Isolation of host plant induced cDNAs from Nostoc sp strain PCC 9229 forming symbiosis with the angiosperm Gunnera spp.

The identification of three genes differentially expressed in a symbiotical ly competent cyanobacterium during an early stage of its infection of the a ngiosperm Gunnera is reported. The symbiotic isolate Nostoc sp. PCC 9229 wa s treated for 6 and 16 hours with the carbohydrate rich mucilage excreted b y Gunnera stem glands, the infection organ known to possess multiple induct ion potential. Subtractive unique cDNA libraries were constructed. The cyan obacterial clones, identified after screening the libraries were tentativel y termed hieA, hieB and hieC. The clones were specifically expressed in res ponse to the secreted Gunnera gland mucilage as shown by RT-PCR. Analysis o f the deduced amino acid sequences suggest that hieA encodes a putative pre cursor of a pheromone-like signalling peptide; hieB encodes an outer membra ne or secreted glycoprotein, possibly involved in cell-identification, and the product of hieC is probably involved in adaptation to the acidic enviro nment offered by the mucilage. The nucleotide sequences are deposited in Ge nBank (accession numbers AF291436, AF293349, AF292396). The potential role of these clones in symbiosis is discussed.