SPATIAL AND TEMPORAL ACCUMULATION OF DEFENSE GENE TRANSCRIPTS IN BEAN(PHASEOLUS-VULGARIS) LEAVES IN RELATION TO BACTERIA-INDUCED HYPERSENSITIVE CELL-DEATH
Bm. Meier et al., SPATIAL AND TEMPORAL ACCUMULATION OF DEFENSE GENE TRANSCRIPTS IN BEAN(PHASEOLUS-VULGARIS) LEAVES IN RELATION TO BACTERIA-INDUCED HYPERSENSITIVE CELL-DEATH, Molecular plant-microbe interactions, 6(4), 1993, pp. 453-466
Primary leaves of 7- to 9-day-old Red Mexican bean plants were inocula
ted with virulent or avirulent isolates of Pseudomonas syringae pv. ph
aseolicola, or saprophytic P. fluorescens either by vacuum infiltratio
n of the whole leaf lamina, or by syringe-inoculation of selected leaf
panels. In the incompatible combination, resistance was associated wi
th a hypersensitive response (HR). Syringe-inoculated leaves were samp
led in three zones: zone 1, the inoculated leaf area; zone 2, the surr
ounding 0.5-0.7 cm of leaf tissue; and zone 3, the remainder of the le
af. Northern blots of RNA from zones 1, 2, and 3 were probed with bean
cDNAs for phenylalanine ammonia-lyase (PAL), chalcone synthase (CHS),
chitinase (CHT), and lipoxygenase (LOX). Accumulation of PAL, CHS, an
d CHT transcripts was more rapid and generally of greater magnitude in
the incompatible than in the compatible interaction and, in both case
s, was observed essentially only in zone 1 tissues. Similarly, antibac
terial phytoalexins were only detected in zone 1 from the incompatible
interaction. Young primary leaves have a background level of LOX tran
scripts, which declines as leaves age. This decline was accelerated ov
er the first 12 hr postinoculation (hpi) with avirulent bacteria, wher
eas a weak transient induction, peaking at 5-6 hpi, was observed in th
e compatible interaction. A subsequent, strong accumulation of LOX tra
nscripts was seen in both the compatible and incompatible interactions
outside the inoculation site starting about 14 hpi. LOX transcripts d
id not accumulate at the inoculation site itself in the incompatible i
nteraction compared to a relatively strong induction in the compatible
interaction. Interestingly, inoculation of leaves with cells of the s
aprophyte P. fluorescens also induced the accumulation of transcripts
for CHS, CHT, and LOX, but generally to a lesser degree than in the in
compatible interaction. No HR occurred and no macroscopic cell damage
was apparent in leaves inoculated with P. fluorescens. However, at the
microscopic level individual, trypan blue-stained, necrotic plant cel
ls were visible. In spite of this and the accumulation of CHS transcri
pts, no phytoalexin accumulation was found up to 48 hr after inoculati
on. The spatial and temporal relationship of the hypersensitive reacti
on to defense gene transcript and phytoalexin accumulation is discusse
d.