Phosphatidylinositol 4‐phosphate is associated to extracellular lipoproteic fractions and is detected in tomato apoplastic fluids

Abstract

We have recently detected phosphatidylinositol-4-phosphate (PI4P) in the extracellular medium of tomato cell suspensions. Extracellular PI4P was shown to trigger the activation of defence responses induced by the fungal elicitor xylanase. In this study, by applying a differential centrifugation technique, we found that extracellular PI4P is associated with fractions composed of diverse phospholipids and proteins, which were pelleted from the extracellular medium of tomato cell suspensions grown under basal conditions. Using mass spectrometry, we identified the proteins present in these pelleted fractions. Most of these proteins have previously been characterised as having a role in defence responses. Next, we evaluated whether PI4P could also be detected in an entire plant system. For this, apoplastic fluids of tomato plants grown under basal conditions were analysed using a lipid overlay assay. Interestingly, PI4P could be detected in intercellular fluids obtained from tomato leaflets and xylem sap of tomato plants. By employing electrospray ionisation tandem mass spectrometry (ESI-MS/MS), other phospholipids were also found in intercellular fluids of tomato plants. These had a markedly different profile from the phospholipid pattern identified in entire leaflets. Based on these results, the potential role of extracellular phospholipids in plant intercellular communication is discussed.

From the sap's perspective: The nature of vessel surfaces in angiosperm xylem

“…2E, G), which cleaves phospholipids into their lipophilic and hydrophilic components. Therefore, almost certainly, the lipid layer on vessels at least partially consists of phospholipids, which have been found in xylem sap (Gonorazky et al, 2012;Schenk et al, 2017). Under TEM, this same lipid layer along vessel walls appears broken up into black OsO 4 -stained nanoparticles (Fig.…”

“…4) had few surprises; carbon and oxygen are components of cellulose and potassium, and calcium, chloride, phosphorus, and sulfur are commonly found in xylem sap. The phosphorus peak observed in Distictis, Geijera, and Encelia could be from inorganic phosphate or organic molecules that contain phosphate groups, including phospholipids (Gonorazky et al, 2012;Schenk et al, 2017) or phosphocholine (Martin and Tolbert, 1983). The sulfur peak in Encelia was remarkable for being apparent only in that one species; it could stem from inorganic sulfate or possibly from proteins.…”

Phosphoinositide signaling in plant development

“…It has also been reported that under certain conditions PIs may occur in the luminal/extracellular leaflets of vesicles or the plasma membrane (Gonorazky et al, 2012(Gonorazky et al, , 2008Kale et al, 2010), although it is currently unclear how PIs would manifest in these locations. Within the membrane, PIs act as regulators of integral membrane proteins or mark membrane areas for the recruitment of peripheral membrane proteins.…”

Apoplastic exosome-like vesicles: A new way of protein secretion in plants?

“…Hence, extracellular lipoprotein particles have been detected in tomato apoplastic fluids. 13 Interestingly, they contain several defense-related proteins (including a leaderless aspartyl protease) and also the phospholipid phosphatidylinositol 4-phosphate, previously shown to trigger the activation of defense responses.…”