Resource allocation to growth or luxury consumption drives mycorrhizal responses

Carbon and phosphorus exchange rates in arbuscular mycorrhizas depend on environmental context and differ among co-occurring plants

Phosphorus (P) for carbon (C) exchange is the pivotal function of arbuscular mycorrhiza (AM), but how this exchange varies with soil P availability and among co-occurring plants in complex communities is still largely unknown. We collected intact plant communities in two regions differing c. 10-fold in labile inorganic P. After a 2-month glasshouse incubation, we measured 32P transfer from AM fungi (AMF) to shoots and 13C transfer from shoots to AMF using an AMF-specific fatty acid. AMF communities were assessed using molecular methods. AMF delivered a larger proportion of total shoot P in communities from high-P soils despite similar 13C allocation to AMF in roots and soil. Within communities, 13C concentration in AMF was consistently higher in grass than in blanketflower (Gaillardia aristata Pursh) roots, that is P appeared more costly for grasses. This coincided with differences in AMF taxa composition and a trend of more vesicles (storage structures) but fewer arbuscules (exchange structures) in grass roots. Additionally, 32P-for-13C exchange ratios increased with soil P for blanketflower but not grasses. Contrary to predictions, AMF transferred proportionally more P to plants in communities from high-P soils. However, the 32P-for-13C exchange differed among co-occurring plants, suggesting differential regulation of the AM symbiosis.

High-throughput shoot phenotyping reveals temporal growth responses to nitrogen and inorganic and organic phosphorus sources in tomato

The application of nitrogen (N) and phosphorus (P) fertilizers to soils is required to maintain crop yields, so the sufficient and timely delivery of nutrients to match crop demand is important in fertilizer management. We quantified temporal growth responses of tomato plants with different rates of N and P application using high-throughput shoot phenotyping. The tomato plants were grown in soil that had organic, inorganic or a combination of sources of P incorporated. Additional N was added to each pot at low and high rates, 13 days after planting. At the same rate of total P application, the inorganic P source resulted in greater shoot growth at the early time points. Later on, the plants supplied with organic or mixed P sources grew faster than those that received the inorganic P source, resulting in comparable shoot biomass in all treatments at the time of destructive harvest. The shoot phenotyping data demonstrated that readily available soil P was important for early tomato growth while available N was more important in later stages of vegetative growth. These results suggest that a fertilizer formulation of combined inorganic and organic P sources may be able to sustain rapid and great shoot growth in tomato plants, while also reducing additional N input.

Ecological stoichiometry and fungal community turnover reveal variation among mycorrhizal partners in their responses to warming and drought

Symbiotic fungi mediate important energy and nutrient transfers in terrestrial ecosystems. Environmental change can lead to shifts in communities of symbiotic fungi, but the consequences of these shifts for nutrient dynamics among symbiotic partners are poorly understood. Here, we assessed variation in carbon (C), nitrogen (N) and phosphorus (P) in tissues of arbuscular mycorrhizal (AM) fungi and a host plant (Medicago sativa) in response to experimental warming and drought. We linked compositional shifts in AM fungal communities in roots and soil to variation in hyphal chemistry by using high-throughput DNA sequencing and joint species distribution modelling. Compared to plants, AM hyphae was 43% lower in (C) and 24% lower in (N) but more than nine times higher in (P), with significantly lower C:N, C:P and N:P ratios. Warming and drought resulted in increases in (P) and reduced C:P and N:P ratios in all tissues, indicating fungal P accumulation was exacerbated by climate-associated stress. Warming and drought modified the composition of AM fungal communities, and many of the AM fungal genera that were linked to shifts in mycelial chemistry were also negatively impacted by climate variation. Our study offers a unified framework to link climate change, fungal community composition, and community-level functional traits. Thus, our study provides insight into how environmental change can alter ecosystem functions via the promotion or reduction of fungal taxa with different stoichiometric characteristics and responses.

Brachypodium: 20 years as a grass biology model system; the way forward?

It has been 20 years since Brachypodium distachyon was suggested as a model grass species, but ongoing research now encompasses the entire genus. Extensive Brachypodium genome sequencing programmes have provided resources to explore the determinants and drivers of population diversity. This has been accompanied by cytomolecular studies to make Brachypodium a platform to investigate speciation, polyploidisation, perenniality, and various aspects of chromosome and interphase nucleus organisation. The value of Brachypodium as a functional genomic platform has been underscored by the identification of key genes for development, biotic and abiotic stress, and cell wall structure and function. While Brachypodium is relevant to the biofuel industry, its impact goes far beyond that as an intriguing model to study climate change and combinatorial stress.

Growth, Nutrient Accumulation, and Nutritional Efficiency of a Clonal Eucalyptus Hybrid in Competition with Grasses

Invasive grasses reduce resource availability, mainly nutrients in the soil, and the growth of eucalyptus plants. Efficient management to increase productivity depends on understanding levels of weed interference in eucalyptus plantations. The nutritional efficiency of eucalyptus plants in competition has been evaluated by plant tissue analysis. The objective was to evaluate the growth, relative accumulation of nutrients, and nutritional efficiency of the eucalyptus clonal hybrid I144 (Eucalyptus urophylla × Eucalyptus grandis), in competition with Megathyrsus maximus cv. BRS zuri, Urochloa brizantha cv. marandu, Urochloa decumbens cv. basilisk and in the control (eucalyptus plants without weed competition). The experiment was carried out with a completely randomized design, with four treatments and ten replications. The height, stem diameter, number of leaves, leaf area, dry matter of leaves and stem, nutrient content in leaves and uptake, transport, and N, P, and K utilization efficiency of the eucalyptus clonal hybrid were evaluated at 110 days after transplantation. The growth parameters and relative contents of macro and micronutrients in the eucalyptus clonal hybrid were lower in competition with M. maximus, U. brizantha and U. decumbens. The efficiency of N, P, and K uptake and transport by the eucalyptus clonal hybrid was 29.41 and 7.32% lower in competition with U. decumbens than in the control treatments, respectively. The efficiency of N, P, and K utilization by eucalypts was 13.73, 9.18, and 22.54% lower in competition with M. maximus, U. brizantha, and U. decumbens, respectively. The reduced growth and nutritional parameters of the eucalyptus clonal hybrid were more evident in competition with U. decumbens. Plant tissue analyses efficiently determined the level of competition for nutrients between species. Crop competition with grasses can decrease the efficiency and use of nutrients, which consequently reduces plant development and productivity.

The Costs and Benefits of Plant-Arbuscular Mycorrhizal Fungal Interactions

The symbiotic interaction between plants and arbuscular mycorrhizal (AM) fungi is often perceived as beneficial for both partners, though a large ecological literature highlights the context dependency of this interaction. Changes in abiotic variables, such as nutrient availability, can drive the interaction along the mutualism-parasitism continuum with variable outcomes for plant growth and fitness. However, AM fungi can benefit plants in more ways than improved phosphorus nutrition and plant growth. For example, AM fungi can promote abiotic and biotic stress tolerance even when considered parasitic from a nutrient provision perspective. Other than being obligate biotrophs, very little is known about the benefits AM fungi gain from plants. In this review, we utilize both molecular biology and ecological approaches to expand our understanding of the plant-AM fungal interaction across disciplines.

Seasonal Characterization of the Endophytic Fungal Microbiome of Mulberry (Morus spp.) Cultivars Resistant and Susceptible to Sclerotiniosis

The endophytic microbiome is thought to play an important role in promoting plant growth and health. Using culture-independent and culture-dependent protocols, this study characterized the seasonal shifts in the endophytic fungal microbiota of four mulberry (Morus L.) cultivars having different levels of resistance to mulberry fruit sclerotiniosis. Core endophytes can be obtained by two approaches, and they were divided into two clusters by season. Spring samples harbored higher operational taxonomic units (OTUs) and α-diversity, while autumn samples had more sequences or isolates of the fungal class Dothideomycetes with the representative orders Capnodiales and Pleosporales. While comparing different mulberry cultivars, we found that the total number of OTUs in susceptible cultivars was higher than that of resistant cultivars, and Cladosporium sp. were observed in all. Notably, the causal agent of fruit sclerotiniosis (Scleromitrula shiraiana) was only detected in susceptible cultivars. Collectively, our work elucidated significant variations in the mulberry endophytic microbiome, mainly because of seasonal shifts, and the fact that the host cultivars and mulberry endophytic fungal community appeared to have a certain connection with the resistance level of mulberry fruit to sclerotiniosis. These results provided valuable information on the isolation and culturing of mulberry endophytes that could be applied to improve mulberry fruit production and health.

Metalloid hazards: From plant molecular evolution to mitigation strategies

Metalloids such as boron and silicon are key elements for plant growth and crop productivity. However, toxic metalloids such as arsenic are increasing in the environment due to inputs from natural sources and human activities. These hazardous metalloids can cause serious health risks to humans and animals if they enter the food chain. Plants have developed highly regulated mechanisms to alleviate the toxicity of metalloids during their 500 million years of evolution. A better understanding the molecular mechanisms underlying the transport and detoxification of toxic metalloids in plants will shed light on developing mitigation strategies. Key transporters and regulatory proteins responsive to toxic metalloids have been identified through evolutionary and molecular analyses. Moreover, knowledge of the regulatory proteins and their pathways can be used in the breeding of crops with lower accumulation of metalloids. These findings can also assist phytoremediation by the exploration of plants such as fern species that hyperaccumulate metalloids from soils and water, and can be used to engineer plants with elevated uptake and storage capacity of toxic metalloids. In summary, there are solutions to remediate contamination due to toxic metalloids by combining the research advances and industrial technologies with agricultural and environmental practices.

Factors affecting plant responsiveness to arbuscular mycorrhiza

Arbuscular mycorrhiza (AM) is an ancient, widespread symbiosis between most land plants and fungi of the Glomeromycotina, which receives increasing interest for agricultural application because it can promote plant growth and yield. The ability of plants to react to AM with changes in morphology and/or performance in terms of yield is called 'AM responsiveness'. Its amplitude depends on the plant- fungal genotype combination and the abiotic and biotic environment. A molecular understanding of AM responsiveness is key for enabling rational application of AM in agriculture, for example through targeted breeding of AM-optimised crops. However, the genetic and mechanistic underpinnings of AM responsiveness variation remain still unknown. Here, we review current knowledge on AM responsiveness, with a focus on agricultural crops, and speculate on mechanisms that may contribute to the variation in AM response.

Myristate and the ecology of AM fungi: significance, opportunities, applications and challenges

A recent study by Sugiura and coworkers reported the non-symbiotic growth and spore production of an arbuscular mycorrhizal (AM) fungus, Rhizophagus irregularis, when the fungus received an external supply of certain fatty acids, myristates (C:14). This discovery follows the insight that AM fungi receive fatty acids from their hosts when in symbiosis. If this result holds up and can be repeated under nonsterile conditions and with a broader range of fungi, it has numerous consequences for our understanding of AM fungal ecology, from the level of the fungus, at the plant community level, and to functional consequences in ecosystems. In addition, myristate may open up several avenues from a more applied perspective, including improved fungal culture and supplementation of AM fungi or inoculum in the field. We here map these potential opportunities, and additionally offer thoughts on potential risks of this potentially new technology. Lastly, we discuss the specific research challenges that need to be overcome to come to an understanding of the potential role of myristate in AM ecology.

Smoothing and extraction of traits in the growth analysis of noninvasive phenotypic data

BACKGROUND

Non-destructive high-throughput plant phenotyping is becoming increasingly used and various methods for growth analysis have been proposed. Traditional longitudinal or repeated measures analyses that model growth using statistical models are common. However, often the variation in the data is inappropriately modelled, in part because the required models are complicated and difficult to fit. We provide a novel, computationally efficient technique that is based on smoothing and extraction of traits (SET), which we compare with the alternative traditional longitudinal analysis methods.

RESULTS

The SET-based and longitudinal analyses were applied to a tomato experiment to investigate the effects on plant growth of zinc (Zn) addition and growing plants in soil inoculated with arbuscular mycorrhizal fungi (AMF). Conclusions from the SET-based and longitudinal analyses are similar, although the former analysis results in more significant differences. They showed that added Zn had little effect on plants grown in inoculated soils, but that growth depended on the amount of added Zn for plants grown in uninoculated soils. The longitudinal analysis of the unsmoothed data fitted a mixed model that involved both fixed and random regression modelling with splines, as well as allowing for unequal variances and autocorrelation between time points.

CONCLUSIONS

A SET-based analysis can be used in any situation in which a traditional longitudinal analysis might be applied, especially when there are many observed time points. Two reasons for deploying the SET-based method are (i) biologically relevant growth parameters are required that parsimoniously describe growth, usually focussing on a small number of intervals, and/or (ii) a computationally efficient method is required for which a valid analysis is easier to achieve, while still capturing the essential features of the exhibited growth dynamics. Also discussed are the statistical models that need to be considered for traditional longitudinal analyses and it is demonstrated that the oft-omitted unequal variances and autocorrelation may be required for a valid longitudinal analysis. With respect to the separate issue of the subjective choice of mathematical growth functions or splines to characterize growth, it is recommended that, for both SET-based and longitudinal analyses, an evidence-based procedure is adopted.

Life's rich tapestry : A tribute to Sally E. Smith (1941-2019)

With the recent passing away of Emeritus Professor Sally (Sarah) E. Smith, the mycorrhiza research community has lost one of its most outstanding members. Sally's contribution to knowledge in the fields of soil microbiology and root physiology will continue to be an inspiration to present and future scientists.