Modulates resistance against pathogens including oomycetes (e.g. Hyaloperonospora parasitica and Phytophthora brassicae) and fungi (e.g. Phytophthora brassicae). Controls the abscisic acid-mediated (ABA) signaling pathways. Regulator of the flowering time in response to stress (e.g. UV-C). Regulates polar lipid content; promotes phosphatidylinositol (PI) and 18:0 but prevents 18:2 and 18:3 polar lipids accumulation. Expressed at very low levels in seedlings and petioles, and at higher levels in leaves. Also present in phloem sap. Low expression after germination followed by an abrupt level increase in 10-days old seedlings. Accumulates in senescent leaves. Rapidly up-regulated after pathogen exposure (e.g. avirulent and virulent Pseudomonas syringae pv. tomato) in a salicylic acid (SA) defense-signaling pathway-dependent manner. Circadian-regulation with accumulation during the light period, peaks of expression at the end of the day, and low levels during the dark period. Up-regulated by UV-C light through a SA-dependent process and in a CONSTANS- (CO) dependent manner. Late flowering and defective in UV-C light acceleration of flowering. Strong reduction of FLOWERING LOCUS T (FT) expression. Hypersensitivity to abscisic acid (ABA) and alterations in polar lipid contents and their corresponding fatty acids; reduced levels of phosphatidylinositol (PI) and of 18:0, but increased levels of 18:2 and 18:3 polar lipids. Increased susceptibility to the hemi-biotrophic oomycete pathogen Phytophthora brassicae but enhanced resistance to the necrotrophic fungal pathogen Botrytis cinerea. Belongs to the CYSTM1 family.