Involvement of sphingosine kinase in plant cell signalling

Abstract

SummaryIn mammalian cells sphingosine-1-phosphate (S1P) is a well-established messenger molecule that participates in a wide range of signalling pathways. The objective of the work reported here was to investigate the extent to which phosphorylated long-chain sphingoid bases, such as sphingosine-1-phosphate and phytosphingosine-1-phosphate (phytoS1P) are used in plant cell signalling. To do this, we manipulated Arabidopsis genes capable of metabolizing these messenger molecules. We show that Sphingosine kinase1 (SPHK1) encodes an enzyme that phosphorylates sphingosine, phytosphingosine and other sphingoid long-chain bases. The stomata of SPHK1-KD Arabidopsis plants were less sensitive, whereas the stomata of SPHK1-OE plants were more sensitive, than wild type to ABA. The rate of germination of SPHK1-KD was enhanced, whereas the converse was true for SPHK1-OE seed. Reducing expression of either the putative Arabidopsis S1P phosphatase (SPPASE) or the DPL1 gene, which encodes an enzyme with S1P lyase activity, individually, had no effect on guard-cell ABA signalling; however, stomatal responses to ABA in SPPASE DPL1 RNAi plants were compromised. Reducing the expression of DPL1 had no effect on germination; however, germination of SPPASE RNAi seeds was more sensitive to applied ABA. We also found evidence that expression of SPHK1 and SPPASE were coordinately regulated, and discuss how this might contribute to robustness in guard-cell signalling. In summary, our data establish SPHK1 as a component in two separate plant signalling systems, opening the possibility that phosphorylated long-chain sphingoid bases such as S1P and phytoS1P are ubiquitous messengers in plants.

Functional Characterization of a Higher Plant Sphingolipid Δ4-Desaturase: Defining the Role of Sphingosine and Sphingosine-1-Phosphate in Arabidopsis

“…We believe that our approach, in which the in vivo capacity to synthesize sphingosine (and hence S-1-P) is abolished by insertional gene inactivation and biochemically confirmed by a high-resolution HPLC-electrospray ionization-MS/MS analysis of Arabidopsis sphingolipids, provides definitive evidence that D4-unsaturated sphingolipids are nonessential in this plant species. In addition, in view of the absence of sphingosine and S-1-P in virtually all Arabidopsis tissues, we would suggest that activities recently described as "sphingosine kinase" (Worrall et al, 2008) should be renamed "LCB kinase. "…”

Sphingosine-1-phosphate (S1P) mediates darkness-induced stomatal closure through raising cytosol pH and hydrogen peroxide (H2O2) levels in guard cells in Vicia faba

“…In animal cells, the sphingolipid metabolite S1P has been shown to be involved in the mediation of numerous cellular responses, including proliferation, survival, cytoskeletal organization, motility, differentiation, neurite retraction, and rounding [55,56]. Presently, S1P and LCBKs (referred to as SphKs) are known to participate in ABA-regulated stomatal movement [5,14,15]. However, A. thaliana mutants dis- rupted in ∆4-desaturase neither contain S1P nor display any perturbation to ABA-induced stomatal closure, indicating that S1P is not involved in the stomatal closure by ABA [8].…”

“…Further, A. thaliana insertion mutants with disrupted in ∆4-desaturase genes lack any detectable ∆4-unsaturated LCBP, such as S1P, and display no perturbation to ABA-induced stomatal closure [8], suggesting that S1P is not involved in stomatal closure caused by ABA in A. thaliana. However, ∆4-unsaturated sphingolipids are abundant in some plant species, such as Glycine max and Lycopersicon esculentum [16,17], whereas research has been limited to the role of S1P in ABA signaling in A. thaliana and C. communis guard cells [5,8,14,15]. It remains unknown whether S1P mediates stomatal closure in response to other internal or external factors, including darkness.…”

“…In higher plants, S1P has been shown to be a new calcium-mobilizing molecule and involved in abscisic acid (ABA)-induced stomatal closure in Commelina communis [14] and Arabidopsis thaliana [5]. Worrall et al [15] have reported that the stomata of an A. thaliana LCBK mutant are less sensitive than wild type to ABA, whereas the stomata of LCBK-overexpression plants are more sensitive. Because S1P is often found at trace levels in plants [1618] and phytosphingosine-1-phosphate (Phyto-S1P), a predominant ∆4-saturated LCBP, is also capable of promoting stomatal closure in A. thaliana [19], the role of S1P in guard-cell signaling is doubted.…”

Hope for Humpty Dumpty: Systems Biology of Cellular Signaling

“…A gene may be deemed to be of interest because it has homology to a protein of relevant function in another organism. For example, certain phosphorylated lipid metabolites have been implicated as secondary messengers in guard cell ABA signaling, and candidates for the genes that encode the associated kinases were identified in the Arabidopsis genome based on homology to such kinases in mammals (Worrall et al, 2008). A gene may also be deemed to be of interest because it has been shown to function in an analogous signaling pathway in a different cell type in the same organism.…”