Impact of grafting on cold responsive gene expression in Satsuma mandarin (Citrus unshiu) Ying Huang • Ying Si • Fenny Dane Received: 20 August 2009 / Accepted: 20 August 2010 / Published online: 4 September 2010 Ó Springer Science+Business Media B.V. 2010 Abstract The major limitation preventing the expansion of Satsuma mandarin (Citrus unshiu) pro- duction in the Gulf Coast regions of the US is the risk of freeze damage. Poncirus trifoliata (L.) Raf has been widely used as a cold hardy rootstock for citrus production in freeze-prone environments. In this study, relative quantitative real-time RT–PCR was used to study cold responsive genes in Satsuma mandarin (C. unshiu), C. unshiu grafted on P. trifoliata rootstock and P. trifoliata following exposure to cold shock or cold acclimation treatments. Major changes in gene expression occurred in the different species during cold stress especially with respect to nitrate trans- porter, auxin responsive GH3, UDP-glycosyltransfer- ase (UGT) and chalcone-flavanone isomerase. Very high levels of nitrate transporter were reached early in Poncirus following 60 h of cold shock treatment at 4°C, and in C. unshiu after 3 weeks of gradual cold acclimation. Also, high levels of auxin responsive GH3 were reached in Poncirus following both cold shock and cold acclimation, while levels in C. unshiu were low or reduced. Poncirus rootstock significantly affected gene expression in C. unshiu scion, resulting in a reduction of nitrate transporter, UGT and trehalose 6-phosphate synthase expression levels, and an increase in auxin responsive GH3 gene during cold acclimation. The impact of rootstock on citrus scion during cold stress might be associated with partitioning of nutrients and hormones. Keywords Cold stress Á Poncirus trifoliata Á Citrus unshiu Á Grafting Á Quantitative real-time RT–PCR Á Gene expression Abbreviations CBF C-repeat binding factor CHI Chalcone–flavanone isomerase COR Cold responsive DREB Dehydration-responsive element-binding factor IAA Indole acetic acid MAPK Mitogen activated protein kinase RT–PCR Reverse transcriptase polymerase chain reaction RQ Relative quantitative TPS Trehalose 6-phosphate synthase UGT UDP-glycosyltransferase Introduction Commercial citrus production in the southeastern US is severely limited by low temperature conditions. Economic losses to citrus crop commodities from cold stress in the last 10 years can be conservatively Y. Huang Á Y. Si Á F. Dane (&) Department of Horticulture, Auburn University, Auburn, AL 36849, USA e-mail: danefen@auburn.edu 123 Euphytica (2011) 177:25–32 DOI 10.1007/s10681-010-0243-7