Nonclimacteric Ripening Characteristics of ‘Jen-Ju Bar’ Guava Conferred by a Defect in the Expression of the System-2 ACC Synthase Gene PgACS1

Nonclimacteric varieties of guava (Psidium guajava L.), such as ‘Jen-Ju Bar’ (‘JJB’), have been cultivated in tropical areas due to their crispy texture and long storability. However, the molecular mechanism resulting in different ripening behaviors among guava varieties is still obscure. To elucidate the underlying mechanism, four cDNA clones of two ACC synthases (ACS) and two ACC oxidases (ACO), the key enzymes of ethylene biosynthesis, were isolated based on the conserved nucleotide sequences. Their enzymatic activities were confirmed by heterologous expression in Escherichia coli. The expression levels of PgACS1, PgACO1, and PgACO2 increased dramatically with the advance of ripening in the climacteric cultivar ‘Li-Tzy Bar’ (‘LTB’), whereas the transcripts of these genes, especially PgACS1 and PgACO2, were barely detected in ‘JJB’ throughout the phase. In addition, PgACS1 and PgACO2 transcription were induced by ethylene and repressed by 1-methylcyclopropene, an ethylene action inhibitor, which exhibited the features of system-2 ethylene production genes. These results suggested that PgACS1 and PgACO2 are two important coordinators for controlling autocatalytic ethylene biosynthesis in guava fruit. The major defect of system-2 ethylene production in ‘JJB’ is caused by the silencing of PgACS1 during ripening. Consequently, ‘JJB’ behaves nonclimacterically and has a long storability.