Arachidonic acid alters tomato HMG expression and fruit growth and induces3-hydroxy-3-methylglutaryl coenzyme A reductase-independent lycopene accumulation

Regulation of isoprenoid end-product synthesis required for normal growth a nd development in plants is not well understood. To investigate the extent to which specific genes for the enzyme 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR) are involved in end-product regulation, we manipulated e xpression of the HMG1 and HMG2 genes in tomato (Lycopersicon esculentum) fr uit using arachidonic acid (AA). In developing young fruit AA blocked fruit growth, inhibited HMG1, and activated HMG2 expression. These results are c onsistent with other reports indicating that HMG1 expression is closely cor related with growth processes requiring phytosterol production. In mature-g reen fruit AA strongly induced the expression of HMG2, PSY1 (the gene for p hytoene synthase), and lycopene accumulation before the normal onset of car otenoid synthesis and ripening. The induction of lycopene synthesis was not blocked by inhibition of HMGR activity using mevinolin, suggesting that cy toplasmic HMGR is not required for carotenoid synthesis. Our results are co nsistent with the function of an alternative plastid isoprenoid pathway (th e Rohmer pathway) that appears to direct the production of carotenoids duri ng tomato fruit ripening.