Plant cell-surface GIPC sphingolipids sense salt to trigger Ca2+ influx

Headnote

Salinity is detrimental to plant growth, crop production and food security worldwide. Excess salt triggers increases in cytosolic Ca²⁺ concentration, which activate Ca²⁺-binding proteins and upregulate the Na⁺/H⁺ antiporter in order to remove Na⁺. Salt-induced increases in Ca²⁺ have long been thought to be involved in the detection of salt stress, but the molecular components of the sensing machinery remain unknown. Here, using Ca²⁺-imaging-based forward genetic screens, we isolated the Arabidopsis thaliana mutant monocation-induced [Ca²⁺]i increases 1 (moca1), and identified MOCA1 as a glucuronosyltransferase for glycosyl inositol phosphorylceramide (GIPC) sphingolipids in the plasma membrane. MOCA1 is required for salt-induced depolarization of the cell-surface potential, Ca²⁺ spikes and waves, Na⁺/H⁺ antiporter activation, and regulation of growth. Na⁺ binds to GIPCs to gate Ca²⁺ influx channels. This salt-sensing mechanism might imply that plasma-membrane lipids are involved in adaption to various environmental salt levels, and could be used to improve salt resistance in crops.

More than 6% of the world's total land area and about 20% of irrigated land (which produces one-third of the world's food) are increasingly affected by salt buildup1. Excessive salt is detrimental to plant growth and development, and causes agricultural loss and severe deterioration of plant ecosystems1,2. Sodium chloride is the most soluble and widespread salt found in soils. Sodium is not an essential nutrient in plants, and plants have evolved mechanisms to reduce intracellular sodium buildup1,3. In plants, high salinity triggers early short-term responses for perceiving and transducing the stress signal, and subsequent longterm responses for remodelling the transcriptional network to regulate growth and development. Although several molecular components in the early signalling pathway have been identified, plant salt sensors remain unknown3-8.

Salt stress triggers increases in cytosolic free Ca2+ concentration ([Ca2+]i)9,10, and the expulsion of excess intracellular Na+ involves the Ca2+-related salt-overly-sensitive (SOS) pathway3,5. The SOS pathway comprises the Ca2+ sensor SOS3 (a calcineurin B-like protein (also known as CBL4)), the protein kinase SOS2 (also known as CIPK24), and the Na+/H+ antiporter SOS1. Although salt-induced increases in [Ca2+]i are thought to act as a detection mechanism, the molecular components involved in these increases are unknown3-8,11. In animals, sodium is an essential nutrient,...