Rootstock effects on phloem amino acid composition: Role in aphid resistance

Modern tree fruit production systems depend on rootstocks to control important traits of the scion including plant size, yield and biotic and abiotic stress tolerance. In apple and cherry rootstock studies that are being conducted as part of the USDA NC-140 Regional Research Project, rootstocks have been observed to affect aphid resistance of trees. Since aphids obtain their nutrition from the phloem of these plants, we explored the effect rootstocks had on the amino acid composition of phloem exudates from apple and cherry scions with the objective of identifying relationships with aphid resistance. To conduct this study, phloem exudates were collected from leaves excised in 5 mM EDTA from both ‘Honeycrisp’ apple and ‘Skeena’ sweet cherry trees. Apple leaves were sampled from trees grown on 10 different rootstocks from experimental orchards in Nova Scotia, Ontario and British Columbia. Cherry leaves were sampled from trees grown on 3 different rootstocks and 3 training systems from experimental orchards in Nova Scotia and British Columbia. Free amino acid composition of phloem exudates was analyzed using ultra high performance liquid chromatography (UHPLC) with fluorescence detection following derivatization with o-phthalaldehyde-ethanethiol-9-fluorenylmethyl chloroformate (OPA-ET-FMOC). Of the 20 amino acids analyzed, the relative abundance of 13 and 15 amino acids differed significantly among rootstocks for apple and cherry, respectively. Cherry training systems significantly affected the relative abundance of 6 amino acids. For apple, the top 6 amino acids accounted for 86% of the free amino acids and included proline(Pro), lysine (Lys), glutamate (Gln), aspartate (Asp), alanine (Ala) and valine (Val). For cherry, the top 6 amino acids accounted for 79% and included Gln, Pro, glutamine (Gln), Ala, Asp and serine (Ser). Principle component analysis (PCA) was conducted to identify differences amino acid profiles among rootstocks. The rootstocks B71-7-22 and PiAu9-90in apple and Gisela 3 in cherry were associated with higher concentrations of Pro in the phloem then other rootstocks. These apple rootstocks were also observed to have greater resistance to aphid infestation. The impact of rootstocks on phloem amino acids on and their possible impact on aphid resistance will be discussed.