Rootstock-regulated gene expression patterns in apple tree scions

Apple trees (Malus x domestica) do not reproduce true-to-type from seed. Therefore, desirable cultivars are clonally propagated by grafting vegetative material onto rootstocks. Although cloned cultivars are genetically identical, rootstocks influence horticulturally important cultivar traits, including tree size, disease resistance, and abiotic stress tolerance. Here, 'Gala' scions were grafted to seven different rootstocks that produce a range of tree sizes and grown in a greenhouse. Global gene expression patterns in the scions were compared using a DNA microarray representing 55,230 apple transcripts. Each rootstock triggered a distinct, reproducible scion gene expression pattern. Two thousand nine hundred thirty-four scion transcripts were differentially regulated, by a factor of two or greater, by one or more rootstocks. Transcripts from genes predicted to be involved in responses to stress and biotic and abiotic stimuli were disproportionately represented among the rootstock-regulated transcripts. Microarray data analysis based on tree size identified 116 transcripts whose expression levels were correlated with tree size. The correlation of transcript level with tree size was tested for 14 of these transcripts using quantitative polymerase chain reaction in a population of orchard-grown 'Mutsu' cultivar trees grafted onto rootstocks from a breeding population of multiple crosses. Of those tested, transcripts encoding predicted sorbitol dehydrogenase, homeobox-leucine zipper, and hevein-like proteins were confirmed as being expressed at higher levels in larger trees, while a transcript predicted to encode an extensin-like protein was confirmed as being expressed at higher levels in smaller trees. This study illustrates the utility of using rootstock-regulated phenotypes to identify genes potentially associated with horticulturally important traits.