SPATIAL AND TEMPORAL ACCUMULATION OF DEFENSE GENE TRANSCRIPTS IN BEAN(PHASEOLUS-VULGARIS) LEAVES IN RELATION TO BACTERIA-INDUCED HYPERSENSITIVE CELL-DEATH

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.