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

BACKGROUND

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.

RESULTS

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.

CONCLUSIONS

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.

Cytochemistry and immunolocalisation of polysaccharides and proteoglycans in the endosperm of green Arabica coffee beans

The major noncellulosic polysaccharides and proteoglycans in the coffee bean (Coffea arabica) cell wall are (galacto)mannans and arabinogalactan proteins. Immunological and chemical probes demonstrated that the mannans and arabinogalactan proteins were located continuously across the width of the cell wall, but that the concentration of different structural epitopes within these polysaccharide types showed considerable spatial variation. For the mannans this was implied by the striated pattern demonstrated by fluctuation of the affinity between the mannan monoclonal antibody BGM C6 and (galacto)mannan. The arabinogalactan proteins labelled by the Yariv reagent and the arabinogalactan protein-specific antibody LM2 appeared to be located in all regions of the wall except the middle lamella, but showed some differences in intensity of labelling. However, the LM6 antibody, specific for (1-->5)-alpha-arabinan epitopes, was located only as a compact region adjacent to the cell lumen in the body of the endosperm; though, it did label throughout the wall of epidermal cells. This implied that either some of the more highly arabinosylated arabinogalactan proteins contained contiguous 5-arabinosyl residues or that a rhamnogalacturonan which contained 5-arabinosyl residues as side chains existed in the cell wall. In either case the polymers were very restricted in their distribution. A second category of pectin, a homogalacturonan detected by JIM7, was located only in the middle lamella region. The architecture of the wall, as revealed by resin etching, appeared to reflect the chemical heterogeneity, with three distinct physical zones identifiable in a cross section across a single wall.