16-Systemic acquired resistance (SAR) prepares infected plants for faster and stronger 17 defense activation upon subsequent attacks. SAR requires an information relay from primary 18 infection to distal tissue and the initiation and maintenance of a self-maintaining 19 phytohormone salicylic acid (SA)-defense loop. 20 -In spatial and temporal resolution we show that calcium-dependent protein kinase CPK5 21 contributes to immunity and SAR. In local basal resistance CPK5 functions upstream of SA-22 synthesis, -perception, and -signaling. In systemic tissue, enhanced CPK5 signaling leads to 23 an accumulation of SAR marker genes including transcription factor Systemic Acquired 24Resistance Deficient 1 (SARD1). 25 -Plants of enhanced CPK5-, but not CPK6-, signaling display a 'super-priming' phenotype of 26 enhanced resistance toward a secondary bacterial infection. In sard1 background, CPK5-27 mediated basal resistance is still mounted but systemic 'super-priming' is lost. 28 -The biochemical analysis determines CPK5 half maximal kinase activity for calcium K50 29[Ca 2+ ] to ~100 nM close to the cytoplasmic resting level. This low activation threshold 30 uniquely qualifies CPK5 to decode subtle changes in calcium prerequisite to immune signal 31 relay and to onset and maintenance of priming at later time points in distal tissue. Our data 32 explain why CPK5 functions as a hub in basal and systemic plant immunity. 33 34

The calcium-dependent protein kinase CPK1 regulates a pivotal developmental switch in senescence and leaf cell death through direct phosphorylation of transcription factor ORE1.

As a critical part of plant immunity, cells that are attacked by pathogens undergo rapid transcriptional reprogramming to minimize virulence. Many bacterial phytopathogens use type III effector (T3E) proteins to interfere with plant defense responses, including this transcriptional reprogramming. Here, we show that XopS, a T3E of Xanthomonas campestris pv. vesicatoria (Xcv), interacts with and inhibits proteasomal degradation of WRKY40, a transcriptional regulator of defense gene expression. Virus-induced gene silencing of WRKY40 in pepper (Capsicum annuum) enhanced plant tolerance to Xcv infection, indicating that WRKY40 represses immunity. Stabilization of WRKY40 by XopS reduces the expression of its targets, which include salicylic acid (SA)-responsive genes and the jasmonic acid (JA) signaling repressor JAZ8. Xcv bacteria lacking XopS display significantly reduced virulence when surface inoculated onto susceptible pepper leaves. XopS delivery by Xcv, as well as ectopic expression of XopS in Arabidopsis thaliana or Nicotiana benthamiana, prevented stomatal closure in response to bacteria and biotic elicitors. Silencing WRKY40 in pepper or N. benthamiana abolished XopS’s ability to prevent stomatal closure. This suggests that XopS interferes with both preinvasion and apoplastic defense by manipulating WRKY40 stability and downstream gene expression, eventually altering phytohormone crosstalk to promote pathogen proliferation.

Sucrose nonfermenting related kinase1 (SnRK1) is a conserved energy sensor kinase that regulates cellular adaptation to energy deficit in plants. Activation of SnRK1 leads to the down-regulation of ATP-consuming biosynthetic processes and the stimulation of energy-generating catabolic reactions by transcriptional reprogramming and posttranslational modifications. Although considerable progress has been made during the last years in understanding the SnRK1 signaling pathway, many of its components remain unidentified. Here, we show that the catalytic a-subunits KIN10 and KIN11 of the Arabidopsis (Arabidopsis thaliana) SnRK1 complex interact with the STOREKEEPER RELATED1/G-Element Binding Protein (STKR1) inside the plant cell nucleus. Overexpression of STKR1 in transgenic Arabidopsis plants led to reduced growth, a delay in flowering, and strongly attenuated senescence. Metabolite profiling revealed that the transgenic lines exhausted their carbohydrates during the dark period to a greater extent than the wild type and accumulated a range of amino acids. At the global transcriptome level, genes affected by STKR1 overexpression were broadly associated with systemic acquired resistance, and transgenic plants showed enhanced resistance toward a virulent strain of the biotrophic oomycete pathogen Hyaloperonospora arabidopsidis Noco2. We discuss a possible connection of STKR1 function, SnRK1 signaling, and plant immunity.Plants are the prime example of photoautotrophic organisms, using photosynthesis to harness energy from sunlight for the conversion of CO 2 into energyrich carbohydrates. These photoassimilates are then directed to fuel plant growth and development. However, as sessile organisms, plants usually have to cope with strongly fluctuating environmental conditions, many of which negatively impact on energy availability, as they interfere with the production or distribution of photoassimilates. In order to maintain energy homeostasis and to promote survival, energy shortage triggers a massive cellular reprogramming characterized by nutrient remobilization, suppression of biosynthetic processes, and growth arrest (Baena-González, 2010). A central component of low-energy signaling in plants is the sucrose nonfermenting related kinase1 (SnRK1), which is orthologous to the AMP-dependent kinase (AMPK) and sucrose nonfermenting1 (SNF1) in mammals and yeast, respectively. Similar to its opisthokont counterparts, the SnRK1 holoenzyme is a heterotrimeric complex consisting of a catalytic a-subunit and noncatalytic b-and plant-specific bg-subunits (Ramon et al., 2013;Emanuelle et al., 2015). In Arabidopsis (Arabidopsis thaliana), the catalytic a-subunit is represented by two isoforms, SnRK1a1 (AKIN10; At3g01090) and SnRK1a2 (AKIN11; At3g29160), both of which appear to be expressed ubiquitously, although KIN10 accounts for the majority of SnRK1 activity (Jossier et al., 2009). Activation of SnRK1 involves the phosphorylation/ dephosphorylation of a T-loop Thr (Thr-172 of Arabidopsis SnRK1.1a) involving the upstream kinas...

Summary Systemic acquired resistance (SAR) prepares infected plants for faster and stronger defense activation upon subsequent attacks. SAR requires an information relay from primary infection to distal tissue and the initiation and maintenance of a self‐maintaining phytohormone salicylic acid (SA)‐defense loop. In spatial and temporal resolution, we show that calcium‐dependent protein kinase CPK5 contributes to immunity and SAR. In local basal resistance, CPK5 functions upstream of SA synthesis, perception, and signaling. In systemic tissue, CPK5 signaling leads to accumulation of SAR‐inducing metabolite N‐hydroxy‐L‐pipecolic acid (NHP) and SAR marker genes, including Systemic Acquired Resistance Deficient 1 (SARD1) Plants of increased CPK5, but not CPK6, signaling display an ‘enhanced SAR’ phenotype towards a secondary bacterial infection. In the sard1‐1 background, CPK5‐mediated basal resistance is still mounted, but NHP concentration is reduced and enhanced SAR is lost. The biochemical analysis estimated CPK5 half maximal kinase activity for calcium, K50 [Ca2+], to be c. 100 nM, close to the cytoplasmic resting level. This low threshold uniquely qualifies CPK5 to decode subtle changes in calcium, a prerequisite to signal relay and onset and maintenance of priming at later time points in distal tissue. Our data explain why CPK5 functions as a hub in basal and systemic plant immunity.

Upon pathogen recognition, a transient rise in cytoplasmic calcium levels is one of the earliest events in plants and a prerequisite for defense initiation and signal propagation from a local site to systemic plant tissues. However, it is unclear if calcium signaling differs in the context of priming: Do plants exposed to a first pathogen stimulus and have consequently established systemic acquired resistance (SAR) display altered calcium responses to a second pathogen stimulus? Several calcium indicator systems including aequorin, YC3.6 or R-GECO1 have been used to document local calcium responses to the bacterial flg22 peptide but systemic calcium imaging within a single plant remains a technical challenge. Here, we report on an experimental approach to monitor flg22-induced calcium responses in systemic leaves of primed plants. The calcium-dependent protein kinase CPK5 is a key calcium sensor and regulator of the NADPH oxidase RBOHD and plays a role in the systemic calcium-ROS signal propagation. We therefore compared flg22-induced cytoplasmic calcium changes in Arabidopsis wild-type, cpk5 mutant and CPK5-overexpressing plants (exhibiting constitutive priming) by introgressing the calcium indicator R-GECO1-mTurquoise that allows internal normalization through mTurquoise fluorescence. Aequorin-based analyses were included for comparison. Based on the R-GECO1-mTurquoise data, CPK5-OE appears to reinforce an “oscillatory-like” Ca2+ signature in flg22-treated local tissues. However, no change was observed in the flg22-induced calcium response in the systemic tissues of plants that had been pre-challenged by a priming stimulus – neither in wild-type nor in cpk5 or CPK5-OE-lines. These data indicate that the mechanistic manifestation of a plant immune memory in distal plant parts required for enhanced pathogen resistance does not include changes in rapid calcium signaling upstream of CPK5 but rather relies on downstream defense responses.

This article comments on: Schnake A, Hartmann M, Schreiber S, Malik J, Brahmann L, Yildiz I, von Dahlen J, Rose LE, Schaffrath U, Zeier J. 2020. Inducible biosynthesis and immune function of the systemic acquired resistance inducer N-hydroxypipecolic acid in monocotyledonous and dicotyledonous plants. Journal of Experimental Botany 71, 6444–6459.

A critical component of plant immunity against invading pathogens is the rapid transcriptional reprogramming of the attacked cell to minimize virulence. Many adapted plant bacterial pathogens use type III effector (T3E) proteins to interfere with plant defense responses, including the induction of immunity genes. The elucidation of effector function is essential to understanding bacterial pathogenesis. Here, we show that XopS, a T3E of Xanthomonas campestris pv. vesicatoria (Xcv), interacts with and inhibits the proteasomal degradation of the transcriptional regulator of defense gene expression WRKY40. Virus-induced gene silencing of WRKY40 in pepper enhanced plant tolerance towards Xcv infection, indicating it represses immunity. Stabilization of WRKY40 by XopS reduces the expression of its targets including salicylic acid (SA)-responsive genes and the jasmonic acid (JA) signaling repressor JAZ8. Xcv bacteria lacking XopS display significantly reduced virulence when surface inoculated onto susceptible pepper leaves. XopS delivery by Xcv, as well as ectopic expression of XopS in Arabidopsis or Nicotiana benthamiana prevented stomatal closure in response to bacteria and biotic elicitors in a WRKY40 dependent manner. This suggests that XopS interferes with preinvasion as well as with apoplastic defense by manipulating WRKY40 stability and gene expression eventually altering phytohormone crosstalk to promote pathogen proliferation.