Systemic signalling in the plant pathogen response

During their life, plants face a multitude of non-predictable and harmful environmental conditions, requiring them to promptly translate perceived stress signals into transcriptional re-programming and subsequent stress adaptation. The attack by pathogens triggers a range of local defence responses that restrict further damage near the infection site. In addition, phloem-translocated ("systemic") signals are released from the infection site and put non-infected tissue into a "primed" state which allows a more rapid and stronger response to subsequent attack. Despite its unquestionable role as a major survival strategy and the associated agricultural importance, systemic acquired resistance (SAR) and the molecular mechanisms leading to its establishment are still poorly understood. By acting downstream of stress receptors and upstream of stress-dependent transcription factors, Mitogen-activated protein kinase (MAPK) cascades are key players in the early stress signal transduction. Yet, in vivo evidence for MAPK substrates is very limited. We identified a bZIP transcription factor VIP1, which undergoes cytoplasmic- nuclear translocation upon phosphorylation by MPK3 and induces stress gene expression under conditions that stimulate the MPK3 pathway. One VIP1 target gene is thioredoxin Trxh8. Azelaic acid accumulates in the exudates of pathogen-exposed plants and induces the expression of AZI1, encoding a lipid transfer protein (LTP). azi1 mutants show a normal local response to pathogen treatment but are impaired in SAR. Similarly, SAR is blocked in mpk3 mutants. Moreover, MPK3 transcript and inactive protein accumulate in systemic tissue of stress-treated plants and contribute to priming. Preliminary studies revealed i) a mis-regulation of several LTP genes in stress-treated mpk3 mutants (microarray analysis). ii) enrichment of a peptide motif in the protein sequence of AZI1/AZI1-related proteins that matches the target motifs of known MAPK substrates. iii) high correlation of gene expression between AZI1 and - specifically - Trxh8 over a wide range of environmental conditions - suggesting functional relatedness of the corresponding proteins. In this project, a hypothetical interconnection between MPK3, AZI1 and Trxh8 shall be studied, in vitro, in vivo and in planta. mpk3 mutants will be assessed for a possible impairment in AZI1 gene expression, protein accumulation and/or activity. Likewise, a possible positive feedback regulation, i.e. involvement of (the mobile) AZI1 in the systemic accumulation of MPK3 will be studied in azi1 mutants. Post-translational modification of AZI1 through MPK3 phosphorylation, direct or indirect redox-regulation through Trxh8 and the effect of such modification on the stability, localisation or antimicrobial activity of AZI1 will be investigated. Pathogen tolerance tests on genetic crossings of mpk3 mutants, Trxh8 and AZI1 overexpressing plants shall further dissect the interdependence of these signalling components. Finally, overexpression of AZI1 variants - mimicking the constitutively phosphorylated state of the protein - may yield highly stress-resistant plants. As LTPs are proteins commonly found in dicotyledonous species, the outcomes of this project might be of agricultural relevance.

Plants only can survive under hostile conditions if they know how to adapt. At the cellular level, stress signals are transduced via evolutionary highly conserved protein kinases. The stress-induced activation enables kinases to phosphorylate their specific target proteins and thus to pass on the stress information. On key stress kinase in the model plant Arabidopsis is MPK3. Only few of its target proteins are known by now. Our project lead to identification of the MPK3 target AZI1. MPK3 interacts with AZI1, phosphorylates it and enhances AZI1 stability. AZI1-levels are reduced in MPK3-deficient mutants, and such mutants perform poor under stress. In contrast, transgenics with elevated AZI1 levels show high stress resistance (towards e.g. high salinity. Interestingly, proteins very similar to AZI1 and MPK3 are wide-spread in the plant kingdom. If we produce labelled AZI1 and MPK3 in other species, the two proteins find to each other, just as in Arabidopsis. Whether high levels of AZI1 homologs in plants, in general, provide better stress protection is an exciting question. Future search should focus on agricultural species/cultivars with a naturally high abundance of AZI1 homologs. Such plants might be better adapted to climate change-related adversities, i.e. soil salinization and desertification.