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Numbers don't count when coming through the rye
Fungal endophytes occur frequently in grasses where the relationship between fungus and host is described as symbiotic. As discussed by Spiering et al. (Palmerston North, New Zealand and Wagga Wagga, Australia, pp. 379–387), the fitness of perennial ryegrass (Lolium perenne) is improved by fungal production of alkaloid anti-feedants, reducing the likelihood of herbivory. The endophyte is presumed to benefit from uptake of photosynthate and other nutrients from the host. However, there is much that remains unknown, especially about the dynamics of the interaction. The authors have worked on two genotypes of L. perenne that harbour different concentrations of Neotyphodium lolii: genotype Niu D has approximately twice the concentration of endophyte as genotype Nui UIV. In actively growing plants, the endophyte had no effect on net photosynthesis at low light intensities but at high light intensities, when CO2 is limiting, net photosynthesis was reduced by approx. 17 %. The reason for this reduction in net photosynthesis (which did not occur in older, non-growing plants) is not clear. There was no change in transpiration (except in the dark), no increase in dark respiration and no effect on photon yield. However, it was clear that, in actively growing plants, the presence of the endophyte slowed leaf expansion and tillering rate but without affecting final plant biomass. For older plants that had ceased growing, the endophyte stimulated the growth of replanted tillers. Thus, as the authors rightly say, ‘infection by N. lolii... both increases and decreases growth rates of its host'. Finally, none of the effects of the endophyte showed any significant differences based on endophyte concentration. Effects on the host plant are mediated more subtly than just by ‘weight of numbers’ and almost certainly involve an interplay of signals between host and endophyte, as has already been observed in relation to the growth synchrony of the two organisms.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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