Novel roles of ARR2 in pathogen response and flowering time

Abstract

Two-component system (TCS) and MAPK signalling cascades play essential roles in plant signal transduction. The Arabidopsis Response Regulator 2 (ARR2), a member of B‐type response regulators in the two‐component circuit, serves as a molecular hub integrating several incoming signals. A comparable hub function is also known for the MAPK signalling cascade. Based on preliminary experiments we have started to address the hypothesis that there exists a link between ARR2‐dependent TCS and MAPK signalling cascade, i.e. a crosstalk of those two evolutionary divergent signal transduction systems. The results revealed that ARR2 as well as other tested B‐type ARRs, but not the A-type ARRs, strongly interacted with MKKs and some MPKs where none of them interacted with MKKKs. Interestingly, despite a high sequence similarity with ARR2, ARR1 did not interact with the all those MAPK members ARR2 is interacting with. ARR2 was found to interact preferentially with MKK4 and MKK5, which play an important role in the biotic stress defence specially including responses to pathogen attacks. Pathogenesis assays applied on ARR2 and ARR1 mutants lines, using the necrothrophic, semi biotrophic and biotrophic pathogens revealed us that ARR2 mutants showed difference only in necrotrophic stress responses but not in biotrophic stresses.

The detailed analysis of arr1 and arr2 single loss-of-function mutants as well as an arr1 arr2 double mutant revealed an early flowering phenotype of these plants especially visible under short day conditions (SDs) with the double mutant having an additive effect compared to the single parents. Despite very similar sequence homology of the ARR1 and ARR2 genes, the loss of ARR2 effects the flowering time was quite stronger than ARR1 but they still function in the same signalling pathway. From the mechanistic point of view the lack of ARR1/ARR2 activity causes down-regulation of specific floral repressor genes but not of genes that act as floral activators and this miss-regulation of expression is not due developmental defects but directly linked to floral induction. ARR2 expressed either in the shoot apical meristem (SAM) or in phloem companion cells recues the early flowering phenotype of the arr2 the arr1 arr2 mutants. The crossing of ARR1 and ARR2 mutants with flowering time-related mutants revealed that ARR1 and ARR2 function predominantly independent of Flowering Locus C (FLC). However, this approach revealed that Flowering Locus M (FLM) acts epistatic to ARR1 and ARR2.

These findings are revealing and describing special features of the TCS elements and their crosstalks with other signalling pathways and contributing to a better understanding of their physiological role.