Microbial expansins act on plant cell walls similarly to plant expansins, albeit their loosening activity levels are tenfold lesser compared to plant expansins. We report the characterization of an expansin-like gene from the plant pathogen Pectobacterium carotovorum, named exl1. PcExl1 is an acidic protein that binds cellulose (Avicel), and weakens filter paper. The acidic nature of PcExl1 confers different binding properties when compared to Bacillus subtilis BsEXLX1, which is a basic protein. PcExl1 binding to wheat cell wall increased when acidic components were depleted, reaching a similar level to the binding to Avicel, indicating that cellulose is the target of PcExl1.
The plant xylem is a preferred niche
for some important bacterial
phytopathogens, some of them encoding expansin proteins, which bind
plant cell walls. Yet, the identity of the substrate for bacterial
expansins within the plant cell wall and the nature of its interaction
with it are poorly known. Here, we determined the localization of
two bacterial expansins with differing isoelectric points (and with
differing binding patterns to cell wall extracts) on plant tissue
through in vitro fluorophore labeling and confocal imaging. Differential
localization was observed, in which Exl1 from Pectobacterium
carotovorum located into the intercellular spaces
between xylem vessels and adjacent cells of the plant xylem, whereas
EXLX1 from Bacillus subtilis bound
cell walls of most cell types. In isolated vascular tissue, however,
both PcExl1 and BsEXLX1 preferentially
bound to tracheary elements over the xylem fibers, even though both
are composed of secondary cell walls. Fluorescence correlation spectroscopy,
employed to analyze the interaction of expansins with isolated xylem,
indicates that binding is governed by more than one factor, which
could include interaction with more than one type of polymer in the
fibers, such as cellulose and hemicellulose or pectin. Binding to
different polysaccharides could explain the observed reduction of
cellulolytic and xylanolytic activities in the presence of expansin,
possibly because of competition for the substrate. Our findings are
relevant for the comprehensive understanding of the pathogenesis by P. carotovorum during xylem invasion, a process in
which Exl1 might be involved.
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