Desiccation and survival in the recalcitrant seeds of Avicennia marina: DNA replication, DNA repair and protein synthesis

An autoradiographic study was made of leucine and thymidine incorporation i nto the meristematic root primordia and hypocotyl tips of seeds of the reca lcitrant mangrove species, Avicennia marina. The investigations show that a lthough there is a temporary reduction of protein synthesis at shedding, ro ot primordia and surrounding hypocotyl cells of the axis never wholly cease incorporation of [H-3]leucine and regain pre-shedding levels of activity w ithin a day. Precursor studies using methyl-[H-3]thymidine show that, at sh edding, there is a temporary cessation of incorporation into root meristem nuclei that lasts no longer than 48 h and, within a day, pre-shedding level s are regained in the meristem nuclei. Analysis of DNA fragmentation patter ns in root tips at the time of shedding, and their ability to repair radiat ion-induced DNA damage, indicate that DNA repair processes are markedly com promised in these cells if water loss reaches 22%. Protein synthesis and DN A replication are reduced by more than half by a water loss of 18% and 16%, respectively. DNA replication does not fully recover on rehydration after only 8% water loss. DNA fragmentation to nucleosomes indicates a programme of cell death at a water loss of 10%. We suggest that the feature of contin uous protein synthesis activity with only a temporary interruption in activ e cell cycling in A. marina root primordia helps to explain both the rapidi ty in seedling establishment and the extreme vulnerability to desiccation.