CHANGES IN SYNTHESIS AND LOCALIZATION OF MEMBERS OF THE 70-KDA CLASS OF HEAT-SHOCK PROTEINS ACCOMPANY THE INDUCTION OF EMBRYOGENESIS IN BRASSICA-NAPUS L MICROSPORES

Elevation of the culture temperature to 32 degrees C for approximately 8 h can irreversibly change the developmental fate of isolated Brassi ca napus microspores from pollen development to embryogenesis. This st ress treatment was accompanied by de-novo synthesis of a number of hea t-shock proteins (HSPs) of the 70-kDa class: HSP68 and HSP70. A detail ed biochemical and cytological analysis was performed of the HSP68 and HSP70 isoforms. Eight HSP68 isoforms, one of which was induced three fold by the stress treatment, were detected on two-dimensional immunob lots. Immunocytochemistry revealed a co-distribution of HSP68 with DNA -containing organelles, presumably mitochondria. Six HSP70 isoforms we re detected, one of which was induced six fold under embryogenic cultu re conditions. During normal pollen development, HSP70 was localized i n the nucleoplasm during the S phase of the cell cycle, and predominan tly in the cytoplasm during the remainder. Induction of embryogenic de velopment in late unicellular microspores was accompanied by an intens e anti-HSP70 labeling of the nucleoplasm during an elongated S phase. In early bicellular pollen the nucleus of the vegetative cell, which n ormally does not divide and never expresses HSP70, showed intense labe ling of the nucleoplasm with anti-HSP70 after 8 h of culture under emb ryogenic conditions. These results demonstrate a strong correlation be tween the phase of the cell cycle, the nuclear localization of HSP70 a nd the induction of embryogenesis. As temperature stress alone is resp onsible for the induction of embryogenic development, and causes an al tered pattern of cell division, there might be a direct involvement of HSP70 in this process.