Trifolium repens and T. subterraneum modify their nodule microbiome in response to soil pH.

AIMS: The influence of soil edaphic factors on recruitment and composition of bacteria in the legume nodule is unknown. Typically, low (acidic) pH soils have a negative effect on the plant-rhizobia symbiosis and thereby reduce clover growth. However, the specific relationship between soil pH and the ecology of rhizobia is unknown, in either their free-living or nodule-inhabiting states. We used New Zealand pasture systems with soils of different pH, and white (WC) and subterranean (SC) clovers, to examine the relationship between soil pH and the diversity of bacteria that inhabit the nodules. METHODS AND RESULTS: Amplicon sequencing (16S rRNA) assessed the bacterial community in 5299 nodules recovered from both legume species grown in 47 soils of different edaphic (including pH) properties. Fewer nodules were formed on both clovers at low soil pH. As expected, rhizobia comprised ∼ 92% of the total reads in both clovers, however 28 non-rhizobia genera were also present. Soil pH influenced the community structure of bacteria within the nodule, and this was more evident in non-Rhizobium taxa than Rhizobium. Host strongly influenced the diversity of bacteria in the nodules. The alpha diversity of nodule microbiome in SC nodules was higher than in WC nodules and SC nodules also harbored a higher relative abundance of non-Rhizobium bacteria than WC. Beta diversity of Rhizobium and non-Rhizobium bacteria was influenced more by clover species rather than edaphic factors. CONCLUSIONS: The results indicate that these clover species modified their nodule biomes in response to pH-stress. SIGNIFICANCE AND IMPACT OF THE STUDY: The non-Rhizobium bacteria may have some functional significance (such as improved clover persistence in low pH soils) in legume nodules.

Measurements of carbon utilization by single bacterial species in sterile soil: insights into Rhizobium spp.

AIM: The aim of this work was to develop a tool to investigate the influence of soil factors on carbon utilization activity of single micro-organisms. METHODS AND RESULTS: The assay for Rhizobium leguminosarum bv. trifolii in γ-irradiated soil, using the MicroResp(™) system, was optimized for sterility, incubation time, and moisture level. The optimized method was validated with experiments that assessed (i) differences in C utilization of different rhizobia strains and (ii) how this was affected by soil type. Carbon utilization differed among strains of the same species (and symbiovar), but some strains were more responsive to the soil environment than others. CONCLUSIONS: This novel modification of the MicroResp(™) has enabled the scope of carbon-utilization patterns of single strains of bacteria, such as Rh. leguminosarum bv. trifolii, to be studied in soil. SIGNIFICANCE AND IMPACT OF THE STUDY: The system is a new tool with applications in microbial ecology adaptable to the study of many culturable bacterial and fungal soil-borne taxa. It will allow measurement of a micro-organism's ability to utilize common C sources released in rhizosphere exudates to be measured in a physical soil background. This knowledge may improve selection efficiency and deployment of commercial microbial inoculants.

The use of a principal axis model to examine individual plant harvest index in four grain legumes.

BACKGROUND AND AIMS: A principal axis model (PAM) has been proposed to enable the selection of crop ideotypes. The PAM enables plant-to-plant variability within crops to be quantified and compared. The aim of this paper is to validate the PAM for four grain legumes. METHODS: Four grain legumes (Cicer arietinum, Lens culinaris, Lupinus angustifolius, Pisum sativum) were used to quantify the influence of plant-to-plant variability on crop yields. To create variability, populations of 10, 100 and 400 plants m(-2) were established 'on-the-square' with sowing depths of 2, 5 and 10 cm. Further, a central plant was treated with nitrogen and the impact of this on its four neighbouring plants was examined. Seeds were sown and plants harvested individually by hand. KEY RESULTS: Mean individual plant seed weight (SWT) and plant weight (PWT) decreased as plant population increased but there was a consistent and strong (R2 > 0.90) linear relationship between SWT and PWT, with a negative SWT-axis intercept in all species. These components form the basis of the principal axis model (PAM). The PAM was used to summarize the performance of individual plants within a crop and quantify the variability caused by N treatment and the lowest and highest yielding individual plants. A negative SWT-axis intercept indicated that a minimum plant weight (MPW) was required for seed production and therefore the relationship between plant harvest index (PHI) and PWT was asymptotic. The heaviest MPW was calculated for plants grown at the lowest plant population and it was species-dependent, being higher in the larger seeded species. CONCLUSIONS: Agronomic or physiological characteristics that lead to variability in PWT within a population will decrease PHI, and crop yield. The PAM may be useful in breeding programmes to identify plant phenotypes that minimize this plant-to-plant variability.