Colonization of wheat (Triticum vulgare L.) by N2‐fixing cyanobacteria: III. The role of a hormogonia‐promoting factor

Abstract

SUMMARYInitial colonization of the roots of wheat plants by Nostoc 2S9B in liquid culture was by motile hormogonia, irrespective of whether co‐culture was started using a cyanobacterial inoculum consisting of hormogonia or of aseriate packages. In the latter case the aseriate packages differentiated into hormogonia and this was most rapid and extensive when co‐culture was carried out in NO3−‐free BG‐11 medium. Hormogonia‐promoting activity appears to be due to a plant product that can be extracted from roots and is released into the growth medium in the presence or absence of the cyanobacterium. Induction of hormogonia was inhibited by 17 mM NaNO3, the concentration in BG‐11 medium. This effect was apparently not due to the presence of a combined N source (NO3−) per se but appears to be an ionic effect on the functioning of the hormogonia‐promoting factor, since other salts (NaCl and KNO3) had the same effect as NaNO3. The presence in aseriate packages of an autoinhibitor of hormogonia differentiation is inferred from the effects on hormogonia differentiation of washing aseriate packages. The hormogonia‐promoting factor from wheat roots appears to act antagonistically with respect to the autoinhibitor of hormogonia differentiation. Following initial colonization of plants roots by hormogonia, the cyanobacterial filaments progressed through a complete developmental cycle of hormogonia, long filaments and tightly bound aseriate packages, with subsequent differentiation of hormogonia from these aseriate packages giving rise to further colonization of the growing roots. The attachment of Nostoc 2S9B to the root surface may involve an initial attachment of hormogonia via fimbriae, and a subsequent tight binding of the cyanobacterium by means of the mucilaginous sheath.