HIGH STEROL ESTER 1 is a key factor in plant sterol homeostasis

Plants strictly regulate the levels of sterol in their cells, as high sterol levels are toxic. However, how plants achieve sterol homeostasis is not fully understood. We isolated an Arabidopsis thaliana mutant that abundantly accumulated sterol esters in structures of about 1 µm in diameter in leaf cells. We designated the mutant high sterol ester 1 (hise1) and called the structures sterol ester bodies. Here, we show that HISE1, the gene product that is altered in this mutant, functions as a key factor in plant sterol homeostasis on the endoplasmic reticulum (ER) and participates in a fail-safe regulatory system comprising two processes. First, HISE1 downregulates the protein levels of the β-hydroxy β-methylglutaryl-CoA reductases HMGR1 and HMGR2, which are rate-limiting enzymes in the sterol synthesis pathway, resulting in suppression of sterol overproduction. Second, if the first process is not successful, excess sterols are converted to sterol esters by phospholipid sterol acyltransferase1 (PSAT1) on ER microdomains and then segregated in SE bodies. Sterols are essential structural components of cell membranes and have functions in many different physiological processes. For example, brassinosteroids are a class of sterol phytohormones that are important in regulating plant growth and development. This study describes the identification of a protein factor that regulates sterol homeostasis in Arabidopsis.