On how auxin, ethylene and IDA-peptide relate during mature Citrus fruit abscission

Abstract

While the ethylene-auxin interactions are well documented in model dicots such as tomato (climacteric fruit) during mature fruit abscission, the process is not clearly understood in citrus (non-climacteric fruit). The mature fruit produces very little ethylene but is sensitive to ethylene treatments to induce abscission. By contrast, auxin treatments delay fruit abscission, but the particular role of auxin in the process is unknown. Since the IDA-HAE/HSL2 ethylene-independent pathway seem to regulate organ abscission in both model and crop species, we proposed that auxin treatment delays citrus fruit abscission by reactivating the basipetal auxin flux and reducing CitIDA3 expression, without modifying ethylene synthesis.Comparing orange (C. sinensis) genotypes which differ in their abscission rate, ‘Navelate’ vs. ‘Valencia Late’, we found that the force needed to detach the fruit from the tree (FDF) declines in parallel with 1) an increase of ethylene synthesis and CitIDA3 gene expression, and 2) a reduction of PIN1-like (auxin transporter) gene expresion. Further, auxin (2,4-D) treatment maintains a higher force in the abscission zone upregulating PIN1-like and AUX1-like (auxin transporter) gene expression, and downregulating CitIDA3 gene expression, but without modifying ethylene production. We conclude that the 2,4-D treatment delays citrus mature fruit abscission through an ethylene-independent pathway.