Title : Dissection of heat induced morphological changes of reproductive tissues in Arabidopsis thaliana
Abstract:
eat stress can seriously impact on yield production or quality of crops. Many studies uncovered the molecular mechanisms that regulate heat stress responses in plants. Nevertheless, detailed morphological changes of heat treated plants were still not extensively studied. In this study, we observed the detailed morphological changes of reproductive tissues in Arabidopsis thaliana caused by heat stress. Larger area of stigma, and shorter length of anther, filament and petals were observed in heat treated plants compared to those in controlled conditions. Furthermore, we observed reproductive tissues of plants grown under controlled or heat stressed conditions by scanning electron microscopy (SEM). Although previous studies indicated that pollen is more sensitive to heat stress compared to other tissues, pollen morphology under heat stress condition was almost comparable to that in controlled conditions. In addition, we revealed that pollen attachment to stigma was clearly decreased by heat stress. Furthermore, a length of papilla cells with no pollens seemed to be larger than that with pollens. According to these results, we hypothesized that pollen attachment is involved in size reduction of stigma. To test this hypothesis, we analysed proportion of pollen attachment area on stigma and stigma diameter under controlled and heat stressed conditions. We classified patterns of pollen attachment on stigma into 4 types (type1: pollens on whole stigma, type2: pollens on a half of stigma, type3: pollens only on an edge of stigma, type4: few pollens on stigma). Approximately 60% of stigmas indicates type1 under the controlled conditions. In contrast, approximately 60 % of stigma under heat stressed condition indicates type 4. Furthermore, we compared the diameters of stigma with or without artificial pollination under controlled and heat stressed conditions. We observed enlargement of stigma without pollen, but, size reduction of stigma with pollens under both conditions, suggesting that pollen attachment is clearly involved in stigma size reduction. Finally, we investigated the distribution of O2- and Ca2+, which are involved in pollen-pistil interaction before or after pollination under controlled and heat stressed conditions. Before pollination, O2- accumulated in stigma, and Ca2+ accumulated in anther under both conditions. After pollination, O2- accumulation spread to style and ovary, but, decreased in stigma under controlled conditions. In addition, under heat stressed condition, O2- accumulation was observed in style, but no O2- accumulation was found in ovary. Interestingly, despite the decrease of O2- accumulation in stigma under both conditions after pollination, Ca2+ highly accumulated in stigma after pollination under both conditions. These results indicate that O2- and Ca2+ might be involved in enlargement and reduction of stigma before or after pollination, respectively. Thus, we concluded that heat stress caused the enlargement of stigma that might be associated with decreased pollen attachment to stigma. Furthermore, O2- might be involved in enlargement of stigma before pollination, and Ca2+ might be involved in reduction of stigma size after pollination.
