Searching for Early Fruit Abscission-Related Genes in Self-Abscising Apple (Malus x domestica)

Abstract

Based on the fruit abscission pattern from full bloom to 30 days after full bloom, 48 apple genetic resources were classified into three groups: 1) a non-abscising group, in which no fruit is abscised

2) a June drop group, in which the abscission process occurs non-selectively in clusters

and 3) a self-abscising group, in which only central or last fruit in a cluster remains to grow, and the others are abscised. Most cultivars in the self-abscising group showed a fruit abscission pattern in which central fruit dominated over lateral fruits in a cluster, inducing their growth inhibition and final abscission. Exceptionally, in Malus coronaria Charlottae, central fruit could not dominate lateral fruits, but was abscised first in a cluster. The date of full bloom and fruit weight were significantly related to fruit abscission. The earlier the date of full bloom and the smaller the fruit in the abscission groups were, the less early fruit abscission occurred. To search the MADS-box gene which regulates abscission zone development in pedicels, like JOINTLESS in tomato, lateral pedicels (LP) were investigated in self-abscising apple Saika during early abscission. A novel MADS-box gene, MdJOINTLESS, was identified in young pedicels through the amplification using degenerate primers annealing to a highly conserved domain. Through DNA sequence comparison with the MADS-box gene family, MdJOINTLESS was classified into the SHORT VEGETATIVE PHASE (SVP) clade composed of SVP (Arabidopsis thaliana), IbMADS3 (Ipomoea batatas), StMADS16 (Solanum tuberosum), PsSVP (Pisum sativum), and JOINTLESS (Solanum lycopersicum) which share high sequence similarity. However, MdJOINTLESS did not function as JOINTLESS in transgenic tomato, although these MADS-box homologues have high homology. The other MADS-box genes analogous to SEPALLATA and MACROCALYX might be involved in the formation of AZ in apple as SLMBP21 and MACROCALYX genes in tomato. A comparative analysis was conducted between central pedicels (CP) and LP in the self-abscising apple Saika to identify the genes that trigger the abscission mechanism, as the destinies of the pedicels in a cluster are obvious. Transcriptome analysis was performed using RNA-Seq to compare expression profiles between surviving CP and abscised LP. A total of 797,647 expressed sequence tags were assembled into 65,876 contigs, which were annotated and analyzed through gene ontology analysis. A total of 1,585 differentially expressed genes (DEGs) were identified in CP and LP. Analyses on DEGs and qRT-PCR suggested that IAA3/SHY2 was up-regulated in CP and IAA14/SLR was differentially expressed in LP. IAA3/SHY2 may participate in meristem development which is mediated through auxin signal transduction and IAA14/SLR is at least partly involved in the abscission mechanism of LP. The AUX/IAA genes may be related to the roles of determining the fate of pedicels in early abscission.