**5. Stress tolerant transgenic potato over-expressing modified carbohydrates as signalling factors**

Abiotic stresses, such as drought, salinity, extreme temperatures, chemical toxicity and oxidative stresses are serious threats to agriculture besides their deteriorative impact to the environment. Drought and salinity are the most important environmental stress factors that limit food production worldwide, and may cause a serious salinisation on more than 50% of all arable lands by the year 2050 [50]. In China, almost half of the land is arid or semi-arid and the crop production is strongly affected by drought and salinity seasonally even if in the irrigated farm land. Trehalose is a non-reducing disaccharide of glucose. A plant that produces trehalose is often highly tolerant to desiccation stress. Genetic engineering of potato has done for trehalose biosynthesis in potato by introducing the *otsA* and *otsB* genes from *E. coli*, which encode trehalose-6-phosphate synthase and trehalose-6-phosphate phosphatase, respectively [51]. The plants also report to contain sucrose-non-fermenting-1- (SNF1) linked protein, analogous to that of the protein kinase (SNF-) yeast-signalling pathway. The role of SNF1related protein in sugar signalling has been analysed experimentally by developing the transgenic potato expressing an antisense SNF1-related protein kinase. Analysis of these plants revealed that the SNF1-linked protein plays a role in transduction of the sugar signal, triggering the initiation of sucrose synthase in potato leaves [52]. It has been well established that the plants share with few yeast elements involved in sugar sensing, however, several aspects of sugar receptors are likely to be unusual to higher plants. It has been reported that the abiotic stress factors may bring forth the synthesis of stress-related hormones particularly ABA and ethylene, which appear to be implicated in sugar-sensing mechanisms [53, 54]. The level of complexity in sugar signalling and protein involved depicted that despite the successful explanation of some sugar-sensing signalling mechanisms in recent years, added efforts are required to achieve a complete picture of sugar sensing in plants and thus, increase our knowledge of the mechanisms for plant abiotic stress tolerance and adaptation.
