Innate immune perception is the first line of inducible defense against invading pathogens. Plants lack specialized circulating immune cells. Therefore, diverse cell types are able to recognize and respond to pathogens. Surface localized and intracellular plant innate immune receptors are capable of recognizing diverse pathogen components. Intracellular nucleotide-binding leucine-rich repeat (NLR) receptors recognize pathogen effectors delivered inside host cells. Recent advances shed light onto NLR activation, phosphorylation of defense signaling nodes and overlap in transcriptional responses between pathogen perception and abiotic stress.

To establish infection, pathogens deploy effectors to modify or remove host proteins. Plant immune receptors with nucleotide-binding, leucine-rich repeat domains (NLRs) detect these modifications and trigger immunity. Plant NLRs thus guard host "guardees." A corollary is that autoimmunity may result from inappropriate NLR activation because mutations in plant guardees could trigger corresponding NLR guards. To explore these hypotheses, we expressed 108 dominant-negative (DN) Arabidopsis NLRs in various lesion mimic mutants, including camta3, which exhibits autoimmunity. CAMTA3 was previously described as a negative regulator of immunity, and we find that autoimmunity in camta3 is fully suppressed by expressing DNs of two NLRs, DSC1 and DSC2. Additionally, expression of either NLR triggers cell death that can be suppressed by CAMTA3 expression. These findings support a model in which DSC1 and DSC2 guard CAMTA3, and they suggest that other negative regulators of immunity may similarly represent guardees.

Organophosphonate utilization by Escherichia coli requires the 14 cistrons of the phnCDEFGHIJKLMNOP operon, of which the carbon-phosphorus lyase has been postulated to consist of the seven polypeptides specified by phnG to phnM . A 5,660-bp DNA fragment encompassing phnGHIJKLM is cloned, followed by expression in E. coli and purification of Phn-polypeptides. PhnG, PhnH, PhnI, PhnJ, and PhnK copurify as a protein complex by ion-exchange, size-exclusion, and affinity chromatography. The five polypeptides also comigrate in native-PAGE. Cross-linking of the purified protein complex reveals a close proximity of PhnG, PhnI, PhnJ, and PhnK, as these subunits disappear concomitant with the formation of large cross-linked protein complexes. Two molecular forms are identified, a major form of molecular mass of approximately 260 kDa, a minor form of approximately 640 kDa. The stoichiometry of the protein complex is suggested to be PhnG 4 H 2 I 2 J 2 K. Deletion of individual phn genes reveals that a strain harboring plasmid-borne phnGHIJ produces a protein complex consisting of PhnG, PhnH, PhnI, and PhnJ, whereas a strain harboring plasmid-borne phnGIJK produces a protein complex consisting of PhnG and PhnI. We conclude that phnGHIJK specify a soluble multisubunit protein complex essential for organophosphonate utilization.

Multi-layered defense responses are activated in plants upon recognition of invading pathogens. Transmembrane receptors recognize conserved pathogen-associated molecular patterns (PAMPs) and activate MAP kinase cascades, which regulate changes in gene expression to produce appropriate immune responses. For example, Arabidopsis MAP kinase 4 (MPK4) regulates the expression of a subset of defense genes via at least one WRKY transcription factor. We report here that MPK4 is found in complexes in vivo with PAT1, a component of the mRNA decapping machinery. PAT1 is also phosphorylated by MPK4 and, upon flagellin PAMP treatment, PAT1 accumulates and localizes to cytoplasmic processing (P) bodies which are sites for mRNA decay. Pat1 mutants exhibit dwarfism and de-repressed immunity dependent on the immune receptor SUMM2. Since mRNA decapping is a critical step in mRNA turnover, linking MPK4 to mRNA decay via PAT1 provides another mechanism by which MPK4 may rapidly instigate immune responses.

Somatic cells acclimate to changes in the environment by temporary reprogramming. Much has been learned about transcription factors that induce these cell‐state switches in both plants and animals, but how cells rapidly modulate their proteome remains elusive. Here, we show rapid induction of autophagy during temporary reprogramming in plants triggered by phytohormones, immune, and danger signals. Quantitative proteomics following sequential reprogramming revealed that autophagy is required for timely decay of previous cellular states and for tweaking the proteome to acclimate to the new conditions. Signatures of previous cellular programs thus persist in autophagy‐deficient cells, affecting cellular decision‐making. Concordantly, autophagy‐deficient cells fail to acclimatize to dynamic climate changes. Similarly, they have defects in dedifferentiating into pluripotent stem cells, and redifferentiation during organogenesis. These observations indicate that autophagy mediates cell‐state switches that underlie somatic cell reprogramming in plants and possibly other organisms, and thereby promotes phenotypic plasticity.

Two cDNA clones of the segment of Saccharomyces cerevisiae M1 double‐stranded RNA, which codes for the yeast killer toxin, have been expressed in yeast using the expression vector pYT760. Toxin expression and secretion depended upon the presence of a yeast promoter. Transformants not only contain an authentic preprotoxin precursor, as determined by precipitation of intracellular proteins with antitoxin antisera, but also display an immunity phenotype. The evidence is that the immunity protein is part of the preprotoxin and may act by masking toxin binding sites. Neither cDNA clone had a complete 5′ terminus and the preprotoxin translational start was missing. The promoter and the initiator ATG were supplied by the expression vector. One clone with a full‐length preprotoxin but altered N‐terminal amino acids gave a normal glycosylated intracellular precursor. A clone with an N‐terminal nine amino acid deletion gave a precursor which was not glycosylated but toxin was still secreted.

Plant response to pathogen infection varies within a leaf, yet this heterogeneity is not well resolved. We exposed Arabidopsis to Pseudomonas syringae or mock treatment and profiled >11,000 individual cells using single-cell RNA sequencing. Integrative analysis of cell populations from both treatments identified distinct pathogen responsive cell clusters exhibiting transcriptional responses ranging from immunity to susceptibility. Pseudotime analyses through pathogen infection revealed a continuum of disease progression from an immune to susceptible state. Confocal imaging of promoter reporter lines for transcripts enriched in immune cell clusters expressed surrounding substomatal cavities colonized or in close proximity to bacterial colonies, suggesting cells within immune clusters represent sites of early pathogen invasion. Susceptibility clusters exhibited more general localization and were highly induced at later stages of infection. Overall, our work uncovers cellular heterogeneity within an infected leaf and provides unique insight into plant differential response to infection at a single-cell level.

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