Water deficit inhibits cell division and expression of transcripts involved in cell proliferation and endoreduplication in maize endosperm

Water deficit at the early post-pollination stage in cereal grains decreases endosperm cell division and, in turn, decreases the capacity for storage material accumulation. Post-mitotic replication of nuclear DNA (endoreduplication) may also play a role in stress effects. To gain a better understanding of the extent to which cell proliferation and endoreduplication are affected by water deficit, nuclear numbers and size were examined in endosperms of maize (Zea mays L.) by flow cytometry and the transcript levels of genes which have recognized roles in the cell cycle were quantified. Water deficit from 5-13 d after pollination (DAP) decreased the rate of endosperm cell division by 90% and inhibited [H-3]-thymidine incorporation into DNA from 9-13 DAP. The proportion of nuclei engaging in endoreduplication and nuclear DNA content increased steadily from 9-13 DAP in controls, but water deficit initially increased the proportion of endoreduplicating nuclei at 9 DAP, then halted further entry into endoreduplication and S-phase cycling from 9-13 DAP. Transcript levels of alpha -tubulin, and the S-phase gene products histone H3 and PCNA were not affected by water deficit until 13 DAP, whereas those of ZmCdc2, a cyclin dependent kinase (CDK) with regulatory roles in mitosis, were inhibited substantially from 9-13 DAP. Cell proliferation and associated processes were inhibited at initial stages of the stress episode, whereas endoreduplication and associated S-phase processes were not inhibited until the stress was more advanced. It was concluded that endosperm mitosis has greater sensitivity than endoreduplication to water deficit.