Nonclimacteric ‘Jen-Ju Bar’ guava ripening behavior is caused by Copia LTR retrotransposon insertion in the promoter region of PgACS1, a System-2 ACC synthase gene

Abstract

‘Jen-Ju Bar’ (‘JJB’) guava (Psidium guajava L.), a nonclimacteric guava variety featuring delayed peel coloration and pulp softening, exhibits very low ethylene production during the fruit ripening stage and long storability due to the barely detectable activity of 1-aminocyclopropane-1-carboxylate synthase (ACS) and 1-aminocyclopropane-1-carboxylic acid (ACC) content in the pulp. PgACS1, a System-2 ACS gene, has been demonstrated to be a major, if not the only, responsive ACS associated with massive ethylene production during ripening in climacteric guava, such as ‘Li-Tzy Bar’ (‘LTB’). After alignment of the nucleotide sequences of PgACS1 genomic clones between ‘JJB’ and ‘LTB’, the promoter region of ‘JJB’ PgACS1 was found to be interrupted by a retrotransposon. We cloned and characterized the full-length retrotransposon in the promoter region of ‘JJB’ PgACS1. Nucleic acid sequence analysis indicated that the 5487 bp insertion was a Copia long terminal repeat (LTR) retrotransposon, which is oriented in the antisense transcriptional direction of PgACS1 at − 6 upstream of the putative transcription start site (TSS). These results imply that retrotransposon insertion in the promoter region of PgACS1 renders a massive frameshift of the ethylene response element (ERE) cis-element; therefore, the gene was almost silent at the ripening stage. Consequently, ‘JJB’ behaved in a nonclimacteric ripening manner and is feasible for distant marketing. Specific PCR primers targeting the PgACS1 promoter were designed and successfully discriminated the ripening behavior among ten guava cultivars. The primers were suggested for application as molecular markers to assist in early ripening-behavior screening among progenies of a guava breeding project.