Physiological sources of reductant for nitrogen fixation activity in Nostoc sp. strain UCD 7801 in symbiotic association with Anthoceros punctatus

Abstract

Pure cultures of the symbiotic cyanobacterium-bryophyte association with Anthoceros punctatus were reconstituted by using Nostoc sp. strain UCD 7801 or its 3-(3,4-dichlorophenol)-1,1-dimethylurea (DCMU)-resistant mutant strain, UCD 218. The cultures were grown under high light intensity with CO2 as the sole carbon source and then incubated in the dark to deplete endogenous reductant pools before measurements of nitrogenase activities (acetylene reduction). High rates of light-dependent acetylene reduction wpre obtained both before starvation in the dark and after recovery from starvation, regardless of which of the two Nostoc strains was reconstituted in the association. Rates of acetylene reduction by symbiotic tissue with the wild-type Nostoc strain decreased 99 and 96% after 28 h of incubation in the dark and after reexposure to light in the presence of 5 ,IM DCMU, respectively. Supplementation of the medium with glucose restored nitrogenase activity in the dark to a rate that was 64% of the illuminated rate. In the light and in the presence of 5 ,uM DCMU, acetylene reduction could be restored to 91% of the uninhibited rate by the exogenous presence of various carbohydrates. The rate of acetylene reduction in the presence of DCMU was 34% of the uninhibited rate of tissue in association with the DCMU-resistant strain UCD 218. This result implies that photosynthates produced immediately by the cyanobacterium can supply at least one-third of the reductant required for nitrogenase activity on a short-term basis in the symbiotic association. However, high steady-state rates of nitrogenase activity by symbiotic Nostoc strains appear to depend on endogenous carbohydrate reserves, which are presumably supplied as photosynthate from both A. punctatus tissue and the Nostoc strain.

Regulation of Cellular Differentiation in Filamentous Cyanobacteria in Free-Living and Plant-Associated Symbiotic Growth States

“…Second, when the Anthoceros-Nostoc association is grown under high light intensity without glucose, the growth rate of the associated tissue and that of N 2 fixation per unit of gametophyte tissue is identical to that of tissue grown under low light intensity in glucose-supplemented medium (177). However, both the number and the biomass of individual high-lightintensity-grown Nostoc colonies are approximately half those of low-light-intensity-grown colonies, and the specific activity of nitrogenase is fivefold higher.…”

Tansley Review No. 116

“…A similar pattern is found from young to mature parts of the lichen thallus (Rowell et al, 1985 ;Hill, 1989 ;Rai, 1990c ;Janson et al, 1993), the coralloid roots of cycads , Gunnera plants ( So$ derba$ ck et al, 1990 ;Bergman et al, 1992a) and the thalli of liverworts\hornworts Meeks, 1990). Slime cavities, coralloid roots or stem glands are produced as a fixed proportion of the host biomass, and the cyanobiont population in them is controlled (Halliday & Pate, 1976 ;Stewart et al, 1983 ;Meeks, 1990 ;So$ derba$ ck et al, 1990 ;Steinberg & Meeks, 1991 ;Osborne et al, 1992 ;Meeks et al, 1999).…”

“…The overall rate of N # fixation per unit host biomass remains constant owing to host control on number and size of colonies. Treatments that decrease colony number increase their individual size (Meeks, 1990 ;Steinberg & Meeks, 1991).…”

Evidence for plant-mediated regulation of nitrogenase expression in theAnthoceros-Nostoc symbiotic association

“…C . Meeks photosynthetic inhibitor 3-(3,4-dichloropheno1)-l,l-dimethylurea (DCMU), provided unequivocal evidence that the Nostoc is capable of complete photosynthesis when in association with Anthoceros, albeit at a low rate (Steinberg & Meeks, 1989), and can use its immediate photosynthate to support nitrogenase activity (Steinberg & Meeks, 1991). We are interested in the chemical interactions between partners of the Anthoceros-Nostoc association that regulate Nostoc gene expression and cellular differentiation.…”