Dwarfing rootstocks are widely used in modern apple production to control the size of apple trees in orchards and to increase overall fruit production. Despite their widespread use, and a century of research, the underlying mechanism of rootstock-induced dwarfing is still largely unknown. One of the best supported hypotheses of the dwarfing effect is altered concentrations and transport of signalling plant hormones such as auxins and cytokinins. To improve our understanding this dwarfing effect we undertook an analysis of the metabolites in stem tissue using a metabolomics work flow.
Grafted apple trees were grown with ‘Royal Gala’ scions and either ‘Royal Gala’ (non-dwarfing) or ‘M9’ (dwarfing) rootstocks. The metabolites in ‘Royal Gala’ and ‘M9’ stem tissues were measured using Liquid Chromatography-Quadrupole-Time of Flight-Mass Spectrometry (LC-Q-TOF-MS).
Metabolites were detected as molecular features (MF) and each MF was tagged with its LC retention time and accurate m/z. The MFs detected in each sample were combined into a bucket table that contained approximately 200 MFs (DataAnalysis and ProfileAnalysis, Bruker Daltonics).
Of the 200 metabolites detected in apple stem tissue, 44 differed in concentration (fold change >2; p-value < 0.05) between ‘Royal Gala’ and ‘M9’. The majority of these metabolites were tentatively identified as flavonoids. Arginine had a 10-fold increase in concentration in ‘M9’ compared with ‘Royal Gala’. Interestingly, different isomers of a metabolite, tentatively identified as phloretin coumarylglucoside, over accumulated in both ‘M9’ and ‘Royal Gala’.
Our results further support the hypothesis that plants over-accumulating flavonoids have reduced auxin transport and dwarfed genotype. The difference in arginine concentrations suggests there is also an effect on primary metabolism in addition to the effects on secondary metabolism involving flavonoid metabolites. This study was conducted with only two apple genotypes, one dwarfing (‘M9’) and one non-dwarfing (‘Royal Gala’), and the results need to be confirmed with additional dwarfing and non-dwarfing genotypes.