A genomics approach to understanding the role of auxin in apple (Malus x domestica) fruit size control
1 The New Zealand Institute for Plant & Food Research Limited (PFR), Private Bag 92169, Auckland 1142, New Zealand
2 School of Biological Sciences, University of Auckland, Private Bag 92019, Auckland 1142, New Zealand
3 PFR, Private Bag 11600, Palmerston North 4442, New Zealand
4 School of Plant Science, University of Tasmania, GPO Box 252-55, Hobart, Tasmania 7001, Australia
5 PFR, Private Bag 1401, Havelock North 4157, New Zealand
6 PFR, Old Mill Road, RD3, Motueka 7198, New Zealand
7 INRA, UMR AGAP, Equipe Architecture et Fonctionnement des Espèces Fruitières, Avenue Agropolis - TA-A-108/03, 34398 Montpellier Cedex 01, France
BMC Plant Biology 2012, 12:7 doi:10.1186/1471-2229-12-7Published: 13 January 2012
Auxin 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.
High 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.
The 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.