A genomics approach to understanding the role of auxin in apple (Malus x domestica)fruit size control

Abstract

BackgroundAuxin is an important phytohormone for fleshy fruit development, having been shown to be involved in the initial signal for fertilisation, fruit size through the control of cell division and cell expansion, and ripening related events. There is considerable knowledge of auxin-related genes, mostly from work in model species. With the apple genome now available, it is possible to carry out genomics studies on auxin-related genes to identify genes that may play roles in specific stages of apple fruit development.ResultsHigh amounts of auxin in the seed compared with the fruit cortex were observed in 'Royal Gala' apples, with amounts increasing through fruit development. Injection of exogenous auxin into developing apples at the start of cell expansion caused an increase in cell size. An expression analysis screen of auxin-related genes involved in auxin reception, homeostasis, and transcriptional regulation showed complex patterns of expression in each class of gene. Two mapping populations were phenotyped for fruit size over multiple seasons, and multiple quantitative trait loci (QTLs) were observed. One QTL mapped to a region containing an Auxin Response Factor (ARF106). This gene is expressed during cell division and cell expansion stages, consistent with a potential role in the control of fruit size.ConclusionsThe application of exogenous auxin to apples increased cell expansion, suggesting that endogenous auxin concentrations are at least one of the limiting factors controlling fruit size. The expression analysis of ARF106 linked to a strong QTL for fruit weight suggests that the auxin signal regulating fruit size could partially be modulated through the function of this gene. One class of gene (GH3) removes free auxin by conjugation to amino acids. The lower expression of these GH3 genes during rapid fruit expansion is consistent with the apple maximising auxin concentrations at this point.

Transcript Profiles of Auxin Efflux Carrier and IAA-Amido Synthetase Genes Suggest the Role of Auxin on Apple (<i>Malus</i> × <i>domestica</i>) Fruit Maturation Patterns

“…The role of auxin in cell division and cell enlargement during fruit development has been well elucidated with auxin treatment studies [13] [33]. Specific distribution of auxin in fruit tissues by the function of auxin transporters has been shown to regulate fruit development and ripening [34]- [39].…”

“…Name of the genes were followed [13] [37]. Based on the corresponding amino acid sequence for each gene, protein domain structure was predicted with SMART and Pfam, and the potential subcellular location was predicted with PSORT and TargetP (www.expasy.org).…”

“…However, ethylene itself is most likely just one node in the hormone regulation network. Recent transcriptomic and genomic studies indicated the roles of auxin and its potential interactions with ethylene during apple fruit maturation and ripening [13] [14], but the details of the interactions are unknown.…”

SlARF4, an Auxin Response Factor Involved in the Control of Sugar Metabolism during Tomato Fruit Development

“…Auxin is, however, an important phytohormone for initiation of fleshy fruit development, since it was shown to play a key role in triggering fruit set upon flower fertilization (Pandolfini et al, 2002;Wang et al, 2005;de Jong et al, 2009ade Jong et al, , 2009b. Auxin is also essential in determining final fruit size through the control of cell division and cell expansion (Devoghalaere et al, 2012). In support of the potential role of auxin in fruit development is the finding that the highest auxin concentrations in different parts of the plant were found in developing fruit (Müller et al, 2002).…”

A standard nomenclature for gene designation in the Rosaceae

“…Some effort has been made to give specific names to two genes derived from the whole genome duplication in apple by using related numerical suffixes. For example, two apple genes were named ARF1 and ARF101 following the strawberry homoeologous gene ARF1 (Devoghalaere et al 2012). This solution was chosen at the time as a few studies had shown homoeologous genes maintain a degree of conservation in function, for example MYB10 (Espley et al 2007) and MYB110 (Chagné et al 2013) both control the anthocyanin accumulation.…”