Studies of vegetative compatibility-incompatibility in higher plants - III. The involvement of acid phosphatase in the lethal cellular senescence associated with an incompatible heterograft

In order to elucidate the events that lead to cellular autolysis, and thus better understand the mechanism of cellular incompatibility between Sedum telephoides and Solanum pennellii stems, we have followed the appearance and fate of the hydrolytic enzyme acid phosphatase in both the compatible Sedum autograft and the incompatible Sedum/Solanum heterograft. Acid phosphatase was localized by a modified Gomori-type reaction. Following an initial association with the endoplasmic reticulum and dictyosomes by 6-10 hours after grafting, acid phosphatase activity in the compatible Sedum autograft was associated primarily with the plasmalemma, tonoplast, and vacuole. This strict compartmentation in membranes or organelles and absence of enzyme from the cytosol was maintained throughout the development of the compatible autograft in Sedum. Although acid phosphatase activity in the incompatible heterograft between Sedum and Solanum was initially similar to the compatible autograft in Sedum, a marked difference in enzyme localization occurred in the two graft partners over time. Solanum cells accumulated increased amounts of acid phosphatase, but the enzyme remained sequestered in the plasmalemma, tonoplast, and vacuole. In comparable Sedum cells, however, there was a dramatic increase in acid phosphatase activity in the cytosol, often without any prior compartmentation within the vacuole. This high activity of acid phosphatase in the Sedum cytosol was correlated with cellular autolysis, death, and eventual cell collapse to form the characteristic necrotic layer that insulates the stock from the scion. These results suggest that the lethal cellular senescence associated with Sedum cells of the incompatible heterograft is correlated with a cytoplasmic release of acid phosphatase. A similar release of the enzyme does not occur in the Solanum stock or in the compatible Sedum autograft. Thus, while acid phosphatase synthesis and/or activation is induced in both the compatible and incompatible grafts, incompatibility between Sedum and Solanum involves a failure of Sedum cells to isolate hydrolytic enzymes from the cytosol, which subsequently leads to cellular necrosis.