Rescuing ovaries from abortion at low w
Gengenbach (1977) discovered a medium for culturing isolated embryos of maize, and Boyle et al. (1991) fed it to the stems of plants while they were being exposed to abortion-inducing w. The idea was to test whether the medium could keep the embryos alive when photosynthesis was inhibited at low w. Stems were fed an amount containing the dry mass normally produced by photosynthesis because the entire plant was starved for photosynthetic products. This amount was fed each day to a different stem position. The plants were then rewatered and allowed to develop for the rest of the season. At maturity, the low w treatment had caused nearly complete abortion in the unfed plants, but 80% of the ovaries developed in the fed plants. The rescued kernels appeared normal at maturity. If water was fed to the stems without the ingredients of the Gengenbach (1977) medium, no kernels developed.
This rescue indicated that low w was not in itself lethal. Instead, abortion was initiated because the parent did not supply an ingredient in the medium, and the missing ingredient caused the abortion. It should be noted that the feeding kept the kernels alive at low w so that photosynthesis and normal kernel development could resume when water was resupplied. Most of the kernel development occurred after rewatering.
Boyle et al. (1991) and Zinselmeier et al. (1995a) conducted deletion and addition experiments to identify the active component(s). Plant growth regulators (auxin, cytokinin), amino acids, cofactors, and salts had no activity. Sucrose alone was the compound that rescued the ovaries from abortion. A typical result, shown in Fig. 1B, indicates that about 68% of the kernels were rescued by the daily feeding of sucrose to the stems in this particular experiment. The plants were subjected to low w, photosynthesis had been nearly completely inhibited at the low w, and enough sucrose was fed to replace the missing sucrose completely. Photosynthetic activity recovered fully when the parent was rewatered, and kernel development continued. Typically, the feeding resulted in 40–80% rescue of the ovaries that otherwise would have completely aborted.
Because sucrose is the main translocation form for the carbon fixed in photosynthesis, this result indicated that abortion was controlled by sucrose in the sugar stream delivered from the leaves to the ovaries. Whether the change in sucrose delivery itself signals the pedicel and/or ovary to initiate the abortion process has not been resolved. But at the molecular level it is clear that these two tissues respond differently to water deficits. The pedicel tissue was more responsive in terms of gene expression with many stress tolerance proteins up-regulated. Genes associated with cell division and growth, however, apparently were unaffected (Zinselmeier et al., 2002; Yu and Setter, 2003). These studies confirm the importance of sucrose in the sugar stream and suggest a possible triggering effect of changes in sucrose delivery on molecular events in the pedicel or ovaries.