No abstract

Summary Phospholipids have recently been found to be integral elements of hormone signalling pathways. An Arabidopsis thaliana double mutant in two type III phosphatidylinositol‐4‐kinases (PI4Ks), pi4kIIIβ1β2, displays a stunted rosette growth. The causal link between PI4K activity and growth is unknown. Using microarray analysis, quantitative reverse transcription polymerase chain reaction (RT‐qPCR) and multiple phytohormone analysis by LC‐MS we investigated the mechanism responsible for the pi4kIIIβ1β2 phenotype. The pi4kIIIβ1β2 mutant accumulated a high concentration of salicylic acid (SA), constitutively expressed SA marker genes including PR‐1, and was more resistant to Pseudomonas syringae. pi4kIIIβ1β2 was crossed with SA signalling mutants eds1 and npr1 and SA biosynthesis mutant sid2 and NahG. The dwarf phenotype of pi4kIIIβ1β2 rosettes was suppressed in all four triple mutants. Whereas eds1 pi4kIIIβ1β2, sid2 pi4kIIIβ1β2 and NahG pi4kIIIβ1β2 had similar amounts of SA as the wild‐type (WT), npr1pi4kIIIβ1β2 had more SA than pi4kIIIβ1β2 despite being less dwarfed. This indicates that PI4KIIIβ1 and PI4KIIIβ2 are genetically upstream of EDS1 and need functional SA biosynthesis and perception through NPR1 to express the dwarf phenotype. The slow root growth phenotype of pi4kIIIβ1β2 was not suppressed in any of the triple mutants. The pi4kIIIβ1β2 mutations together cause constitutive activation of SA signalling that is responsible for the dwarf rosette phenotype but not for the short root phenotype.

Phosphoglycerolipids are essential structural constituents of membranes and some also have important cell signalling roles. In this review, we focus on phosphoglycerolipids that are mediators in hormone signal transduction in plants. We first describe the structures of the main signalling phosphoglycerolipids and the metabolic pathways that generate them, namely the phospholipase and lipid kinase pathways. In silico analysis of Arabidopsis transcriptome data provides evidence that the genes encoding the enzymes of these pathways are transcriptionally regulated in responses to hormones, suggesting some link with hormone signal transduction. The involvement of phosphoglycerolipid signalling in the early responses to abscisic acid, salicylic acid and auxins is then detailed. One of the most important signalling lipids in plants is phosphatidic acid. It can activate or inactivate protein kinases and/or protein phosphatases involved in hormone signalling. It can also activate NADPH oxidase leading to the production of reactive oxygen species. We will interrogate the mechanisms that allow the activation/deactivation of the lipid pathways, in particular the roles of G proteins and calcium. Mediating lipids thus appear as master players of cell signalling, modulating, if not controlling, major transducing steps of hormone signals.