EVIDENCE THAT THE PSEUDOMONAS-SYRINGAE PV SYRINGAE HRP-LINKED HRMA GENE ENCODES AN AVR-LIKE PROTEIN THAT ACTS IN AN HRP-DEPENDENT MANNER WITHIN TOBACCO CELLS
Jr. Alfano et al., EVIDENCE THAT THE PSEUDOMONAS-SYRINGAE PV SYRINGAE HRP-LINKED HRMA GENE ENCODES AN AVR-LIKE PROTEIN THAT ACTS IN AN HRP-DEPENDENT MANNER WITHIN TOBACCO CELLS, Molecular plant-microbe interactions, 10(5), 1997, pp. 580-588
A 25-kb DNA region, previously cloned from Pseudomonas syringae pv. sy
ringae 61 in cosmid pHIR11, enables nonpathogenic bacteria such as Pse
udomonas fluorescens and Escherichia coli to elicit the hypersensitive
response (HR) in tobacco (Nicotiana tabacum). hrmA is located within
this region, adjacent to a conserved cluster of hrp genes, and is esse
ntial for nonpathogens to elicit the HR. DNA sequence analysis suggest
ed that hrmA was the second of two genes in an operon and was preceded
by an open reading frame (ORF), ORF1, which is predicted to encode a
10.9-kDa protein. DNA gel blot analysis revealed that sequences hybrid
izing with a DNA fragment internal To hrmA were absent from P. syringa
e pv. syringae B728a, P. syringae pv. tabaci 11528, and P. syringae pv
. glycinea race 4 U1, but present in P. syringae pv. tomato DC3000. A
2.4-kb BamHI-AvrII fragment carrying hrmA, ORF1, and native regulatory
sequences was subcloned into broad-host-range vector pDSK519 and elec
troporated into P. syringae pv. syringae B728a and P. syringae pv. tab
aci 11528. The presence of the hrmA locus had no apparent effect on th
e ability of P. syringae pv. syringae B728a to cause brown spot of bea
n, but it caused P. syringae pv. tabaci 11528 to elicit the defense-as
sociated HR rather than disease in N. tabacum cvs. Xanthi N and Xanthi
NC and N. clevelandii. Furthermore, N. debeyii, N. glutinosa, N. rust
ica, and N. tabacum cvs. Petit Havana and Samsun responded with the HR
to P. fluorescens (pHIR11). In contrast, N. benthamiana-P. syringae p
v. tabaci interactions were unaffected by the presence of HrmA, and P.
fluorescens (pHIR11) did not elicit the HR in N. benthamiana. The hrm
A ORF was subcloned into pFLAG-CTC, which expressed HrmA with a C-term
inal FLAG synthetic epitope fusion. Escherichia coli MC4100 cells carr
ying the functional hrp cluster and the hrmA-FLAG derivative secreted
the HrpZ harpin, but not HrmA-FLAG, to the medium, as indicated by imm
unoblot analysis with M2 anti-FLAG and polyclonal anti-HrpZ antibodies
. The hrmA ORF was also subcloned into plant expression vector pFF19 a
nd then biolistically delivered, along with pFF19G (expressing beta-gl
ucuronidase), into suspension-cultured tobacco cells, Histochemical st
aining 24 h later revealed substantial beta-glucuronidase activity in
cells receiving pFF19G and pFF19 but not in those receiving pFF19G and
pFF19-HrmA, Thus, internal production of HrmA was deleterious to toba
cco cells.