Nitric oxide-sphingolipid interplays in plant signalling: a new enigma from the Sphinx?

Cotton sphingosine kinase GhLCBK1 participates in fiber cell elongation by affecting sphingosine-1-phophate and auxin synthesis

Sphingolipids serve as essential components of biomembrane and possess significant bioactive properties. Sphingosine-1-phophate (S1P) plays a key role in plant resistance to stress, but its specific impact on plant growth and development remains to be fully elucidated. Cotton fiber cells are an ideal material for investigating the growth and maturation of plant cells. In this study, we examined the content and composition of sphingosine (Sph) and S1P throughout the progression of fiber cell development. The content of S1P elevated gradually during fiber elongation but declined during the transition stage. Exogenous application of S1P promoted fiber elongation while using of FTY720 (an antagonist of S1P), and DMS (an inhibitor of LCBK) hindered fiber elongation. Cotton Long Chain Base Kinase 1 (GhLCBK1) was notably expressed during the fiber elongation stage, containing all conserved domains of LCBK protein and localized in the endoplasmic reticulum. Overexpression GhLCBK1 increased the S1P content and promoted fiber elongation while retarded secondary cell wall (SCW) deposition. Conversely, downregulation of GhLCBK1 reduced the S1P levels, and suppressed fiber elongation, and accelerated SCW deposition. Transcriptome analysis revealed that upregulating GhLCBK1 or applying S1P induced the expression of GhEXPANSIN and auxin related genes. Furthermore, the levels of IAA were elevated and reduced in the fibers when up-regulating or down-regulating GhLCBK1, respectively. Our investigation demonstrated that GhLCBK1 and its product S1P facilitated the elongation of fiber cells by affecting auxin biosynthesis. This study contributes novel insights into the intricate regulatory pathways involved in fiber cell elongation, identifying GhLCBK1 as a potential target gene and laying the groundwork for enhancing fiber quality via genetic manipulation.

Plant defence mechanisms against mycotoxin Fumonisin B1

Fumonisin B1 (FB1) is the most harmful mycotoxin which prevails in several crops and affects the growth and yield as well. Hence, keeping the alarming consequences of FB1 under consideration, there is still a need to seek other more reliable approaches and scientific knowledge for FB1-induced cell death and a comprehensive understanding of the mechanisms of plant defence strategies. FB1-induced disturbance in sphingolipid metabolism initiates programmed cell death (PCD) through various modes such as the elevated generation of reactive oxygen species, lipid peroxidation, cytochrome c release from the mitochondria, and activation of specific proteases and nucleases causing DNA fragmentation. There is a close interaction between sphingolipids and defence phytohormones in response to FB1 exposure regulating PCD and defence. In this review, the model plant Arabidopsis and various crops have been presented with different levels of susceptibility and resistivity exposed to various concentration of FB1. In addition to this, regulation of PCD and defence mechanisms have been also demonstrated at the physiological, biochemical and molecular levels to help the understanding of the role and function of FB1-inducible molecules and genes and their expressions in plants against pathogen attacks which could provide molecular and biochemical markers for the detection of toxin exposure.

Interaction between nitric oxide and storage temperature on sphingolipid metabolism of postharvest peach fruit

Both nitric oxide (NO) and cold storage have positive effects on the maintenance of fruit quality during storage. However, the roles of NO and storage temperatures in regulating the responses of sphingolipids metabolism to chilling injury of peach fruit during storage remain unknown. Peaches were treated by immersion in distilled water and 15 μmol L^-1 NO solution, then stored at 25 °C and 0 °C, respectively. The effects of NO-treatment and storage temperature on the activities of enzymes in sphingolipid metabolism and the contents of sphingolipids in peach fruits were studied. NO maintained higher activities of acid phosphatase (AP) and alkaline phosphatase (ALP) in peach fruits at 25 °C, but promoted the decrease in the activities of AP and ALP at 0 °C, suggesting the regulation by NO on AP and ALP could be modulated by temperature. Compared with the storage at 25 °C, cold storage at 0 °C decreased the activities of phospholipase A (PLA), alkaline phosphatase (ALP), 3-ketodihydrosphingosine reductase (KDSR), sphingosine kinase (SPHK), ceramide synthase (CERS), ceramide kinase (CERK), and the contents of sphingosine (SPH), ceramide (CER), sphingosine-1-phosphate (S1P), ceramide-1-phosphate (C1P), sphingomyelin (SM), and increased the activities of phospholipase C (PLC), phospholipase D (PLD), sphingomyelin synthase (SMS). NO significantly increased the contents of sphingolipid metabolites, and the activities of PLA, KDSR, SPHK, CERS, CERK, but decreased the activities of PLC, PLD, SMS of peaches. The results suggested that NO could maintain sphingolipid metabolism to relieve the response of the postharvest fruit to low temperature.

Modulation of sphingolipid long-chain base composition and gene expression during early olive-fruit development, and putative role of brassinosteroid

Sphingolipids are abundant membrane components and signalling molecules in various aspects of plant development. However, the role of sphingolipids in early fleshy-fruit growth has rarely been investigated. In this study, we first investigated the temporal changes in sphingolipid long-chain base (LCB) content, composition, and gene expression that occurred during flower opening and early fruit development in olive (Olea europaea L. cv Picual). Moreover, the interaction between sphingolipid and the plant hormone, brassinosteroid (BR), during the early fruit development was also explored. For this, BR levels were manipulated through the application of exogenous BRs (24-epibrassinolide, EBR) or a BR biosynthesis inhibitor (brassinazole, Brz) and their effects on early fruit development, sphingolipid LCB content, and gene expression were examined in olive fruit at 14 days post-anthesis (DPA). We here show that sphingolipid with C-4 hydroxylation and Δ8 desaturation with a preference for (E)-isomer formation are quantitatively the most important sphingolipids in olive reproductive organs. In this work, the total LCB amount significantly decreased at the anthesis stage, but olive sphingosine-1-phosphate lyase (OeSPL) gene was expressed exclusively in flower and upregulated during the anthesis, revealing an association with the d18:1(8E) accumulation. However, the LCB content increased in parallel with the upregulation of the expression of genes for key sphingolipid biosynthetic and LCB modification enzymes during early fruit development in olive. Likewise, we found that EBR exogenously applied to olive trees significantly stimulated the fruit growth rate whereas Brz inhibited fruit growth rate after 7 and 14 days of treatment. In addition, this inhibitory effect could be counteracted by the application of EBR. The promotion of early fruit growth was accompanied by the down-regulation of sphingolipid LCB content and gene expression in olive fruit, whereas Brz application raised levels of sphingolipid LCB content and gene expression in olive fruit after 7 and 14 days of treatment. Thus, our data indicate that endogenous sphingolipid LCB and gene-expression levels are intricately controlled during early fruit development and also suggest a possible link between BR, the sphingolipid content/gene expression, and early fruit development in olive.

Stomatal closure induced by phytosphingosine-1-phosphate and sphingosine-1-phosphate depends on nitric oxide and pH of guard cells in Pisum sativum

MAIN CONCLUSION

Phyto-S1P and S1P induced stomatal closure in epidermis of pea ( Pisum sativum ) by raising the levels of NO and pH in guard cells. Phosphosphingolipids, such as phytosphingosine-1-phosphate (phyto-S1P) and sphingosine-1-phosphate (S1P), are important signaling components during drought stress. The biosynthesis of phyto-S1P or S1P is mediated by sphingosine kinases (SPHKs). Although phyto-S1P and S1P are known to be signaling components in higher plants, their ability to induce stomatal closure has been ambiguous. We evaluated in detail the effects of phyto-S1P, S1P and SPHK inhibitors on signaling events leading to stomatal closure in the epidermis of Pisum sativum. Phyto-S1P or S1P induced stomatal closure, along with a marked rise in nitric oxide (NO) and cytoplasmic pH of guard cells, as in case of ABA. Two SPHK inhibitors, DL-threo dihydrosphingosine and N',N'-dimethylsphingosine, restricted ABA-induced stomatal closure and prevented the increase of NO or pH by ABA. Modulators of NO or pH impaired both stomatal closure and increase in NO or pH by phyto-S1P/S1P. The stomatal closure by phyto-S1P/S1P was mediated by phospholipase D and phosphatidic acid (PA). When present, PA elevated the levels of pH, but not NO of guard cells. Our results demonstrate that stomatal closure induced by phyto-S1P and S1P depends on rise in pH as well as NO of guard cells. A scheme of signaling events initiated by phyto-S1P/S1P, and converging to cause stomatal closure, is proposed.

Open Field Study of Some Zea mays Hybrids, Lipid Compounds and Fumonisins Accumulation

Lipid molecules are increasingly recognized as signals exchanged by organisms interacting in pathogenic and/or symbiotic ways. Some classes of lipids actively determine the fate of the interactions. Host cuticle/cell wall/membrane components such as sphingolipids and oxylipins may contribute to determining the fate of host-pathogen interactions. In the present field study, we considered the relationship between specific sphingolipids and oxylipins of different hybrids of Zea mays and fumonisin by F. verticillioides, sampling ears at different growth stages from early dough to fully ripe. The amount of total and free fumonisin differed significantly between hybrids and increased significantly with maize ripening. Oxylipins and phytoceramides changed significantly within the hybrids and decreased with kernel maturation, starting from physiological maturity. Although the correlation between fumonisin accumulation and plant lipid profile is certain, the data collected so far cannot define a cause-effect relationship but open up new perspectives. Therefore, the question-"Does fumonisin alter plant lipidome or does plant lipidome modulate fumonisin accumulation?"-is still open.

Long-chain bases, phosphatidic acid, MAPKs, and reactive oxygen species as nodal signal transducers in stress responses in Arabidopsis

Due to their sessile condition, plants have developed sensitive, fast, and effective ways to contend with environmental changes. These mechanisms operate as informational wires conforming extensive and intricate networks that are connected in several points. The responses are designed as pathways orchestrated by molecules that are transducers of protein and non-protein nature. Their chemical nature imposes selective features such as specificity, formation rate, and generation site to the informational routes. Enzymes such as mitogen-activated protein kinases and non-protein, smaller molecules, such as long-chain bases, phosphatidic acid, and reactive oxygen species are recurrent transducers in the pleiotropic responses to biotic and abiotic stresses in plants. In this review, we considered these four components as nodal points of converging signaling pathways that start from very diverse stimuli and evoke very different responses. These pleiotropic effects may be explained by the potentiality that every one of these four mediators can be expressed from different sources, cellular location, temporality, or magnitude. Here, we review recent advances in our understanding of the interplay of these four specific signaling components in Arabidopsis cells, with an emphasis on drought, cold and pathogen stresses.

Deciphering the link between salicylic acid signaling and sphingolipid metabolism

The field of plant sphingolipid biology has evolved in recent years. Sphingolipids are abundant in cell membranes, and genetic analyses revealed essential roles for these lipids in plant growth, development, and responses to abiotic and biotic stress. Salicylic acid (SA) is a key signaling molecule that is required for induction of defense-related genes and rapid and localized cell death at the site of pathogen infection (hypersensitive response) during incompatible host-pathogen interactions. Conceivably, while levels of SA rapidly increase upon pathogen infection for defense activation, they must be tightly regulated during plant growth and development in the absence of pathogens. Genetic and biochemical evidence suggest that the sphingolipid intermediates, long-chain sphingoid bases, and ceramides, play a role in regulating SA accumulation in plant cells. However, how signals generated from the perturbation of these key sphingolipid intermediates are transduced into the activation of the SA pathway has long remained to be an interesting open question. At least four types of molecules - MAP kinase 6, reactive oxygen species, free calcium, and nitric oxide - could constitute a mechanistic link between sphingolipid metabolism and SA accumulation and signaling.

Nitric oxide in guard cells as an important secondary messenger during stomatal closure

The modulation of guard cell function is the basis of stomatal closure, essential for optimizing water use and CO2 uptake by leaves. Nitric oxide (NO) in guard cells plays a very important role as a secondary messenger during stomatal closure induced by effectors, including hormones. For example, exposure to abscisic acid (ABA) triggers a marked increase in NO of guard cells, well before stomatal closure. In guard cells of multiple species, like Arabidopsis, Vicia and pea, exposure to ABA or methyl jasmonate or even microbial elicitors (e.g., chitosan) induces production of NO as well as reactive oxygen species (ROS). The role of NO in stomatal closure has been confirmed by using NO donors (e.g., SNP) and NO scavengers (like cPTIO) and inhibitors of NOS (L-NAME) or NR (tungstate). Two enzymes: a L-NAME-sensitive, nitric oxide synthase (NOS)-like enzyme and a tungstate-sensitive nitrate reductase (NR), can mediate ABA-induced NO rise in guard cells. However, the existence of true NOS in plant tissues and its role in guard cell NO-production are still a matter of intense debate. Guard cell signal transduction leading to stomatal closure involves the participation of several components, besides NO, such as cytosolic pH, ROS, free Ca(2+), and phospholipids. Use of fluorescent dyes has revealed that the rise in NO of guard cells occurs after the increase in cytoplasmic pH and ROS. The rise in NO causes an elevation in cytosolic free Ca(2+) and promotes the efflux of cations as well as anions from guard cells. Stomatal guard cells have become a model system to study the signaling cascade mechanisms in plants, particularly with NO as a dominant component. The interrelationships and interactions of NO with cytosolic pH, ROS, and free Ca(2+) are quite complex and need further detailed examination. While assessing critically the available literature, the present review projects possible areas of further work related to NO-action in stomatal guard cells.