DNA endoreduplication in maize endosperm cells is reduced by high temperature during the mitotic phase

DNA endoreduplication is the replication of nuclear DNA without subsequent mitosis and cell division, and is believed to be important in maize (Zea mays L.) kernel development. DNA endoreduplication has been shown to be negatively affected by high temperature treatments (HTTs) imposed during early kernel development in maize; however, the specific period of endosperm development at which the process is most sensitive to HTT has not been determined. To address this issue, HTTs (4 and 6 d at 35 degreesC) were applied to in vitro-grown maize kernels starting at 4, 6, 8, 10, or 12 d after pollination (DAP). Our approach was to isolate endosperms to determine the effect of elevated temperature on endosperm fresh weight (FW), number of endosperm cells, mitotic index, and DNA endoreduplication. The 4- and 6-d HTTs imposed during the mitotic phase of the endosperm cell cycle (4, 6, and 8 DAP) reduced nuclei number and kernel FW, delayed cell division, decreased average DNA content, and reduced the percentage of nuclei in the 24, 48, and 96 C classes (C is the DNA content of a haploid nucleus in maize). In contrast, delaying the imposition of the 4- or 6-d HTTs until 10 or 12 DAP (during the endoreduplication phase of the endosperm cell cycle) did not affect nuclei number, average DNA content, and DNA endoreduplication compared to the control. Thus, HTTs are most deleterious to DNA endoreduplication, endosperm FW, and nuclei number when applied during the mitotic phase of the endosperm cell cycle. These data further show that 4 to 10 DAP is the period during maize endosperm development that is most sensitive to high temperature, and that prolonged exposure restricts entry of mitotic cells into the endoreduplication phase of the cell cycle.