Diazotrophic heterocystous cyanobacteria of the genus Nostoc enter into nitrogen-fixing symbioses with a number of plants. During the initial steps of the infection process, liverwort Blasia pusila and Gunnera spp. induce hormogonia (motile filaments) formation in Nostoc, thus facilitating the infection. In this thesis we used subtractive RNA/DNA hybridisation to study differential gene expression in Nostoc in response to the hormogonia inducing mucilage, secreted by Gunnera symbiotic organs, the stem glands. Another approach employed was two dimensional protein gel electrophoresis of protein extracts from Nostoc, forming hormogonia in response to the fresh medium, Blasia exudates and mucilage from Gunnera. The set of proteins upregulated upon either of treatments was the same, with prolonged time of expression in Gunnera treated cultures. The identified hormogonia induction related genes/proteins in Nostoc may be divided in following groups: Stress related: low pH indueced gene hieC (homologous to lpiA from Sinorhizobium meliloti), a GTP cyclohydrolase I related protein, universal stress protein A (UspA); Proteins earlier assigned a function in nitrogen fixation and heterocyst formation control: NtcA - global nitrogen fixation regulator, undecaprenyl-phosphate galactosephosphotransferase involved inlipopolysaccharide biosynthesis, and a polyketide synthase; Hormogonia specific proteins: structural gas vesicle proteins; Extracellular and outer-membrane: hieB, encoding a protein related to hemolysin type proteins and an S-layer associated multidomain endoglucanase; Low molecular weight proteins with unassigned function: hieA from nostoc sp. PCC 9229, and an unidentified 7 kDa protein from N. punctiforme PCC 73102.

For the first time, we show that a vast range of cyanobacteria of all morphological types, including the majority of symbiotic isolates, is capable of synthesis and release of the plant hormone indole-3-acetic acid (IAA). In several Nostoc strains IAA biosynthesis is tryptophan dependent and is regulated by IAA precursors, anthranilate and tryptophan. An existence of a pathway via indole-3-pyruvic acid in some Nostoc is proposed based on the cyanobacterial genome analyses.

The roles of the novel hormogonia differentiation related host plant-induced genes/proteins and cyanobacterial IAA in establishment of symbioses with plants are discussed.

Manuscripts on which thesis is based
Liaimer A, Matveyev A, Bergman B (2001). Isolation of host plant induced cDNAs from Nostoc sp. strain PCC 9229 forming symbiosis with the angiosperm Gunnera spp. Symbiosis 31:293-307.

Liaimer A, Bergman B (in preparation). Protein expression patterns during hormogonia differentiation in the symbiotically competent cyanobacterium Nostoc punctiforme PCC 73102 elicited by fresh medium and host-plant exudates.

Sergeeva E, Liaimer A, Bergman B (2002). Evidence for production of phytohormone indole-3-acetic acid by cyanobacteria. Planta 215:229-238. Abstract

Liaimer A, Sergeeva E, Bergman B (in preparation). Indole-3-acetic acid in cyanobacateria of the genus Nostoc: Regulation of biosynthesis and influence on growth and cell differentiation.