ABSTRACT
Phyllosphere (above ground plant structures) is an interesting habitat where many plant associated bacteria, fungi and viruses coexist. Leaves, which form the major part of the phyllosphere serve as food resource for many epiphytic bacteria as well as fungi. The paper by Wichmann et al. (2008) describes a unique interaction of a plant pathogenic bacterium Pseudomonas syringae and fi lamentous fungus Neurospora crassa. It was a serendipitous fi nding. The authors observed a N. crassa hetC homologue in the genome of P. syringae by analysing the genome sequence of several P. syringae strains. In Neurospora, the hetC gene is involved in heterokaryon incompatiblity (HI), and the presence of dissimilar hetC allele is suffi cient to trigger a HI mediated programmed cell death (Debets et al. 1994). There is another locus in N. crassa called pinC, which is also involved in HI. For example, a hetC1pinC2, non allelic combination and/or a hetC1 hetC2 allelic interaction can trigger HI mediated programmed cell death. HI has been proposed as a barrier against non-self recognition which in turn may prevent virus transmission within fungal populations and resource plundering between genetically dissimilar individuals (Debets et al. 1994). Wichmann et al. (2008) showed that the P. syringae phcA (P. syringae het-c homologue) is suffi cient to trigger HI in N. crassa and HI induced by heterologous expression of PhcA in N. crassa is dependent on the presence of a fungal counterpart of hetC and pinC. This results in growth inhibition of N. crassa a characteristic hallmark of HI. They further showed by co-immune precipitation and western blotting that PhcA and HetC1 interact with each other and form a hetero-complex in vivo in N. crassa hyphae undergoing HI. The authors did some elegant experiments in which they incubated P. syringae strains labelled with different fl uorescence colours in close proximity to growing hyphae of N. crassa and observed that the bacterium effi ciently colonizes the growing hyphae. By limiting the carbon and nitrogen source, P. syringae could grow only in the presence of only N. crassa, as a sole carbon source. These results indicated that P. syringae exploits the HI phenomena in fi lamentous fungi, thereby inducing programmed cell death, which may in turn leak out nutrients for the feeding of the bacteria. This study indicates an important ecological relationship, in which leaf associated microbes like bacteria and fungi interact and the bacteria try to manipulate a well conserved genetic pathway of non-self recognition in fungi, to get nutrients for growth. DNA analysis of several P. syringae strains indicated that phcA homologues are present in diverse strains isolated from soil, plant as well as water. The phenomena of HI are conserved in many different divisions of fi lamentous fungi, including ascomycete, basidiomycete and zygomycete species. This research work opens up a yet another very interesting aspect of inter-kingdom interactions which occurs in nature where members try to exploit each other resources by manipulating conserved pathways.