Heterogeneous transcription of an indoleacetic acid biosynthetic gene in Erwinia herbicola on plant surfaces

We investigated the spatial pattern of expression of ipdC, a plant inducibl e gene involved in indoleacetic acid biosynthesis in Erwinia herbicola, amo ng individual cells on plants to gain a better understanding of the role of this phenotype in the epiphytic ecology of bacteria and the factors involv ed in the regulation of ipdC, Nonpathogenic E. herbicola strain 299R harbor ing a transcriptional fusion of ipdC to gfp was inoculated onto bean plants , recovered from individual leaves 48 h after inoculation, and subjected to fluorescence in situ hybridization using a 16S rRNA oligonucleotide probe specific to strain 299R. Epifluorescence images captured through a rhodamin e filter were used to distinguish the 5-carboxytetramethylrhodamine-labeled cells of strain 299R from other leaf microflora, Quantification of the gre en fluorescence intensity of individual cells by analysis of digital images revealed that about 65% of the 299R cells recovered from bean leaves had h igher ipdC expression than in culture. Additionally, 10% of the cells exhib ited much higher levels of green fluorescence than the median fluorescence intensity, indicating that they are more heterogeneous with respect to ipdC expression on plants than in culture. Examination of 299R cells in situ on leaf surfaces by confocal laser scanning microscopy after fluorescence in situ hybridization of cells on leaf samples showed that even cells that wer e in close proximity exhibited dramatically different green fluorescence in tensities, and thus, were in a physical or chemical microenvironment that i nduced differential expression of ipdC.