MAP kinase phosphatase1 and protein tyrosine phosphatase1 are repressors of salicylic acid synthesis and SNC1-mediated responses in Arabidopsis.

Mitogen-activated protein (MAP) kinase phosphatases are important negative regulators of the levels and kinetics of MAP kinase activation that modulate cellular responses. The dual-specificity phosphatase MAP KINASE PHOSPHATASE1 (MKP1) was previously shown to regulate MAP KINASE6 (MPK6) activation levels and abiotic stress responses in Arabidopsis thaliana. Here, we report that the mkp1 null mutation in the Columbia (Col) accession results in growth defects and constitutive biotic defense responses, including elevated levels of salicylic acid, camalexin, PR gene expression, and resistance to the bacterial pathogen Pseudomonas syringae. PROTEIN TYROSINE PHOSPHATASE1 (PTP1) also interacts with MPK6, but the ptp1 null mutant shows no aberrant growth phenotype. However, the pronounced constitutive defense response of the mkp1 ptp1 double mutant reveals that MKP1 and PTP1 repress defense responses in a coordinated fashion. Moreover, mutations in MPK3 and MPK6 distinctly suppress mkp1 and mkp1 ptp1 phenotypes, indicating that MKP1 and PTP1 act as repressors of inappropriate MPK3/MPK6-dependent stress signaling. Finally, we provide evidence that the natural modifier of mkp1 in Col is largely the disease resistance gene homolog SUPPRESSOR OF npr1-1, CONSTITUTIVE 1 (SNC1) that is absent in the Wassilewskija accession. Our data thus indicate a major role of MKP1 and PTP1 in repressing salicylic acid biosynthesis in the autoimmune-like response caused by SNC1.

The Arabidopsis protein kinase Pto-interacting 1-4 is a common target of the oxidative signal-inducible 1 and mitogen-activated protein kinases.

In Arabidopsis thaliana, the serine/threonine protein kinase oxidative signal-inducible 1 (OXI1), mediates oxidative stress signalling. Its activity is required for full activation of the mitogen-activated protein kinases (MAPKs), MPK3 and MPK6, in response to oxidative stress. In addition, the serine/threonine protein kinase Pto-interacting 1-2 (PTI1-2) has been positioned downstream from OXI1, but whether PTI1-2 signals through MAPK cascades is unclear. Using a yeast two-hybrid screen we show that OXI1 also interacts with PTI1-4. OXI1 and PTI1-4 are stress-responsive genes and are expressed in the same tissues. Therefore, studies were undertaken to determine whether PTI1-4 is positioned in the OXI1/MAPK signalling pathway. The interaction between OXI1 and PTI1-4 was confirmed by using in vivo co-immunoprecipitation experiments. OXI1 and PTI1-4 were substrates of MPK3 and MPK6 in vitro. Although no direct interaction was detected between OXI1 and MPK3 or MPK6, in vitro binding studies showed interactions between MPK3 or MPK6 with PTI1-4. In addition, PTI1-4 and MPK6 were found in vivo in the same protein complex. These results demonstrate that PTI1-4 signals via OXI1 and MPK6 signalling cascades.

The dark side of the salad: Salmonella typhimurium overcomes the innate immune response of Arabidopsis thaliana and shows an endopathogenic lifestyle.

Salmonella enterica serovar typhimurium contaminated vegetables and fruits are considerable sources of human infections. Bacteria present in raw plant-derived nutrients cause salmonellosis, the world wide most spread food poisoning. This facultative endopathogen enters and replicates in host cells and actively suppresses host immune responses. Although Salmonella survives on plants, the underlying bacterial infection mechanisms are only poorly understood. In this report we investigated the possibility to use Arabidopsis thaliana as a genetically tractable host system to study Salmonella-plant interactions. Using green fluorescent protein (GFP) marked bacteria, we show here that Salmonella can infect various Arabidopsis tissues and proliferate in intracellular cellular compartments. Salmonella infection of Arabidopsis cells can occur via intact shoot or root tissues resulting in wilting, chlorosis and eventually death of the infected organs. Arabidopsis reacts to Salmonella by inducing the activation of mitogen-activated protein kinase (MAPK) cascades and enhanced expression of pathogenesis related (PR) genes. The induction of defense responses fails in plants that are compromised in ethylene or jasmonic acid signaling or in the MKK3-MPK6 MAPK pathway. These findings demonstrate that Arabidopsis represents a true host system for Salmonella, offering unique possibilities to study the interaction of this human pathogen with plants at the molecular level for developing novel drug targets and addressing current safety issues in human nutrition.

Site-specific phosphorylation profiling of Arabidopsis proteins by mass spectrometry and peptide chip analysis.

An estimated one-third of all proteins in higher eukaryotes are regulated by phosphorylation by protein kinases (PKs). Although plant genomes encode more than 1000 PKs, the substrates of only a small fraction of these kinases are known. By mass spectrometry of peptides from cytoplasmic- and nuclear-enriched fractions, we determined 303 in vivo phosphorylation sites in Arabidopsis proteins. Among 21 different PKs, 12 were phosphorylated in their activation loops, suggesting that they were in their active state. Immunoblotting and mutational analysis confirmed a tyrosine phosphorylation site in the activation loop of a GSK3/shaggy-like kinase. Analysis of phosphorylation motifs in the substrates suggested links between several of these PKs and many target sites. To perform quantitative phosphorylation analysis, peptide arrays were generated with peptides corresponding to in vivo phosphorylation sites. These peptide chips were used for kinome profiling of subcellular fractions as well as H 2O 2-treated Arabidopsis cells. Different peptide phosphorylation profiles indicated the presence of overlapping but distinct PK activities in cytosolic and nuclear compartments. Among different H 2O 2-induced PK targets, a peptide of the serine/arginine-rich (SR) splicing factor SCL30 was most strongly affected. SRPK4 (SR protein-specific kinase 4) and MAPKs (mitogen-activated PKs) were found to phosphorylate this peptide, as well as full-length SCL30. However, whereas SRPK4 was constitutively active, MAPKs were activated by H 2O 2. These results suggest that SCL30 is targeted by different PKs. Together, our data demonstrate that a combination of mass spectrometry with peptide chip phosphorylation profiling has a great potential to unravel phosphoproteome dynamics and to identify PK substrates.