Nostoc hormogonia

N2-fixing cyanobacteria, primarily Nostoc, enter into productive symbioses with a broad range of land plants: angiosperms (Gunnera), gymnosperms (cycads), ferns (Azolla), and bryophytes (the hornwort Anthoceros and the liverwort Blasia). Plant colonization relies on the ability of cyanobacteria to form hormogonia, the transiently motile state of Nostoc. Once inside the plant, motile hormogonia revert back to N2-fixing vegetative filaments. Thus far, very little is known about how plant hosts and cyanobacteria interact in order to establish successful symbioses.

Gunnera gland

        Gunnera plants establish endosymbioses with cyanobacteria through specialized mucilage-secreting glands located on the stem (Fig. 1a). Nostoc filaments, in the form of hormogonia, enter Gunnera glands through cells lining channels within the glands. Cells harboring Nostoc continue to divide and form tissue packed with the cyanobiont (Fig. 1b,c). Established Nostoc stem colonies are able to promote healthy plant growth on N-limited condition (Fig. 1d).  Unlike nodulation in legume plants, Gunnera gland development does not require the presence of the symbiotic partner. Instead, gland development is initiated when nitrogen supply becomes limited (Chiu et al. 2005). The N-deprivation induced gland development in Gunnera provides us with a valuable system to study what is required for a plant to form N2-fixing symbioses with cyanobacteria.

        The overall goal of our lab is to identify physiological characters and genes that are crucial for plants to form successful N2-fixing symbioses with cyanobacteria. Toward this goal, we have characterized a normalized cDNA library from Gunnera glands (without cyanobacteria) using both 454 and Illumina sequencing technologies and have begun to analyze genes that may be crucial for gland development and the establishment of endosymbioses. .

Figure 1. Gunnera-Nostoc symbioses. (a) Mature glands on Gunnera manicata stem. (b).Cross section of a Nostoc-colonized stem of G. manicata. (c) Cyanobacteria colonies at the base of a leaf from Gunnera monoica (d) Growth of G. manicata seedlings with (right) or without (left) symbiotic Nostoc.

ACKNOWLEDGEMENT

          This project is partially supported by a grant from the Jeffress Memorial trust.

REFERENCES

Khamar HJ, Breathwaite EK, Prasse CE, Fraley ER, Secor CR, Chibane FL, Elhai J, Chiu WL.
Multiple roles of soluble sugars in the establishment of Gunnera-Nostoc endosymbiosis.
Plant Physiol 154:1381-1389

Prasse CE, Smith M, Chiu W-L (in preparation).
Transformation and regeneration of Gunnera manicata: an angiosperm system for investigating plant-cyanobacteria symbioses.

Chiu W-L, Peters GA, Levieille G, Still PC, Cousins S, Osborne B, Elhai J (2005).
Nitrogen deprivation stimulates symbiotic gland development in Gunnera manicata.
Plant Physiol 139:224-230