br Introduction Sphingolipids are ubiquitous membrane
Introduction Sphingolipids are ubiquitous membrane components of all eukaryotic cells belonging to a group of complex lipids. The basic structure of sphingolipids is composed of a sphingoid long chain base (LCB) linked to a fatty adenosine receptor antagonist via an amide bond to form a ceramide . In animals, sphingolipids play a key role in cell death regulation . Indeed, sphingosine (Sph, d18:1), which is the most abundant free LCB in animals, induces apoptosis in many cell types in response to death physiological activators [2,3]. In plants, it becomes now evident that, similarly to animals, sphingolipids are cell death mediators. For instance, spontaneous cell death phenotypes are observed in Arabidopsis mutants such as accelerated cell death 5 (acd5) and accelerated cell death 11 (acd11) impaired in a ceramide kinase or a LCB transport protein [4,5]. In addition, the mycotoxins fumonisin B1 (FB1) and Alternaria alternata lycopersici (AAL) toxin, two natural mimics of LCBs produced by the necrotrophic fungi Fusarium moniliforme and Alternaria alternata respectively, interfere with plant sphingolipid metabolism by inhibiting the activity of ceramide synthase. This enzyme inhibition leads to plant cell death by a subsequent accumulation of dihydrosphingosine (DHS, d18:0) and phytosphingosine (PHS, t18:0), which are the two major free LCBs in Arabidopsis [, , ]. The FB1 resistant11-1 (fbr11-1) Arabidopsis mutant is impaired in the gene encoding the long-chain base 1 (LCB1) subunit of serine palmitoyltransferase (SPT). This enzyme catalyzes the first rate-limiting step of de novo sphingolipid synthesis. The mutant presents a lower accumulation of free LCBs in response to FB1 than the wild type and fails to initiate programmed cell death (PCD) when challenged with FB1 . Interestingly, inoculation of Arabidopsis plants with an avirulent strain of the bacterial pathogen Pseudomonas syringae pv. tomato (avrRpm1), that typically induces a localized PCD termed the hypersensitive response (HR), leads to a rapid and sustained increase in PHS, due to de novo synthesis from DHS . A similar response was observed with the necrotrophic fungus Sclerotinia sclerotiorum indicating that fungal pathogens as well as bacteria can trigger PHS increases. These data suggest a crucial involvement of sphingolipid metabolism in plant immunity. However, although LCBs are known to induce PCD in plant cells, the signaling pathway leading to this PCD and its actors remains largely uncharacterized. It has been shown that the mitogen-activated protein kinase 6 (MPK6) is rapidly activated when Arabidopsis seedlings are exposed to FB1 or LCBs, such as PHS or DHS . Moreover, using tobacco BY-2 cells, we showed that DHS is able to trigger PCD in a calcium (Ca2+)-dependent manner mainly through nuclear Ca2+ increases . More recently, we highlighted a new sphingolipid-induced Ca2+-dependent mechanism involving the protein kinase CPK3 and 14-3-3 proteins in the context of LCB-induced cell death in Arabidopsis . As plant proteases, and more particularly Papain Like Cysteine Proteases (PLCPs) are key players in plant immunity [12,13] we investigated their role in LCB-mediated PCD. PLCPs belong to MEROPS protease family C1 A of clan CA, and are characterized by the occurrence in their structure of the papain-like fold that displays two lobes delineating a substrate-binding groove which contains the catalytic site Cys-His-Asn [14,15]. In plants, this specific class of protease has been sub-divided into 9 subfamilies according to phylogenetic analysis and conserved structural features. The number of PLCPs is between 20–40 genes per plant genome, with 31 genes found in Arabidopsis . PLCPs are involved at various stage of plant development and two of them RD21 A and AALP are the main active in senescing leaves . More interestingly, they also participate in plant immunity and thus rd21 null mutants are more susceptible to the necrotrophic fungal pathogen Botrytis cinerea whereas they do not display a particular phenotype in response to P. syringae or the oomycete pathogen Hyaloperonospora arabidopsidis .