Modulation of MAPK signaling in plant immunity and development

Plants and animal cells use intricate signaling pathways to respond to a diverse array of stimuli. These stimuli include signals from the environment, such as biotic and abiotic stress signals, as well as cell-to-cell signaling required for pattern formation during development. The transduction of the signal often relies on protein phosphorylation, which in eukaryotic cells occurs mainly on serine (Ser), threonine (Thr) and tyrosine (Tyr) residues. In animal systems Tyr phosphorylation plays a prominent role in signaling, however the relative contribution of Tyr phosphorylation to plant signal transduction has remained an open question. We implemented an approach to selectively measure and quantify Tyr phosphorylation in plant cells, which can also be applied to whole plants. We reproducibly quantified 23 pTyr peptides in two inversely labeled biological replicates, identifying 11 differentially phosphorylated proteins. These include a set of 3 well-characterized flagellin responsive mitogen-activated protein kinases (MAPKs) and 4 novel MAPKs. In plant signal transduction MAPKs are important in transducing signals from the upstream receptor to the downstream targets and they are pivotal signaling modules in pathogen-associated molecular patterns (PAMP)-triggered immunity (PTI). In Arabidopsis thaliana (Arabidopsis), the MEK4/5-MPK3/6 module is an important part of a major MAPK cascade in immune responses. We report the functional analysis of a Arabidopsis MAPK kinase kinase (MAPKKK) as a negative regulator of PTI signaling and basal immunity. We show that this function requires phosphorylation of this protein on specific Ser residues. Based on genetic evidence we observed, that this MAPKKK is also involved in root meristem maintenance and meristem cell proliferation, which are required for root growth. Our results show that through phosphorylation of this MAPKKK, signaling pathways controlling growth and immune response are antagonistically regulated.