'Jen-Ju Bar' guava exhibited a non-climacteric ripening behavior resulting from a defect in the expression of System-2 ACC synthase PgACS1

The non-climacteric ripening characteristics, including delayed peel coloration and pulp softening, of 'Jen-Ju Bar' ('JJB') guava (Psidium guajava L.) is a result of a reduction in the usually massive amount of ethylene produced during the ripening stage. To elucidate the underlying physiological mechanism, four cDNA clones of ACC synthase (PgACS1 and PgACS2) and ACC oxidase (PgACO1 and PgACO2), the key enzymes in the ethylene biosynthetic pathway, from guava pericarp were isolated and were analyzed for their expression patterns in the fruit. The expressions of PgACS1, PgACO1, and PgACO2 increased with the advance of fruit ripening in 'Li-Tzy Bar' ('LTB'), a climacteric cultivar of guava, at 20°C. On the other hand, all three genes, especially PgACS1, displayed a low and steady pattern of gene expression in 'JJB' fruit over 12-day storage. The fact that exogenous ethylene treatment up-regulated the gene expression as well as 1-methylcyclopropene, an ethylene action inhibitor, suppressed the transcript accumulation in mature-green 'LTB' fruit revealed an autocatalysis or System-2 property of PgACS1 expression. Because not enough ACC is produced by PgACS1 to support massive ethylene synthesis at ripening, 'JJB' fruit behaved in a non-climacteric manner.