Study on developmental mechanism of low-temperature (LT) tolerance and expression of LT-induced genes at the whole plant and molecular levels under field and controlled conditions in cereals.

Understanding of mechanisms controlling low-temperature (LT) tolerance would facilitate the cereals productivity and cultivar improvement o in cold areas. The objectives of this study were :1) to determine the relationship between vegetative/reproductive transition which is regulated by vernalization and photoperiod requirements and expression of LT tolerance under field conditions ,and 2) to apply a proteomic approach to study developmental mechanism of LT tolerance and molecular responses of wheat cultivars to LT stress under controlled conditions. Three barley (Rihane-03, Dobrinya and Dicktoo) and four bread wheat cultivars (Kohdasht, Azar2, Sardari and Norstar) with different vernalization response and photoperiod sensitivity were planted in the field at the Maragheh Dryland Agricultural Research Station (37o15&rsquoN, 46o15&rsquoE1720m) in Iran in the autumns of 2002 and 2003. LT tolerance, as measured by LT50, and stage of phenological development, as estimated from final leaf number and shoot apex developmental morphology, were determined during the autumn and winter seasons. Also, a proteomic approach was applied to study molecular responses of Norstar and Azar 2 at various acclimation periods (0, 14 ,28, 42 and 56 days) to LT stress. Protein patterns of leaf were studied by two-dimensional electrophoresis. The short day less sensitive spring-habit cultivar &lsquoRihane-03&rsquohad a limited ability to LT acclimate and it reached its maximum level of LT tolerance very quickly. The winter habit cultivar &lsquoDobrinya&rsquoreached its maximum LT tolerance at the point of vernalization saturation. The very short day (SD) sensitive spring habit cultivar &lsquoDicktoo&rsquoentered the double ridge stage at about the same time as the winterhabit Dobrinya. The delay in the phenological development of both Dobrinya and Dicktoo was accompanied by increased expression of LT tolerance confirming that the length of the vegetative phase determines a plant ability to maintain a high level of cold tolerance expression.. &lsquoKohdasht'pring wheat, which does not have a vernalization or day length requirement, quickly entered the reproductive stage and had a limited ability to LT acclimate. In contrast, a significant decrease in FLN associated with LT treatment and delayed double ridge formation under field conditions indicated that &lsquoNorstar&rsquo, &lsquoSardari&rsquo, and &lsquoAzar2&rsquohad vernalization responses. Sardari increased its FLN from 9 to 12 when grown at 16 h compared to 12 h d indicating that it was also day length sensitive. Azar2 had little day-length sensitivity, but it had a higher unvernalized FLN with the result that both Sardari and Azar2 achieved their vegetative/ reproductive transition and maximum FR (~ -15oC) by early December. Norstar had a longer vernalization requirement and reached vernalization saturation and maximum FR (~ -28oC) near the end December. The delay in the phenological development associated with a vernalization requirement, photoperiod sensitivity and/or increased leaf numbers was accompanied by increased expression of LT tolerance confirming that the length of the vegetative phase determines a plants ability to maintain a high level of LT tolerance gene expression. In molecular analysis, out of about 1095 leaf proteins quantitatively analyzed using software, 121 and 35 protein spots showed significant changes in response to freezing in Norsrtar and Azar 2, respectively, and 37 responsive proteins were common in two genotypes. Some of the protein spots showed significant changes in abundance after vernalization saturation. These observations suggest that transition from the vegetative to the reproductive phase which is influenced by developmental genes such as vernalization and photoperiod regulate the duration of expression of LT induced structural genes, while the level of LT tolerance is determined by both the length of time and the degree that the structural genes are up-regulated.