Infectious and inflammatory diseases have repeatedly shown strong genetic associations within the major histocompatibility complex (MHC); however, the basis for these associations remains elusive. To define host genetic effects on the outcome of a chronic viral infection, we performed genome-wide association analysis in a multiethnic cohort of HIV-1 controllers and progressors, and we analyzed the effects of individual amino acids within the classical human leukocyte antigen (HLA) proteins. We identified >300 genome-wide significant single-nucleotide polymorphisms (SNPs) within the MHC and none elsewhere. Specific amino acids in the HLA-B peptide binding groove, as well as an independent HLA-C effect, explain the SNP associations and reconcile both protective and risk HLA alleles. These results implicate the nature of the HLA–viral peptide interaction as the major factor modulating durable control of HIV infection.

A comparative analysis of daily carbon (C) budgets and aspects of the C physiology of clover (Trifolium repens L.) colonized by vesicular-arbuscular (VA) mycorrhizal fungi was carried out over a 70 d growth period under conditions designed to ensure that shoots of mycorrhizal (M) and non-mycorrhizal (NM) plants were of similar nutrient status. C budgets did not differ on day 24 but by day 42 M plants had a significantly higher rate of photosynthesis than their NM counterparts when expressed on a whole shoot basis or unit dry weight basis. As both sets of plants were of the same size it was concluded that this greater C gain was the result of increased sink strength provided by the mycorrhizal fungus. By day 53 M plants had become larger than their uncolonized counterparts and a sinkinduced stimulation in the rate of photosynthesis was no longer apparent. M plants had higher root sucrose, glucose and fructose pools from day 24. Analyses suggested that these sugars were utilized for trehalose and lipid synthesis, for the production of the large extramatrical mycelium and for the support of the respiratory demands of the M root system. Increased C allocation to roots of M plants was associated with a stimulation of the activities of cell wall and cytoplasmic invertases and of sucrose synthase in roots colonized by VA fungi. Such increases in enzyme activity may provide the mechanism enabling increased partitioning of carbohydrate both to the M root system and the fungal symbiont.Key-words: Trifolium repens L.; carbohydrates; carbon partitioning; clover; invertase; photosynthesis; sucrose synthase; vesicular-arbuscular mycorrhiza. INTRODUCTIONAs obligate symbionts, vesicular-arbuscular (VA) mycorrhizal fungi can exert a substantial effect on the carbon (C) economy of the autotroph (Smith & Read 1997).Comparison of the C economies of mycorrhizal (M) and non-mycorrhizal (NM) roots has shown that support of the symbiosis requires the transfer of an additional 4-20% of the total net C fixed by the plant (Pang & Paul 1980;Paul & Kucey 1981;Kucey & Paul 1982;Snellgrove et al. 1982;Koch & Johnson 1984;Harris, Pacovsky & Paul 1985;Douds, Johnson & Koch 1988;Wang et al. 1989). While this demand can be seen as the 'cost' of the symbiosis, it has been hypothesized that mycorrhizal colonization of the root, by increasing its sink strength, may stimulate the process of C assimilation so that the 'cost' imposed on the plant's C economy is reduced or eliminated and thus contribute to the overall benefit derived from association with mycorrhizal fungi (Fitter 1991;Tinker, Durall & Jones 1994). Some evidence in support of such a hypothesis was provided in an earlier study (Wright, Scholes & Read 1998), but the mechanisms involved in enhancement of sink strength have not been investigated in depth. In our previous study we demonstrated, using M and NM clover (Trifolium repens L.) plants with similar foliar nitrogen (N) and phosphorus (P) contents, size and growth rate, that VA mycorrhizal colonization stimulated the rate of photosynt...

The influence of vesicular-arbuscular mycorrhizal (M) colonization on biomass production and photosynthesis of Trifolium repens L. was investigated in two experiments in which the foliar nitrogen and phosphorus contents of nonmycorrhizal (NM) plants were manipulated to be no lower than that of M plants. Throughout both experiments there was a stimulation in the rate of CO 2 assimilation of the youngest, fully expanded leaf of M compared with NM plants. In addition, M plants exhibited a higher specific leaf area compared with NM plants, a response that maximized the area available for CO 2 assimilation per unit of carbon (C) invested. Despite the increased rate of photosynthesis in M plants there was no evidence that the additional C gained was converted to biomass production of M plants. It is suggested that this additional C gained by colonized plants was allocated to the mycorrhizal fungus and that it is the fungus, by acting as a sink for assimilates, that facilitated the stimulation in the rate of photosynthesis of the plant partner.