Sclareol and linalyl acetate are produced by glandular trichomes through the MEP pathway

Sclareol,an antifungal specialized metabolite produced by clary sage,Salvia sclarea,is the starting plant natural molecule used for the hemisynthesis of the perfume ingredient ambroxide.Sclareol is mainly produced in clary sage flower calyces;however,the cellular localization of the sclareol biosynthesis remains unknown.To elucidate the site of sclareol biosynthesis,we analyzed its spatial distribution in the clary sage calyx epidermis using laser desorption/ionization mass spectrometry imaging(LDI–FTICR-MSI)and investigated the expression profile of sclareol biosynthesis genes in isolated glandular trichomes(GTs).We showed that sclareol specifically accumulates in GTs’gland cells in which sclareol biosynthesis genes are strongly expressed.We next isolated a glabrous beardless mutant and demonstrate that more than 90%of the sclareol is produced by the large capitate GTs.Feeding experiments,using 1-13 C-glucose,and specific enzyme inhibitors further revealed that the methylerythritol-phosphate(MEP)biosynthetic pathway is the main source of isopentenyl diphosphate(IPP)precursor used for the biosynthesis of sclareol.Our findings demonstrate that sclareol is an MEP-derived diterpene produced by large capitate GTs in clary sage emphasing the role of GTs as biofactories dedicated to the production of specialized metabolites.

The miR166-SIHB15A regulatory module controls ovule development and parthenocarpic fruit set under adverse temperatures in tomato

Fruit set is inhibited by adverse temperatures,with consequences on yield.We isolated a tomato mutant producing fruits under non-permissive hot temperatures and identified the causal gene as SIHB15A,belonging to classⅢhomeodomain leucine-zipper transcription factors.SIHB15A,loss-of-function mu-tants display aberrant ovule development that mimics transcriptional changes occurring in fertilized ovules and leads to parthenocarpic fruit set under optimal and non-permissive temperatures,in field and greenhouse conditions.Under cold growing conditions,SIHB15A is subjected to conditional haploinsufficiency and recessive dosage sensitivity controlled by microRNA 166(miR166).Knockdown of SIHB15A alleles by miR166 leads to a continuum of aberrant ovules correlating with parthenocarpic fruit set.Consistent with this,plants harboring an Slhb15a-miRNA166-resisiant allele developed normal ovules and were unable to set parthenocarpic fruit under cold conditions.DNA affinity purification sequencing and RNA-sequencing analyses revealed that SIHB15A is a bifunctional transcription factor expressed in the ovule integument.SIHB15A binds to the promoters of auxin-related genes to repress auxin signaling and to the promoters of ethylene-related genes to activate their expression.A survey of tomato genetic biodiversity identified pat and pat-1,two historical parthenocarpic mutants,as alleles of SIHB15A.Taken together,our findings demonstrate the role of SIHB15A as a sentinel to prevent fruit set in the absence of fertilization and provide a mean to enhance fruiting under extreme temperatures.