Previous work has indicated that the graft incompatihility between
Sedrmi telephoides and Solanum pennellil involves cell necrosis that results
In a thick layer of collapsed cells at the graft Interface. This necrotic
layer insulates the stock from the scion, which results in abscission of the
Sedum scion after 4-6 weeks due to desiccation and starvation. Thus, cell
autolysis (which is restricted to Sedum) characterizes the Incompatibility
response in this system (1).
In order to elucidate the events that lead to cell autolysis, and thus
better understand the cellular site and mode of action of cellular
incompatibility, the appearance and fate of the hydrolytlc enzyme acid
phosphatase (AP) was followed in both the compatible Sedum autograft and the
incompatible Sedum/Solanum heterograft. Acid phosphatase was localized by a
modified Gomori-type reaction; positive (i.e., including NaF inhibitor) and
negative (lacking substrate) controls showed no enzymatic precipitate.
Following an initial association with the endoplasmic reticulum (ER) and
dictyosomes at 6-10 hours after grafting, AP activity in the compatible
Sedum autograft is associated primarily with the plasmalemma (Fig. 1). By
18-24 hours after grafting, the AP activity is restricted to the tono-plast
and vacuole (Fig. 2). This strict compartmentation and absence of enzyme
from the cytosol is maintained throughout the development of the compatible
graft. While AP activity in the incompatible Sedum/Solanum heterograft is
Initially similar to the compatible Sedum autograft (i.e., initially found
on the ER and dictyosomes), there is a marked difference in enzyme
localization in the two graft partners as the incompatibility response
develops. As in the compatible autograft, Solanum cells at the graft
interface show an Increase in AP activity that Is restricted to the vacuole
and tonoplast, with little or no enzyme activity in the cytosol (Fig. 3). In
comparable Sedum cells, however, there is a dramatic Increase In AP activity
in the cytosol (Fig. h); this cytosollc AP activity is associated with thin
fibril-like structures (Fig. 5) measuring approximately 60 A in diameter.
This high cytoplasmic AP activity In Sedum cells results in cell autolysis,
death, and eventual cell collapse to form the characteristic necrotic layer
separating the two graft partners.
The secreted purple acid phosphatase isozymes AtPAP12 and AtPAP26 play a pivotal role in extracellular phosphate-scavenging by Arabidopsis thaliana