 AGC genes in Z. mays , categorising them into three subfamilies based on their conserved domains. We investigated their phylogenetic relationships, gene structures (including intron-exon configurations), and expression patterns. These genes are likely involved in diverse signalling pathways, fulfilling distinct roles when exposed to heat stress conditions. Notably, most ZmAGC1.5, ZmAGC1.9, ZmNDR3, ZmNDR5 and ZmIRE3 exhibited significant changes in expression levels under heat stress, featuring a high G-box ratio. Furthermore, we pinpointed a subset of AGC genes displaying highly coordinated expression, implying their potential involvement in the heat stress response pathway. Our study offers valuable insights into the contribution of AGC genes to Z. mays 's heat stress response, thus facilitating the development of heat-tolerant Z. mays varieties.

An unexpected role for tomato threonine deaminase 2 in host defense against bacterial infection

The hormones salicylic acid (SA) and jasmonic acid (JA) often act antagonistically in controlling plant defense pathways in response to hemibiotrophs/biotrophs (hemi/biotroph) and herbivores/necrotrophs, respectively. Threonine deaminase (TD) converts threonine to α-ketobutyrate and ammonia as the committed step in isoleucine (Ile) biosynthesis and contributes to JA responses by producing the Ile needed to make the bioactive JA-Ile conjugate. Tomato (Solanum lycopersicum) plants have two TD genes: TD1 and TD2. A defensive role for TD2 against herbivores has been characterized in relation to JA-Ile production. However, it remains unknown whether TD2 is also involved in host defense against bacterial hemi/biotrophic and necrotrophic pathogens. Here, we show that in response to the bacterial pathogen-associated molecular pattern (PAMP) flagellin flg22 peptide, an activator of SA-based defense responses, TD2 activity is compromised, possibly through carboxy-terminal cleavage. TD2 knockdown (KD) plants showed increased resistance to the hemibiotrophic bacterial pathogen Pseudomonas syringae but were more susceptible to the necrotrophic fungal pathogen Botrytis cinerea, suggesting TD2 plays opposite roles in response to hemibiotrophic and necrotrophic pathogens. This TD2 KD plant differential response to different pathogens is consistent with SA- and JA-regulated defense gene expression. flg22-treated TD2 KD plants showed high expression levels of SA-responsive genes, whereas TD2 KD plants treated with the fungal PAMP chitin showed low expression levels of JA-responsive genes. This study indicates TD2 acts negatively in defense against hemibiotrophs and positively against necrotrophs and provides insight into a new TD2 function in the elaborate crosstalk between SA and JA signaling induced by pathogen infection.

Early Fruit Development Regulation-Related Genes Concordantly Expressed with TCP Transcription Factors in Tomato (Solanum lycopersicum)

The tomato (Solanum lycopersicum L.) is considered one of the most important vegetable crops globally, both agronomically and economically; however, its fruit development regulation network is still unclear. The transcription factors serve as master regulators, activating many genes and/or metabolic pathways throughout the entire plant life cycle. In this study, we identified the transcription factors that are coordinated with TCP gene family regulation in early fruit development by making use of the high-throughput sequencing of RNA (RNAseq) technique. A total of 23 TCP-encoding genes were found to be regulated at various stages during the growth of the fruit. The expression patterns of five TCPs were consistent with those of other transcription factors and genes. There are two unique subgroups of this larger family: class I and class II TCPs. Others were directly associated with the growth and/or ripening of fruit, while others were involved in the production of the hormone auxin. Moreover, it was discovered that TCP18 had an expression pattern that was similar to that of the ethylene-responsive transcription factor 4 (ERF4). Tomato fruit set and overall development are under the direction of a gene called auxin response factor 5 (ARF5). TCP15 revealed an expression that was in sync with this gene. This study provides insight into the potential processes that help in acquiring superior fruit qualities by accelerating fruit growth and ripening.

Phosphorylation control of PIN auxin transporters

The directional transport of the phytohormone auxin is required for proper plant development and tropic growth. Auxin cell-to-cell transport gains directionality through the polar distribution of 'canonical' long PIN-FORMED (PIN) auxin efflux carriers. In recent years, AGC kinases, MAP kinases, Ca²⁺/CALMODULIN-DEPENDENT PROTEIN KINASE-RELATED KINASEs and receptor kinases have been implicated in the control of PIN activity, polarity and trafficking. In this review, we summarize the current knowledge in understanding the posttranslational regulation of PINs by these different protein kinase families. The proposed regulation of PINs by AGC kinases after salt stress and by the stress-activated MAP kinases suggest that abiotic and biotic stress factors may modulate auxin transport and thereby plant growth.

Cargo Recognition and Function of Selective Autophagy Receptors in Plants

Autophagy is a major quality control system for degradation of unwanted or damaged cytoplasmic components to promote cellular homeostasis. Although non-selective bulk degradation of cytoplasm by autophagy plays a role during cellular response to nutrient deprivation, the broad roles of autophagy are primarily mediated by selective clearance of specifically targeted components. Selective autophagy relies on cargo receptors that recognize targeted components and recruit them to autophagosomes through interaction with lapidated autophagy-related protein 8 (ATG8) family proteins anchored in the membrane of the forming autophagosomes. In mammals and yeast, a large collection of selective autophagy receptors have been identified that mediate the selective autophagic degradation of organelles, aggregation-prone misfolded proteins and other unwanted or nonnative proteins. A substantial number of selective autophagy receptors have also been identified and functionally characterized in plants. Some of the autophagy receptors in plants are evolutionarily conserved with homologs in other types of organisms, while a majority of them are plant-specific or plant species-specific. Plant selective autophagy receptors mediate autophagic degradation of not only misfolded, nonactive and otherwise unwanted cellular components but also regulatory and signaling factors and play critical roles in plant responses to a broad spectrum of biotic and abiotic stresses. In this review, we summarize the research on selective autophagy in plants, with an emphasis on the cargo recognition and the biological functions of plant selective autophagy receptors.

The MAF1 Phosphoregulatory Region Controls MAF1 Interaction with the RNA Polymerase III C34 Subunit and Transcriptional Repression in Plants

MAF1 is a phosphoprotein that plays a critical role in cell growth control as the central regulator of RNA polymerase (Pol) III activity. Citrus MAF1 (CsMAF1) was identified as a direct target of PthA4, a bacterial effector protein required to induce tumors in citrus. CsMAF1 binds to Pol III to restrict transcription; however, exactly how CsMAF1 interacts with the polymerase and how phosphorylation modulates this interaction is unknown. Moreover, how CsMAF1 binds PthA4 is also obscure. Here we show that CsMAF1 binds predominantly to the WH1 domain of the citrus Pol III subunit C34 (CsC34) and that its phosphoregulatory region, comprising loop-3 and α-helix-2, contributes to this interaction. We also show that phosphorylation of this region decreases CsMAF1 affinity to CsC34, leading to Pol III derepression, and that Ser 45, found only in plant MAF1 proteins, is critical for CsC34 interaction and is phosphorylated by a new citrus AGC1 kinase. Additionally, we show that the C-terminal region of the citrus TFIIIB component BRF1 competes with CsMAF1 for CsC34 interaction, whereas the C-terminal region of CsMAF1 is essential for PthA4 binding. Based on CsMAF1 structural data, we propose a mechanism for how CsMAF1 represses Pol III transcription and how phosphorylation controls this process.

SnRK1 Phosphorylates and Destabilizes WRKY3 to Enhance Barley Immunity to Powdery Mildew

Plants recognize pathogens and activate immune responses, which usually involve massive transcriptional reprogramming. The evolutionarily conserved kinase, Sucrose non-fermenting-related kinase 1 (SnRK1), functions as a metabolic regulator that is essential for plant growth and stress responses. Here, we identify barley SnRK1 and a WRKY3 transcription factor by screening a cDNA library. SnRK1 interacts with WRKY3 in yeast, as confirmed by pull-down and luciferase complementation assays. Förster resonance energy transfer combined with noninvasive fluorescence lifetime imaging analysis indicates that the interaction occurs in the barley nucleus. Transient expression and virus-induced gene silencing analyses indicate that WRKY3 acts as a repressor of disease resistance to the Bgh fungus. Barley plants overexpressing WRKY3 have enhanced fungal microcolony formation and sporulation. Phosphorylation assays show that SnRK1 phosphorylates WRKY3 mainly at Ser83 and Ser112 to destabilize the repressor, and WRKY3 non-phosphorylation-null mutants at these two sites are more stable than the wild-type protein. SnRK1-overexpressing barley plants display enhanced disease resistance to Bgh. Transient expression of SnRK1 reduces fungal haustorium formation in barley cells, which probably requires SnRK1 nuclear localization and kinase activity. Together, these findings suggest that SnRK1 is directly involved in plant immunity through phosphorylation and destabilization of the WRKY3 repressor, revealing a new regulatory mechanism of immune derepression in plants.

Screening for potential nuclear substrates for the plant cell death suppressor kinase Adi3 using peptide microarrays

The tomato AGC protein kinase Adi3 is a Ser/Thr kinase that functions as a negative regulator of programmed cell death through cell death suppression (CDS) activity in the nucleus. In this study, to understand the mechanism of Adi3 CDS, peptide microarrays containing random Ser- and Thr-peptide phosphorylation substrates were used to screen for downstream phosphorylation substrates. In the microarray phosphorylation assay, Adi3 showed promiscuous kinase activity more toward Ser-peptides compared to Thr-peptides, and a preference for aromatic and cyclic amino acids on both Ser- and Thr-peptides was seen. The 63 highest phosphorylated peptide sequences from the Ser-peptide microarray were selected as queries for a BLAST search against the tomato proteome. As a result, 294 candidate nuclear Adi3 substrates were selected and categorized based on their functions. Many of these proteins were classified as DNA/RNA polymerases or regulators involved in transcription and translation events. The list of potential Adi3 substrates was narrowed to eleven and four candidates were tested for phosphorylation by Adi3. Two of these candidates, RNA polymerase II 2nd largest subunit (RPB2) and the pathogen defense related transcription factor Pti5, were confirmed as Adi3 phosphorylation substrates by in vitro kinase assays. Using a mutational approach two residues, Thr675 and Thr676, were identified as Adi3 phosphorylation sites on RPB2. This study provides the foundation for understanding Adi3 CDS mechanisms in the nucleus as well as other cellular functions.

In vitro activity characterization of the tomato SnRK1 complex proteins

Plant Sucrose non-Fermenting 1-Related Protein Kinase1 (SnRK1) complexes are members of the Snf1/AMPK/SnRK protein kinase family and play important roles in many aspects of metabolism. In tomato (Solanum lycopersicum, Sl), only one α-subunit of the SnRK1 complex, SlSnRK1.1, has been characterized to date. In this study, the phylogenetic placement and in vitro kinase activity of a second tomato SnRK1 α-subunit, SlSnRK1.2, were characterized. Interestingly, in the phylogenetic analysis of SnRK1 sequences from monocots and dicots SlSnRK1.2 clusters only with other Solanaceae SnRK1.2 sequences, suggesting possible functional divergence of these kinases from other SnRK1 kinases. For analysis of kinase activity, SlSnRK1.2 was able to autophosphorylate, phosphorylate the complex β-subunits, and phosphorylate the SnRK1 AMARA peptide substrate, all with drastically lower overall kinase activity compared to SlSnRK1.1. Activation by the upstream kinase SlSnAK was able to increase the kinase activity of both SlSnRK1.1 and SlSnRK1.2, although the increase is less dramatic for SlSnRK1.2. The highest kinase activity on the AMARA peptide for SlSnRK1.2 was seen when reconstituting the complex in vitro with SlSip1 as the β-subunit. In comparison, SlSnRK1.1 showed the lowest kinase activity on the AMARA peptide when SlSip1 was used. These studies suggest the SlSnRK1.2 phylogenetic divergence and lower SlSnRK1.2 kinase activity compared to SlSnRK1.1 may be indicative of different in vivo roles for each kinase.

To deliver or to degrade - an interplay of the ubiquitin-proteasome system, autophagy and vesicular transport in plants

The efficient utilization and subsequent reuse of cell components is a key factor in determining the proper growth and functioning of all cells under both optimum and stress conditions. The process of intracellular and intercellular recycling is especially important for the appropriate control of cellular metabolism and nutrient management in immobile organisms, such as plants. Therefore, the accurate recycling of amino acids, lipids, carbohydrates or micro- and macronutrients available in the plant cell becomes a critical factor that ensures plant survival and growth. Plant cells possess two main degradation mechanisms: a ubiquitin-proteasome system and autophagy, which, as a part of an intracellular trafficking system, is based on vesicle transport. This review summarizes knowledge of both the ubiquitin-proteasome system and autophagy pathways, describes the cross-talk between the two and discusses the relationships between autophagy and the vesicular transport systems.

Phospholipid composition and a polybasic motif determine D6 PROTEIN KINASE polar association with the plasma membrane and tropic responses

Polar transport of the phytohormone auxin through PIN-FORMED (PIN) auxin efflux carriers is essential for the spatiotemporal control of plant development. The Arabidopsis thaliana serine/threonine kinase D6 PROTEIN KINASE (D6PK) is polarly localized at the plasma membrane of many cells where it colocalizes with PINs and activates PIN-mediated auxin efflux. Here, we show that the association of D6PK with the basal plasma membrane and PINs is dependent on the phospholipid composition of the plasma membrane as well as on the phosphatidylinositol phosphate 5-kinases PIP5K1 and PIP5K2 in epidermis cells of the primary root. We further show that D6PK directly binds polyacidic phospholipids through a polybasic lysine-rich motif in the middle domain of the kinase. The lysine-rich motif is required for proper PIN3 phosphorylation and for auxin transport-dependent tropic growth. Polybasic motifs are also present at a conserved position in other D6PK-related kinases and required for membrane and phospholipid binding. Thus, phospholipid-dependent recruitment to membranes through polybasic motifs might not only be required for D6PK-mediated auxin transport but also other processes regulated by these, as yet, functionally uncharacterized kinases.

The SnRK1 Energy Sensor in Plant Biotic Interactions

Our understanding of plant biotic interactions has grown significantly in recent years with the identification of the mechanisms involved in innate immunity, hormone signaling, and secondary metabolism. The impact of such interactions on primary metabolism and the role of metabolic signals in the response of the plants, however, remain far less explored. The SnRK1 (SNF1-related kinase 1) kinases act as metabolic sensors, integrating very diverse stress conditions, and are key in maintaining energy homeostasis for growth and survival. Consistently, an important role is emerging for these kinases as regulators of biotic stress responses triggered by viral, bacterial, fungal, and oomycete infections as well as by herbivory. While this identifies SnRK1 as a promising target for directed modification or selection for more quantitative and sustainable resistance, its central function also increases the chances of unwanted side effects on growth and fitness, stressing the need for identification and in-depth characterization of the mechanisms and target processes involved. VIDEO ABSTRACT.

The tomato cell death suppressor Adi3 is restricted to the endosomal system in response to the Pseudomonas syringae effector protein AvrPto

The tomato (Solanum lycopersicum) AGC protein kinase Adi3 functions as a suppressor of cell death and was first identified as an interactor with the tomato resistance protein Pto and the Pseudomonas syringae effector protein AvrPto. Models predict that loss of Adi3 cell death suppression (CDS) activity during Pto/AvrPto interaction leads to the cell death associated with the resistance response initiated from this interaction. Nuclear localization is required for Adi3 CDS. Prevention of nuclear accumulation eliminates Adi3 CDS and induces cell death by localizing Adi3 to intracellular punctate membrane structures. Here we use several markers of the endomembrane system to show that the punctate membrane structures to which non-nuclear Adi3 is localized are endosomal in nature. Wild-type Adi3 also localizes in these punctate endosomal structures. This was confirmed by the use of endosomal trafficking inhibitors, which were capable of trapping wild-type Adi3 in endosomal-like structures similar to the non-nuclear Adi3. This suggests Adi3 may traffic through the cell using the endomembrane system. Additionally, Adi3 was no longer found in the nucleus but was visualized in these punctate endosomal-like membranes during the cell death induced by the Pto/AvrPto interaction. Therefore we propose that inhibiting nuclear import and constraining Adi3 to the endosomal system in response to AvrPto is a mechanism to initiate the cell death associated with resistance.

Quantitative peptidomics study reveals that a wound-induced peptide from PR-1 regulates immune signaling in tomato

Many important cell-to-cell communication events in multicellular organisms are mediated by peptides, but only a few peptides have been identified in plants. In an attempt to address the difficulties in identifying plant signaling peptides, we developed a novel peptidomics approach and used this approach to discover defense signaling peptides in plants. In addition to the canonical peptide systemin, several novel peptides were confidently identified in tomato (Solanum lycopersicum) and quantified to be induced by both wounding and methyl jasmonate (MeJA). A wounding or wounding plus MeJA-induced peptide derived from the pathogenesis-related protein 1 (PR-1) family was found to induce significant antipathogen and minor antiherbivore responses in tomato. This study highlights a role for PR-1 in immune signaling and suggests the potential application of plant endogenous peptides in efforts to defeat biological threats in crop production. As PR-1 is highly conserved across many organisms and the putative peptide from At-PR1 was also found to be bioactive in Arabidopsis thaliana, our results suggest that this peptide may be useful for enhancing resistance to stress in other plant species.

An Arabidopsis homolog of importin β1 is required for ABA response and drought tolerance

The import of proteins into the nucleus in response to drought is critical for mediating the reprogramming of gene expression that leads to drought tolerance. However, regulatory mechanisms involved in nuclear protein import remain largely unknown. Here, we have identified an Arabidopsis gene (AtKPNB1) as a homolog of human KPNB1 (importin β1). AtKPNB1 was expressed in multiple organs, and the protein was localized in the cytoplasm and nucleus. AtKPNB1 was able to facilitate nuclear import of a model protein. Null mutation of AtKPNB1 delayed development under normal growth conditions and increased sensitivity to abscisic acid (ABA) during seed germination and cotyledon development. Inactivation of AtKPNB1 increased stomatal closure in response to ABA, reduced the rate of water loss, and substantially enhanced drought tolerance. AtKPNB1 interacted with several importin α proteins, nucleoporin AtNUP62, and the Arabidopsis Ran proteins. Inactivation of AtKPNB1 did not affect the ABA responsiveness or the expression level or subcellular localization of ABI1, ABI2 or ABI5, key regulators of the ABA signaling pathway. Moreover, phenotypic analysis of epistasis revealed that AtKPNB1 modulates the ABA response and drought tolerance through a pathway that is independent of ABI1 and ABI5. Collectively, our results show that AtKPNB1 is an Arabidopsis importin β that functions in ABA signaling.

Two Pdk1 phosphorylation sites on the plant cell death suppressor Adi3 contribute to substrate phosphorylation

The tomato AGC kinase Adi3 is phosphorylated by Pdk1 for activation of its cell death suppression activity. The Pdk1 phosphorylation site for activation of Adi3 is at Ser539. However, there is at least one additional Pdk1 phosphorylation site on Adi3 that has an unknown function. Here we identify an Arabidopsis thaliana sequence homologue of Adi3 termed AGC1-3. Two Pdk1 phosphorylation sites were identified on AGC1-3, activation site Ser596 and Ser269, and by homology Ser212 on Adi3 was identified as a second Pdk1 phosphorylation site. While Ser212 is not required for Adi3 autophosphorylation, Ser212 was shown to be required for full phosphorylation of the Adi3 substrate Gal83.

Ubiquitination of the tomato cell death suppressor Adi3 by the RING E3 ubiquitin ligase AdBiL

Programmed cell death (PCD) is an organized process by which organisms selectively remove cells according to developmental needs or in response to biotic or abiotic stress. Despite recent efforts to understand mechanisms by which cell death takes place in plants, several gaps remain in our understanding of the molecular elements involved. The tomato PCD suppressor Adi3 is an AGC kinase that shares functional homology with the mammalian inhibitor of apoptosis PKB. Regulation of PKB stability, cell localization, and activation state is achieved through post-translational modifications such as ubiquitination. In an effort to understand the regulation of Adi3 function, we studied its interaction with the E3 ubiquitin ligase AdBiL. Using in vitro ubiquitination assays we show that AdBiL is an active E3 ubiquitin ligase using the E2 ubiquitin ligase UBC8 to ubiquitinate Adi3. Adi3 is also degraded in a proteasome-dependent manner. Our data draws additional parallels between Adi3 and PKB to support the functional relationship between these two PCD regulators.

Role of AGC kinases in plant growth and stress responses

AGC kinases are important regulators of cell growth, metabolism, division, and survival in mammalian systems. Mutation or deregulation of members of this family of protein kinases contribute to the pathogenesis of many human diseases, including cancer and diabetes. Although AGC kinases are conserved in the plant kingdom, little is known about their molecular functions and targets. Some of the best-studied plant AGC kinases mediate auxin signaling and are thereby involved in the regulation of growth and morphogenesis. Furthermore, certain members are regulated by lipid-derived signals via the 3-phosphoinositide-dependent kinase 1 (PDK1) and the kinase target of rapamycin (TOR), similar to its animal counterparts. In this review, we discuss recent findings on plant AGC kinases that unravel important roles in the regulation of plant growth, immunity and cell death, and connections to stress-induced mitogen-activated protein kinase signaling cascades.

The β-subunit of the SnRK1 complex is phosphorylated by the plant cell death suppressor Adi3

The protein kinase AvrPto-dependent Pto-interacting protein3 (Adi3) is a known suppressor of cell death, and loss of its function has been correlated with cell death induction during the tomato (Solanum lycopersicum) resistance response to its pathogen Pseudomonas syringae pv tomato. However, Adi3 downstream interactors that may play a role in cell death regulation have not been identified. We used a yeast two-hybrid screen to identify the plant SnRK1 (for Sucrose non-Fermenting-1-Related Protein Kinase1) protein as an Adi3-interacting protein. SnRK1 functions as a regulator of carbon metabolism and responses to biotic and abiotic stresses. SnRK1 exists in a heterotrimeric complex with a catalytic α-subunit (SnRK1), a substrate-interacting β-subunit, and a regulatory γ-subunit. Here, we show that Adi3 interacts with, but does not phosphorylate, the SnRK1 α-subunit. The ability of Adi3 to phosphorylate the four identified tomato β-subunits was also examined, and it was found that only the Galactose Metabolism83 (Gal83) β-subunit was phosphorylated by Adi3. This phosphorylation site on Gal83 was identified as serine-26 using a mutational approach and mass spectrometry. In vivo expression of Gal83 indicates that it contains multiple phosphorylation sites, one of which is serine-26. An active SnRK1 complex containing Gal83 as the β-subunit and sucrose nonfermenting4 as the γ-subunit was constructed to examine functional aspects of the Adi3 interaction with SnRK1 and Gal83. These assays revealed that Adi3 is capable of suppressing the kinase activity of the SnRK1 complex through Gal83 phosphorylation plus the interaction with SnRK1 and suggested that this function may be related to the cell death suppression activity of Adi3.

Opprimo ergo sum--evasion and suppression in the root endophytic fungus Piriformospora indica

The genetically tractable endophytic fungus Piriformospora indica is able to colonize the root cortex of a great variety of different plant species with beneficial effects to its hosts, and it represents a suitable model system to study symbiotic interactions. Recent cytological studies in barley and Arabidopsis showed that, upon penetration of the root, P. indica establishes a biotrophic interaction during which fungal cells are encased by the host plasma membrane. Large-scale transcriptional analyses of fungal and plant responses have shown that perturbance of plant hormone homeostasis and secretion of fungal lectins and other small proteins (effectors) may be involved in the evasion and suppression of host defenses at these early colonization steps. At later stages, P. indica is found more often in moribund host cells where it secretes a large variety of hydrolytic enzymes that degrade proteins. This strategy of colonizing plants is reminiscent of that of hemibiotrophic fungi, although a defined shift to necrotrophy with massive host cell death is missing. Instead, the association with the plant root leads to beneficial effects for the host such as growth promotion, increased resistance to root as well as leaf pathogens, and increased tolerance to abiotic stresses. This review describes current advances in understanding the components of the P. indica endophytic lifestyle from molecular and genomic analyses.

Evolutionary Adaptations of Plant AGC Kinases: From Light Signaling to Cell Polarity Regulation

Signaling and trafficking over membranes involves a plethora of transmembrane proteins that control the flow of compounds or relay specific signaling events. Next to external cues, internal stimuli can modify the activity or abundance of these proteins at the plasma membrane (PM). One such regulatory mechanism is protein phosphorylation by membrane-associated kinases, several of which are AGC kinases. The AGC kinase family is one of seven kinase families that are conserved in all eukaryotic genomes. In plants evolutionary adaptations introduced specific structural changes within the AGC kinases that most likely allow modulation of kinase activity by external stimuli (e.g., light). Starting from the well-defined structural basis common to all AGC kinases we review the current knowledge on the structure-function relationship in plant AGC kinases. Nine of the 39 Arabidopsis AGC kinases have now been shown to be involved in the regulation of auxin transport. In particular, AGC kinase-mediated phosphorylation of the auxin transporters ABCB1 and ABCB19 has been shown to regulate their activity, while auxin transporters of the PIN family are located to different positions at the PM depending on their phosphorylation status, which is a result of counteracting AGC kinase and PP6 phosphatase activities. We therefore focus on regulation of AGC kinase activity in this context. Identified structural adaptations of the involved AGC kinases may provide new insight into AGC kinase functionality and demonstrate their position as central hubs in the cellular network controlling plant development and growth.

The Arabidopsis extracellular UNUSUAL SERINE PROTEASE INHIBITOR functions in resistance to necrotrophic fungi and insect herbivory

Protease inhibitors (PIs) function in the precise regulation of proteases, and are thus involved in diverse biological processes in many organisms. Here, we studied the functions of the Arabidopsis UNUSUAL SERINE PROTEASE INHIBITOR (UPI) gene, which encodes an 8.8 kDa protein of atypical sequence relative to other PIs. Plants harboring a loss-of-function UPI allele displayed enhanced susceptibility to the necrotrophic fungi Botrytis cinerea and Alternaria brassicicola as well as the generalist herbivore Trichoplusia ni. Further, ectopic expression conferred increased resistance to B. cinerea and T. ni. In contrast, the mutant has wild-type responses to virulent, avirulent and non-pathogenic strains of Pseudomonas syringae, thus limiting the defense function of UPI to necrotrophic fungal infection and insect herbivory. Expression of UPI is significantly induced by jasmonate, salicylic acid and abscisic acid, but is repressed by ethylene, indicating complex phytohormone regulation of UPI expression. The upi mutant also shows significantly delayed flowering, associated with decreased SOC1 expression and elevated levels of MAF1, two regulators of floral transition. Recombinant UPI strongly inhibits the serine protease chymotrypsin but also weakly blocks the cysteine protease papain. Interestingly, jasmonate induces intra- and extracellular UPI accumulation, suggesting a possible role in intercellular or extracellular functions. Overall, our results show that UPI is a dual-specificity PI that functions in plant growth and defense, probably through the regulation of endogenous proteases and/or those of biotic invaders.

An ATP analog-sensitive version of the tomato cell death suppressor protein kinase Adi3 for use in substrate identification

Adi3 is a protein kinase from tomato that functions as a cell death suppressor and its substrates are not well defined. As a step toward identifying Adi3 substrates we developed an ATP analog-sensitive version of Adi3 in which the ATP-binding pocket is mutated to allow use of bulky ATP analogs. Met385 was identified as the "gatekeeper" residue and the M385G mutation allows for the use of two bulky ATP analogs. Adi3(M385G) can also specifically utilize N(6)-benzyl-ATP to phosphorylate a known substrate and provides a tool for identifying Adi3 substrates.

The plant cell death suppressor Adi3 interacts with the autophagic protein Atg8h

The tomato AGC protein kinase Adi3 is known to function as a suppressor of PCD and silencing of Adi3 leads to spontaneous cell death on leaves and stems. In an effort to isolate Adi3 interacting proteins, a yeast two-hybrid screen was carried out and identified the autophagy protein Atg8h as an Adi3 interactor. This interaction occurred independent of the kinase activity status of Adi3. Silencing of genes involved in autophagy is known to eliminate the restriction of pathogen-induced PCD to a few cells and leads to run away PCD. Cosilencing Adi3 with several autophagy genes lead to the same run away cell death suggesting Adi3 may be involved in autophagic regulation of PCD.

The role of vacuolar processing enzyme (VPE) from Nicotiana benthamiana in the elicitor-triggered hypersensitive response and stomatal closure

Elicitors/pathogen-associated molecular patterns (PAMPs) trigger the plant immune system, leading to rapid programmed cell death (hypersensitive response, HR) and stomatal closure. Previous reports have shown that the vacuolar processing enzyme (VPE), a cysteine proteinase responsible for the maturation of vacuolar proteins, has caspase-1-like activity and mediates TMV- and mycotoxin-induced cell death. The role of VPE from Nicotiana benthamiana in the response to three elicitors: bacterial harpin, fungal Nep1, and oomycete boehmerin, is described here. Single-silenced (NbVPE1a or NbVPE1b) and dual-silenced (NbVPE1a/1b) N. benthamiana plants were produced by virus-induced gene silencing. Although NbVPE silencing does not affect H(2)O(2) accumulation triggered by boehmerin, harpin, or Nep1, the HR is absent in NbVPE1a- and NbVPE1a/1b-silenced plants treated with harpin alone. However, NbVPE-silenced plants develop a normal HR after boehmerin and Nep1 treatment. These results suggest that harpin-triggered HR is VPE-dependent. Surprisingly, all gene-silenced plants show significantly impaired elicitor-induced stomatal closure and elicitor-promoted nitric oxide (NO) production in guard cells. Dual-silenced plants show increased elicitor-triggered AOS production in guard cells. The accumulation of transcripts associated with defence and cell redox is modified by VPE silencing in elicitor signalling. Overall, these results indicate that VPE from N. benthamiana functions not only in elicitor-induced HR, but also in elicitor-induced stomatal closure, suggesting that VPE may be involved in elicitor-triggered immunity.