Cyanobionts from the coralloid roots of Cycadales: is there any molecular evidence of cyanotoxins?

The gymnosperms of the order Cycadales form a symbiosis with cyanobacteria in a special zone of the coralloid roots called the cyanobacterial zone (a green ring between the inner and outer root cortex). Free-living cyanobacteria synthesise secondary metabolites, including cyanotoxic compounds. Given that cyanotoxins were synthesised by certain symbiotic cyanobacteria from lichens and from the gymnosperm Macrozamia, this study investigated the presence of genes for four cyanotoxins (microcystin, nodularin, cylindrospermopsin and saxitoxin) in the cyanobionts of Encephalartos and Cycas revoluta collected in three Portuguese botanical gardens. PCR reactions indicated that the cyanobionts did not have the genes to synthesise microcystin, nodularin and cylindrospermopsin. However, for the saxitoxin gene, three isolates (accessions fEnc1, fEH1, cEH1) showed a positive amplification with the pair of primers sxtI683F/sxt877R. The BLAST of the nucleotide sequences did indicate a homology to the sxtI gene that encodes the enzyme O-carbamoyltransferase (OCTASE). Nevertheless, the same samples did not amplify with a second pair of primers for the sxtI gene (sxtI-F2/sxtI-R). Most likely, this result indicates that the OCTASE enzyme is related to the synthesis of other compounds rather than saxitoxin. More research on these cyanobionts should be conducted in the future.