Poster: Plant-symbiont interactions

Abs # 598: Nitrogen limitation triggers root development in the Azolla-Anabaena symbiosis - role of the endophyte.

Presenter:	Heimer, Yair M., heimer@bgumail.bgu.ac.il
Authors	Heimer, Yair M. (A)   Kaplan, Drora  (A)   Azeb, Mouhamed  (A)   Akkara,  Yusuf  (A)
Affiliations:	(A): J.Blaustein Institute for Desert Research

The Azolla-Anabaena azollae association is an obligatory symbiotic system between the water floating fern Azolla and the N₂ fixing cyanobacterium Anabaena azollae. Endophyte-free fern cannot be found in nature but can be obtained in the laboratory by curing the fern of the symbiont. The endophyte-free fern is dependent solely on combined nitrogen in the medium yet, it grew much slower and developed a massive root system as compared with the association. We hypothesized that the endophyte-free fern is under chronic nitrogen limitation. When grown under low atmospheric N₂ the association developed many long roots similar to that of the endophyte-free fern. In addition it displayed low Glutamine Synthetase (GS) activity and reduced nitrogen content in the biomass, compared to that of the association grown under ambient conditions. These changes were prevented if combined nitrogen was provided in the medium. In contrary, the endophyte-free fern displayed low GS activity and reduced nitrogen content under all growing conditions in spite the presence of ample combined nitrogen in the growth medium. Western blot analysis of the GS protein revealed two bands corresponding to the GS1 and GS2 isoforms in the association under all growing conditions and the abundance of the GS peptide doublet (GS1 +GS2) was the same. However, only one somewhat diffused band, with a size intermediate between the two GS isoforms of the association, was observed in the endophyte-free fern under all growing conditions. We conclude that: 1) Nitrogen limitation triggers the development of massive root system and 2) The symbiont is essential not only for N₂ fixation but also for the proper expression and processing of GS in the fern and its ability to utilize combined nitrogen regardless of its origin.