|Many adverse environmental conditions can affect the productivity and distribution of plant species. Low temperature is one of the most common stresses faced by many plants. Many plants are able to increase their freezing tolerance ability in response to a period of low, non-freezing temperature, a process called cold acclimation. During this process, various physiological and biochemical changes occurred in order to impart cold tolerance capacity to plants. Cell metabolom and proteome changes due to the adjustment at transcription level have been postulated and investigated in the process of cold acclimation in many plants, especially in Arabidopsis. Many genes have been isolated in response to low temperature stress in many herbaceous plants, while very limited information is available for woody plants, including economically important fruit crops, such as citrus.
Citrus species are cold-tender evergreen plants with a tropical and subtropical origin. Citrus unshiu (one kind of Satsuma) is one of the most cold hardy commercial Citrus species which can tolerate temperature as low as –9 to –10 degree C. Poncirus trifoliata, with a maximum freeze tolerance ability of –30 degree C, is a deciduous relative of Citrus that is often used as rootstock to enhance freeze tolerance of the other Citrus species. In order to gain an understanding of the molecular mechanisms of these two species under cold temperature and compare their responses to low temperature, mRNA differential display and cDNA amplified fragment length polymophism (cDNA-AFLP) were used to identify the cold responsive genes under a gradually declined temperature regime. With relative quantitative RT-PCR, genes as follows were identified as differentially expressed in P. trifoliata and C. unshiu: betaine/proline transporter, water channel protein, aldo-keto reductase, early light induced protein, nitrate transporter, tetratricopeptide-repeat protein, F-box protein, ribosomal protein L15, chlorophyll a/b binding protein, photosystem II OEC 23, carbonic anhydrase, tumor related protein, pyrrolidone-carboxylate peptidase, b-galactosidase, translation initiaton factor eIF1, cytochrome C, trigger factor type chaperone family protein, polyprotein, leucine-rich repeat transmembrane protein kinase/receptor-like protein kinase, PAZ/PIWI domain containing protein, 40S ribosomal protein S23, amino acid permease 6, miraculin-like protein 2, 14-3-3 d-2 protein, nucleoside diphosphate kinase III, regulator of chromosome condensation like protein and glutathione S-transferase C-terminal domain containing protein. Osmotic adjustment, photo-oxidative protection and photosynthesis adaptation were suggested to be the main mechanisms for these plants to acclimate to low temperatures. The full length sequences of carbonic anhydrase, proline transporter and nitrate transporter have been obtained. The detailed characterization of these genes are ongoing.