STRUCTURE OF THE PRIMARY CELL WALL OF HIGHER PLANTS, AND CONTROL OF WALL DEGRADATION BY PATHOGEN SECRETED ENZYMES. Page: 4 of 4

The only enzyme which, by itself, has been found to efficiently attack
native cell walls is an endopolygalacturonase. The endopolygalacturonase
hydrolyzes the homogalacturonan regions of the pectic polymers to tri-, di-,
and monogalacturonic acid. The enzyme also releases galacturonic acid con-
taining fragments rich in rhamnose, galactose and arabinose. Some of these
pectic fragments have small pieces of xyloglucan covalently attached.
The endopolygalacturonase used in these studies is secreted by the plant
pathogen Colletotrichum lindemuthianum; the enzyme has been purified to homo-
geneity. This is the first polysaccharide-degrading enzyme to be secreted
when this fungus is grown in culture on isolated cell walls as the sole car-
bon source. Isolated cell walls which have been treated with the endopoly-
galacturonase have greatly increased susceptibility to subsequent degrada-
tion by cellulolytic and proteolytic enzymes. However, that portion of the
wall released by the action of cellulase originates from the xyloglucan rather
than from cellulose. These results have demonstrated that efficient enzy-
matic hydrolysis of plant cell walls to yield their monomeric constituents
requires the sequential action of a series of glycanases and glycosidases.
A variety of plant tissues, including cultured sycamore cells, possess
proteins capable of inhibiting the endopolygalacturonases secreted by seve-
ral plant pathogens. The inhibitor can be extracted from the plant's cell
wall with buffered 0.5 M salt solutions, and, therefore, these proteins are
not covalently attached to the structural matrix of the cell wall.
The inhibitor of the C. lindemuthianum endopolygalacturonase has been
purified about 600-fold from Red Kidney bean hypocotyl extracts. This puri-
fied inhibitor is 40 times as effective an inhibitor of the C. lindemuthia-
num endopolygalacturonase as of a Fusarium oxysporum polygalacturonase, and
the inhibitor does not demonstrably affect the activity of a Sclerotium rolf-
sii polygalacturonase. A crude Red Kidney bean hypocotyl extract that com-
pletely inhibits these three polygalacturonases does not inhibit C. linde-
muthianum-secreted cellulase, xylanase, a-galactosidase, a-arabinofuranosi-
dase, or exopolygalacturonase. The purified bean hypocotyl protein combines
with the C. lindemuthianum endopolygalacturonase to form a complex with a
dissociation constant of 10-9 or less. Indeed, it appears that it may re-
quire only a single molecule of inhibitor to inactivate completely a single
molecule of endopolygalacturonase.
Thus, plants possess-, in. their cell walls, proteins which are highly
efficient inhibitors of pathogen-secreted endopolygalacturonases, the enzymes
which can initiate degradation of plant cell walls. These inhibitors are
not only efficient but they are highly specific, for the inhibitors distin-
guish between endopolygalacturonases and other degradative enzymes - enzymes
which require the prior action of endopolygalacturonases before they can
attack their cell wall substrates. Moreover, these inhibitors distinguish
between the polygalacturonases secreted by different species of pathogenic
fungi. These results suggest that such inhibitors participate in disease
resistance.
-- This work has been supported by Atomic Energy Commission contract
#AT(ll-1)-1426 and by a University of Colorado Faculty Fellowship.
NOTICE
This report was prepared as an account of work
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Albersheim, P; Anderson, A J; Bauer, W D; Keegstra, K G & Talmadge, K W. STRUCTURE OF THE PRIMARY CELL WALL OF HIGHER PLANTS, AND CONTROL OF WALL DEGRADATION BY PATHOGEN SECRETED ENZYMES., report, October 31, 1972; United States. (https://digital.library.unt.edu/ark:/67531/metadc1035733/m1/4/ocr/: accessed March 26, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.

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