Publikationen - Molekulare Signalverarbeitung

Pflanzen müssen gegen vielfältige biotische und abiotische Umwelteinflusse eine Abwehr aufbauen. Aber gleichzeitig müssen sie wachsen und sich vermehren. Jasmonate sind neben anderen Hormonen ein zentrales Signal bei der Etablierung von Abwehrmechanismen, aber auch Signal von Entwicklungsprozessen wie Blüten‐ und Trichombildung, sowie der Hemmung von Wachstum. Biosynthese und essentielle Komponenten der Signaltransduktion von JA und seinem biologisch aktiven Konjugat JA‐Ile sind gut untersucht. Der Rezeptor ist ein Proteinkomplex, der “JA‐Ile‐Wahrnehmung” mit proteasomalem Abbau von Repressorproteinen verbindet. Dadurch können positiv agierende Transkriptionsfaktoren wirksam werden und vielfältige Genexpressionsänderungen auslösen. Dies betrifft die Bildung von Abwehrproteinen, Enzymen der JA‐Biosynthese und Sekundärstoffbildung, und Proteinen von Signalketten und Entwicklungsprozessen. Die Kenntnisse zur JA‐Ile‐Wirkung werden in Landwirtschaft und Biotechnologie genutzt.

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.

The INDOLE-3-BUTYRIC ACID RESPONSE5 (IBR5) gene encodes a dual specificity phosphatase that regulates plant auxin responses. IBR5 has been predicted to generate two transcripts through alternative splicing, but alternative splicing of IBR5 has not been confirmed experimentally. The previously characterized ibr5-1 null mutant exhibits many auxin related defects such as auxin insensitive primary root growth, defective vascular development, short stature and reduced lateral root development. However, whether all these defects are caused by the lack of phosphatase activity is not clear. Here we describe two new auxin insensitive IBR5 alleles, ibr5-4, a catalytic site mutant, and ibr5-5, a splice site mutant. Characterization of these new mutants indicates that IBR5 is post-transcriptionally regulated to generate two transcripts, AT2G04550.1 and AT2G04550.3, and consequently two IBR5 isoforms, IBR5.1 and IBR5.3. The IBR5.1 isoform exhibits phosphatase catalytic activity that is required for both proper degradation of Aux/IAA proteins and auxin-induced gene expression. These two processes are independently regulated by IBR5.1. Comparison of new mutant alleles with ibr5-1 indicates that all three mutant alleles share many phenotypes. However, each allele also confers distinct defects implicating IBR5 isoform specific functions. Some of these functions are independent of IBR5.1 catalytic activity. Additionally, analysis of these new mutant alleles suggests that IBR5 may link ABP1 and SCFTIR1/AFBs auxin signaling pathways.

Bacillus amyloliquefaciens ssp. plantarum FZB42 represents the prototype of Gram-positive plant growth promoting and biocontrol bacteria. In this study, we applied transposon mutagenesis to generate a transposon library, which was screened for genes involved in multicellular behavior and biofilm formation on roots as a prerequisite of plant growth promoting activity. Transposon insertion sites were determined by rescue-cloning followed by DNA sequencing. As in B. subtilis, the global transcriptional regulator DegU was identified as an activator of genes necessary for swarming and biofilm formation, and the DegU-mutant of FZB42 was found impaired in efficient root colonization. Direct screening of 3,000 transposon insertion mutants for plant-growth-promotion revealed the gene products of nfrA and RBAM_017140 to be essential for beneficial effects exerted by FZB42 on plants. We analyzed the performance of GFP-labeled wild-type and transposon mutants in the colonization of lettuce roots using confocal laser scanning microscopy. While the wild-type strain heavily colonized root surfaces, the nfrA mutant did not colonize lettuce roots, although it was not impaired in growth in laboratory cultures, biofilm formation and swarming motility on agar plates. The RBAM17410 gene, occurring in only a few members of the B. subtilis species complex, was directly involved in plant growth promotion. None of the mutant strains were affected in producing the plant growth hormone auxin. We hypothesize that the nfrA gene product is essential for overcoming the stress caused by plant response towards bacterial root colonization.