Ne. Strobel et al., INDUCTION OF SYSTEMIC ACQUIRED-RESISTANCE IN CUCUMBER BY PSEUDOMONAS-SYRINGAE PV SYRINGAE-61 HRPZ(PSS) PROTEIN, Plant journal, 9(4), 1996, pp. 431-439
Systemic acquired resistance (SAR) is an inducible plant defense respo
nse and is effective against a broad spectrum of pathogens. Biological
induction of SAR usually follows plant cell death resulting from the
plant hypersensitive response (HR) elicited by an avirulent pathogen o
r from disease necrosis caused by a virulent pathogen. The elicitation
of the HR and disease necroses by pathogenic bacteria is controlled b
y hrp genes. Previously, it was shown that the Pseudomonas syringae 61
(Pss61) HrpZ(Pss) protein (formally harpin(Pss)) elicited the HR in p
lants. In this study, it is shown that HrpZ(Pss) induced SAR in cucumb
er to diverse pathogens, including the anthracnose fungus (Colletotric
hum lagenarium), tobacco necrosis virus and the bacterial angular leaf
spot bacterium (P. s. pv. lachrymans). A hrpH mutant of Pss61, which
is defective in the secretion of HrpZ(Pss) and, possibly, other protei
n elicitors, failed to elicit SAR. Pathogenesis-related (PR) proteins,
including peroxidase, beta-glucanase and chitinases, were induced in
cucumber plants inoculated with Pss61, C. lagenarium or HrpZ(Pss). The
induction patterns of PR proteins by HrpZ(Pss) and Pss61 were the sam
e, but were different from that induced by C. lagenarium. Interestingl
y, the hrpH mutant induced two of the three identified PR proteins, de
spite its failure to induce SAR. These results suggest that proteinace
ous elicitors, such as HrpZ(Pss) that traverse the bacterial Hrp secre
tion pathway are involved in the biological induction of SAR and that
at least some PR proteins can be induced by bacterial factors that are
not controlled by hrp genes.