Phospholipases and the Network of Auxin Signal Transduction with ABP1 and TIR1 as Two Receptors: A Comprehensive and Provocative Model

Abstract

Three types of phospholipases, phospholipase D, secreted phospholipase A2, and patatin-related phospholipase A (pPLA) have functions in auxin signal transduction. Potential linkage to auxin receptors ABP1 or TIR1, their rapid activation or post-translational activation mechanisms, and downstream functions regulated by these phospholipases is reviewed and discussed. Only for pPLA all aspects are known at least to some detail. Evidence is gathered that all these signal reactions are located in the cytosol and seem to merge on regulation of PIN-catalyzed auxin efflux transport proteins. As a consequence, auxin concentration in the nucleus is also affected and this regulates the E3 activity of this auxin receptor. We showed that ABP1, PIN2, and pPLA, all outside the nucleus, have an impact on regulation of auxin-induced genes within 30 min. We propose that regulation of PIN protein activities and of auxin efflux transport are the means to coordinate ABP1 and TIR1 activity and that no physical contact between components of the ABP1-triggered cytosolic pathways and TIR1-triggered nuclear pathways of signaling is necessary to perform this.

Mutants of phospholipase A (pPLA‐I) have a red light and auxin phenotype

“…A transcriptional regulation by auxin of pPLA-I should be controlled by TIR1, but was not observed. Obviously, auxin-induced events elicited in less than 10 min need a receptor other than TIR1 (Scherer 2011;Scherer et al 2012;Labusch et al 2013). Therefore, our data rather indicate an auxin triggered post-transcriptional activation/inactivation event on pPLA-I to be evoked in less than 10 min.…”

“…The very same phenotypes in abp1–5 and abp1/ABP1 , and in ppla‐I , together with all the other facets of auxin‐ and red light‐related phenotype elements, provide this hypothesis (Scherer et al . ). Potential interactors of phototropism and gravitropism and red light were discussed earlier and may in part explain delayed tropistic responses.…”

“…Obviously, auxin‐induced events elicited in less than 10 min need a receptor other than TIR1 (Scherer ; Scherer et al . ; Labusch et al . ).…”

“…, ) and the ppla‐I alleles here, and the proven capacity of ABP1 to trigger rapid auxin responses (Scherer ; Scherer et al . ) favour ABP1 as the respective receptor although the specific mechanism remains to be uncovered. Moreover, the co‐localization of pPLA‐I (Fig.…”

Lipid signaling in plants

“…Over recent years, lipids as hormones and signaling messengers have gained increasing attention in the plant biology community. This volume collected 20 articles, including both primary research articles and several timely reviews on lipid signaling pathways, from active researchers addressing various fundamental questions in lipid signaling in plants (Ibrahim et al, 2011; Alford et al, 2012; Benning et al, 2012; Berkey et al, 2012; Dave and Graham, 2012; Dieck et al, 2012; Ge et al, 2012; Guo and Wang, 2012; Jung et al, 2012; Koo and Howe, 2012; Lorenc-Kukula et al, 2012; Maatta et al, 2012; Pleskot et al, 2012; Scherer et al, 2012; Stenzel et al, 2012; Strawn et al, 2012; Teaster et al, 2012; Wager and Browse, 2012; Xia et al, 2012; Arisz et al, 2013). One important question addressed is what lipids act as mediators in plants, and several classes of lipids and their related metabolites have been described, including phosphatidic acid, oxylipins, phosphoinositides, sphingolipids, free fatty acids, lysophospholipids, N -acylethanolamines, oxidatively modified galactolipids, and others.…”