Impact of grafting on cold responsive gene expression in Citrus unshiu

Abstract

Commercial citrus, the most economically important fruit crop in the world, is sensitive to low temperatures. Poncirus trifoliata (L.) Raf., a close relative of Citrus species, has been widely used as a cold hardy rootstock for citrus production in low temperature environments. In this study, the expression profiles of 16 cold responsive genes in leaves of Poncirus trifoliata, Satsuma mandarin (Citrus unshiu), and Citrus unshiu scions grafted on Poncirus trifoliata rootstock were compared following exposure to cold shock or cold acclimation treatments. Using relative quantitative (RQ) real-time RT-PCR analysis, several genes such as UDP-glycosyltransferase (UGT), trehalose-6-phosphate synthase, chalcone isomerase-like protein, proline transporter, plasma membrane intrinsic protein (PIP) and nitrate transporter, involved in osmotic and oxidative stress, were upregulated in response to cold, suggesting an active role for secondary metabolites with respect to osmotic balancing, except for lipoxygenase and phosphomannomutase which were down-regulated in response to low temperature. Species-specific expression profiles were detected especially in response to cold acclimation. Levels of auxin responsive GH3 transcript increased to high levels in Poncirus, but decreased in Satsuma. Levels of UGT did not change in Poncirus, while high increases occurred in Satsuma. Similarly levels of nitrate transporter increased 10 fold in Poncirus, more than 100 fold in Satsuma during cold acclimation. Differences in down regulation of transcript levels were observed for lipoxygenase, with higher levels in Satsuma as compared to Poncirus following cold acclimation. Grafting on Poncirus rootstock did exert a significant effect on gene expression of the Satsuma scion. Expression of genes involved in signal transduction and regulation of gene expression, such as RING finger protein, and auxin-responsive GH3 was positive and increased in grafted as compared to non-grafted Satsuma plants during cold acclimation, while expression of genes involved in nitrate transport, PIP, trehalose 6-P synthase, UGT, were negatively regulated by the rootstock in that expression levels in the grafted Satsuma were reduced as compared to the nongrafted plants during cold acclimation. These results indicate that cessation of growth is more rapidly invoked in cold hardy species than cold sensitive species and that through grafting this response can be activated or induced in the scion. Furthermore, signal transduction between scions and rootstocks leads to significant changes in gene expression during cold stress.