Recognition of pathogen-associated molecular patterns (PAMPs) induces multiple defense mechanisms to limit pathogen growth. Here, we show that the Arabidopsis thaliana tandem zinc finger protein 9 (TZF9) is phosphorylated by PAMP-responsive mitogen-activated protein kinases (MAPKs) and is required to trigger a full PAMP-triggered immune response. Analysis of a tzf9 mutant revealed attenuation in specific PAMP-triggered reactions such as reactive oxygen species accumulation, MAPK activation and, partially, the expression of several PAMP-responsive genes. In accordance with these weaker PAMP-triggered responses, tzf9 mutant plants exhibit enhanced susceptibility to virulent Pseudomonas syringae pv. tomato DC3000. Visualization of TZF9 localization by fusion to green fluorescent protein revealed cytoplasmic foci that co-localize with marker proteins of processing bodies (P-bodies). This localization pattern is affected by inhibitor treatments that limit mRNA availability (such as cycloheximide or actinomycin D) or block nuclear export (leptomycin B). Coupled with its ability to bind the ribohomopolymers poly(rU) and poly(rG), these results suggest involvement of TZF9 in post-transcriptional regulation, such as mRNA processing or storage pathways, to regulate plant innate immunity.

mRNA accumulation is tightly regulated by diverse molecular pathways. The identification and characterization of enzymes and regulatory proteins involved in controlling the fate of mRNA offers the possibility to broaden our understanding of posttranscriptional gene regulation. Processing bodies (P bodies, PB) are cytoplasmic protein complexes involved in degradation and translational arrest of mRNA. Composition and dynamics of these subcellular structures have been studied in animal systems, yeasts and in the model plant Arabidopsis. Their assembly implies the aggregation of specific factors related to decapping, deadenylation, and exoribonucleases that operate synchronously to regulate certain mRNA targets during development and adaptation to stress. Although the general function of PB along with the flow of genetic information is understood, several questions still remain open. This review summarizes data on the composition, potential molecular roles, and biological significance of PB and potentially related proteins in Arabidopsis.

Summary Mitogen‐activated protein kinase (MAPK) cascades are key signalling modules of plant defence responses to pathogen‐associated molecular patterns [PAMPs; e.g. the bacterial peptide flagellin (flg22)]. Tandem zinc finger protein 9 (TZF9) is a RNA‐binding protein that is phosphorylated by two PAMP‐responsive MAPKs, MPK3 and MPK6. We mapped the major phosphosites in TZF9 and showed their importance for controlling in vitro RNA‐binding activity, in vivo flg22‐induced rapid disappearance of TZF9‐labelled processing body‐like structures and TZF9 protein turnover. Microarray analysis showed a strong discordance between transcriptome (total mRNA) and translatome (polysome‐associated mRNA) in the tzf9 mutant, with more mRNAs associated with ribosomes in the absence of TZF9. This suggests that TZF9 may sequester and inhibit the translation of subsets of mRNAs. Fittingly, TZF9 physically interacts with poly(A)‐binding protein 2 (PAB2), a hallmark constituent of stress granules – sites for stress‐induced translational stalling/arrest. TZF9 even promotes the assembly of stress granules in the absence of stress. Hence, MAPKs may control defence gene expression post‐transcriptionally through release from translation arrest within TZF9–PAB2‐containing RNA granules or by perturbing the function of PAB2 in translation control (e.g. in the mRNA closed‐loop model of translation).

Production of antimicrobial metabolites known as phytoalexins is considered as one of the initial and main barriers to inhibit pathogen development in local infected aerial tissues. Capsidiol is the main bicyclic sesquiterpene phytoalexin in tobacco (Nicotiana tabacum) and chili pepper (Capsicum annuum). Production of 5-epi-aristolochene by the corresponding sesquiterpene cyclase enzymes is considered the critical step in capsidiol biosynthesis. To analyze the transcriptional activation of chili pepper 5-epi-aristolochene synthase gene expression in response to several pathogen-associated molecular patterns, a 1,455 bp promoter fragment upstream start codon was fully sequenced and fused to β-glucuronidase reporter gene. Analyses of spatial and temporal patterns of hybrid gene expression were carried out in transgenic tobacco plants. Surprisingly β-glucuronidase was detected in both, the locally treated and the phylotactically adjacent leaves. A particular systemic gene expression was localized in the immediate vascular tissue. The activation patterns of 5-epi-aristolochene synthase transcripts and detection of capsidiol in corresponding tobacco and pepper systemic leaves confirmed these results. This expression pattern might be mediated by reactive oxygen species. This is the first report of a highly localized systemic gene expression of enzymes directly involved in sesquiterpene phytoalexin biosynthesis in leaves, elicited by pathogen-associated molecular patterns.

A major threat for bananas and plantains production is the Panama Disease or Fusarium Wilt caused by Fusarium oxysporum f. sp. cubense. In order to characterize the causal agents of Fusarium wilt in Mexico, a sampling was performed in symptomatic plantains growing in fields of Oaxaca, a coastal southern state of Mexico. A phylogenetic analysis based on the sequences of TEF 1-α and IGS revealed that three isolates belonged to the Fusarium oxysporum species complex, while two other isolates were identified as members of the Fusarium fujikuroi species complex. Furthermore, isolates from the same complex shared the same ITS2 sequence. Inoculation using spores of each isolate on the roots of Musa sp. AAB cv. Manzano produced wilting symptoms of varying severity, suggesting that the Fusarium wilt might not be caused only by Fusarium oxysporum f. sp. cubense. PCR-based detection of Secreted in Xylem (SIX) genes showed that each Fusarium isolate harbored a unique combination of genes typically found in banana pathogens, which might cause the disease.