Abstract:

【Objective】 Previous study revealed that sieve elements (SEs) in the developing caryopsis of Triticum aestivum L. underwent a unique type of programmed cell death (PCD) . In this paper, the dynamic changes and the roles of Ca2+ and Ca2+-ATPase in SEs during the PCD were studied. 【Method】 The ultrastructural aspects of phloem cells in wheat caryopsis were examined by transmission electron microscopy (TEM). Using specific fluorescence staining and potassium pyroantimonate precipitation method, Ca2+ was localized at histological and sub-cellular levels in SEs in the developing wheat caryopsis. TEM and lead nitrate were used to locate Ca2+-ATPase in SEs. 【Result】 TEM studies showed that the cell walls of SEs thickened at the beginning of differentiation, and then became thinner and smoother. Fluorescence staining showed that the fluorescence due to Ca2+ appeared in cell walls of SEs from 6 to 10 d after flowering. The fluorescence due to Ca2+ in cell walls of SEs was most notable on 9 d after flowering and disappeared on 14 d after flowering. Sub-celluar localization of Ca2+ showed that Ca2+ was localized on plasma membrane and in nuclei from 1 to 2 d after flowering. On 4 d after flowering, Ca2+ was localized in cytoplasm and mitochondria of SEs. From 5 to 8 d after flowering, Ca2+ was transported to the cell walls of SEs and no Ca2+ precipitates were observed in mitochondria. From 10 to 18 d after flowering, Ca2+ was transported into the cytoplasm again from cell walls and no Ca2+ precipitates were observed on 20 d after flowering. In intermediary cells (ICs), Ca2+ precipitates were observed from 1 to 18 d after flowering, and Ca2+ mainly distributed on intine and tonoplast. The activity of Ca2+-ATPase changed obviously during the SEs differentiation. There was lowest activity of Ca2+-ATPase on 3 d after flowering in SEs. High levels of Ca2+-ATPase activity were found from 4 to 14 d after flowering in SEs, and the enzyme was mainly localized in cell walls, cytoplasm, plasmodesmata, mitochondria and nuclei. 【Conclusion】 These results showed the dynamic changes of Ca2+ and Ca2+-ATPase in SEs differentiation. Ca2+ might play important roles in SEs during the PCD. In addition, Ca2+ and Ca2+-ATPase might be also related to cell wall thickening and functions of SEs.

Key words: wheat (Triticum aestivum L.), caryopsis, sieve elements, Ca2+, Ca2+-ATPase