Involvement of sphingoid bases in mediating reactive oxygen intermediate production and programmed cell death in Arabidopsis

Abstract

Sphingolipids have been suggested to act as second messengers for an array of cellular signaling activities in plant cells, including stress responses and programmed cell death (PCD). However, the mechanisms underpinning these processes are not well understood. Here, we report that an Arabidopsis mutant, fumonisin B1 resistant11-1 (fbr11-1), which fails to generate reactive oxygen intermediates (ROIs), is incapable of initiating PCD when the mutant is challenged by fumonisin B 1 (FB 1 ), a specific inhibitor of ceramide synthase. Molecular analysis indicated that FBR11 encodes a long-chain base1 (LCB1) subunit of serine palmitoyltransferase (SPT), which catalyzes the first rate-limiting step of de novo sphingolipid synthesis. Mass spectrometric analysis of the sphingolipid concentrations revealed that whereas the fbr11-1 mutation did not affect basal levels of sphingoid bases, the mutant showed attenuated formation of sphingoid bases in response to FB 1 . By a direct feeding experiment, we show that the free sphingoid bases dihydrosphingosine, phytosphingosine and sphingosine efficiently induce ROI generation followed by cell death. Conversely, ROI generation and cell death induced by dihydrosphingosine were specifically blocked by its phosphorylated form dihydrosphingosine-1-phosphate in a dosedependent manner, suggesting that the maintenance of homeostasis between a free sphingoid base and its phosphorylated derivative is critical to determining the cell fate. Because alterations of the sphingolipid level occur prior to the ROI production, we propose that the free sphingoid bases are involved in the control of PCD in Arabidopsis, presumably through the regulation of the ROI level upon receiving different developmental or environmental cues.

Protein Analysis of Sapienic Acid-Treated Porphyromonas gingivalis Suggests Differential Regulation of Multiple Metabolic Pathways

“…RagA also plays a role in the virulence-specifically the soft tissue-destructive abilities-of P. gingivalis. When the rag locus is inactivated, soft tissue destruction in mice is reduced (56).…”

“…The specific function of RagA is unclear, but because of the hemin-binding ability of RagB, it could also be involved in the transport of heme across the periplasmic space. RagA also has strong similarities to TonB-linked receptors in Bacteroides thetaiotaomicron, which transport macromolecules such as carbohydrates or glycoproteins across the periplasmic space (51,56). Because P. gingivalis is asaccharolytic, any carbohydrate uptake would likely be linked to anabolic processes (51).…”

“…Tlr was previously named Tla (for TonB-linked adhesion) because the initial sequence mistakenly included a kgp sequence, which codes for an adhesin (55). TonB-linked receptors are essential for growth at low concentrations of hemin (55,56). RagA is a temperature-regulated TonBlinked receptor that is believed to form a complex on the outer membrane with the hemin-binding lipoprotein RagB.…”

Susceptibility of Phelipanche and Orobanche species to AAL-toxin

“…Moreover, phytosphingosine and sphingosine have been shown to induce cell death in duckweed and tomato leaf . More recently, phytosphingosine, sphinganine, but also sphingosine, have been shown to induce reactive oxygen intermediates generation and PCD in Arabidopsis (Shi et al 2007). AAL treatment may thus also induce PCD in broomrape, since a typical feature of apoptosis, i.e., DNA laddering, is displayed as was previously shown in tomato (Wang et al 1996).…”

Fumonisin B1, a toxin produced by Fusarium verticillioides, modulates maize β-1,3-glucanase activities involved in defense response

“…The signaling role of these LCBs has been documented in plants as they induce reactive oxygen intermediates generation and cell death (Shi et al 2007), probably through a mitogen-activated protein kinase 6 (MPK6) in Arabidopsis (Saucedo-García et al 2011). In addition, FB1 treatment induces the expression in Arabidopsis protoplasts of the SA-regulated genes flavonoid glucosyltransferase, extensions, and vacuolar proteases , as well as the pathogenesis-related genes PR-1, PR-2, and PR-5 in seedlings .…”