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The rhizobiome is an important regulator of plant growth and health. Plants shape their rhizobiome communities through production and release of primary and secondary root metabolites. Benzoxazinoids (BXs) are common tryptophan-derived secondary metabolites in grasses that regulate belowground and aboveground biotic interactions. In addition to their biocidal activity, BXs can regulate plant-biotic interactions as semiochemicals or within-plant defence signals. However, the full extent and mechanisms by which BXs shape the root-associated microbiome has remained largely unexplored. Here, we have taken a global approach to examine the regulatory activity of BXs on the maize root metabolome and associated bacterial and fungal communities. Using untargeted mass spectrometry analysis in combination with prokaryotic and fungal amplicon sequencing, we compared the impacts of three genetic mutations in different steps in the BX pathway. We show that BXs regulate global root metabolism and concurrently influence the rhizobiome in a root type-dependent manner. Correlation analysis between BX-controlled root metabolites and bacterial taxa suggested a dominant role for BX-dependent metabolites, particularly flavonoids, in constraining a range of soil microbial taxa, while stimulating methylophilic bacteria. Our study supports a multilateral model by which BXs control root-microbe interactions via a global regulatory function in root secondary metabolism.

Nutrients are heterogeneously distributed in the soil environment. Plants have evolved a variety of mechanisms to maximise their ability to compete for limited resources, with differential root growth considered among the more important mechanisms. Despite the significant costs of root growth, little data is available regarding carbon (C) allocation to roots growing in heterogeneous conditions. Here, we investigate the allocation of recently assimilated C in Poa annua plants growing in uniform or heterogeneous nutrient conditions. In the first experiment we grew plants in split-root boxes, providing N either equally between the two chambers (0.5 mg/0.5 mg, 8 mg/8 mg) or with one side receiving more N (0.5 mg/8 mg, 8 mg/0.5 mg), and quantified C allocation and N uptake using 13 CO 2 and K 15 NO 3 . Where N was supplied equally to the two chambers, C was allocated equally to the roots irrespective of the total N supply. However, the 13 C label was preferentially allocated (60:40) to high-N roots in the unequal treatments. N uptake was a function of local supply and was not affected by N supply to the roots in the other chamber. C allocation had no discernible effect on N uptake. In the second experiment, we tested whether differential N supply would lead to increased root growth in the high-N side. In this experiment, equal amounts of N were supplied to all plants as ammonium, with half receiving an equal distribution to the two root chambers (50/50), while the other half received an unequal supply (94/6). While no difference in root growth was noted in 50/50 plants, a 60:40 mass allocation was noted from day six onwards in plants receiving the 94/6 N supply. Despite increased root growth in the high-N side, the plants receiving the 94/6 treatment could not achieve the same shoot mass or N concentration as the 50/50 plants. No difference in total C allocation to the roots between treatments was noted in the first experiment, and no difference in total root mass between treatments was found in the second experiment, suggesting that root C supply was source-limited, while allocation to specific roots was strongly influenced by sink strength. Differential C allocation appears to be an important pre-requisite for increased root growth in N-rich patches., (Copyright © 2019 Elsevier GmbH. All rights reserved.)

Investigations were undertaken in the context of the potential environmental impact of carbon capture and storage (CCS) transportation in the form of a hypothetical leak of extreme levels of CO 2 into the soil environment and subsequent effects on plant physiology. Laboratory studies using purpose built soil chambers, separating and isolating the soil and aerial environments, were used to introduce high levels of CO 2 gas exclusively into the rhizosphere. CO 2 concentrations greater than 32% in the isolated soil environment revealed a previously unknown whole plant stomatal response. Time course measurements of stomatal conductance (g s ), leaf temperature and leaf abscisic acid (ABA) show strong coupling between all three variables over a specific period (3 h following CO 2 gassing) occurring as a result of CO 2 -specific detection by roots. The coupling of g s and ABA subsequently breaks down resulting in a rapid and complete loss of turgor in the shoot. Root access to water is severely restricted as evidenced by the inability to counter turgor loss, however, the plant regains some turgor over time. Recovery of full turgor is not achieved over the longer term. Results suggest an immediate perception and whole plant response as changes in measured parameters (leaf temperature, g s and ABA) occur in the shoot, but the response is solely due to detection of very high CO 2 concentration at the root/soil interface which results in loss of stomatal regulation and disruption to control over water uptake., (© 2016 Scandinavian Plant Physiology Society.)

Root hemiparasites are green photosynthetic plants, which parasitically acquire resources from host xylem. Mineral nutrients and water, two principal below-ground abiotic resources, were assumed to affect the interaction between hemiparasites and their hosts. The shape of these effects and the underlying physiological mechanisms have, however, remained unclear. We conducted a glasshouse experiment with root-hemiparasitic Rhinanthus alectorolophus, in which we manipulated the availability of mineral nutrients and water. Biomass production and Chl fluorescence of the hemiparasites and hosts were recorded, together with proportion of host-derived carbon in hemiparasite biomass. The abiotic resources had profound interactive effects on the performance of both the hemiparasite and the hosts, as well as the balance of above-ground biomass between them. These effects were mainly based on an increase of growth and photosynthetic efficiency under high nutrient concentrations, on the hemiparasite's ability to induce strong water stress on the hosts if water is limiting, and on release of the host from parasitism by simultaneous abundance of both resources. Hemiparasitism is a highly variable interaction, in which environmental conditions affect both the parasitic and autotrophic (and thus competitive) components. A hemiparasite's own photosynthesis plays a crucial role in the assimilation of parasitized mineral resources and their transformation into growth and fitness., (© 2014 The Authors. New Phytologist © 2014 New Phytologist Trust.)

Hemiparasitic plants gain virtually all mineral nutrients and water from their host plant whilst organic carbon is provided, at least in part, by their own photosynthetic activity, although their rates of assimilation are substantially lower than that found in non-parasitic plants. Hence, hemiparasites must gain at least some of their organic carbon heterotrophically from the host plant. Despite this, heterotrophic carbon gain by root hemiparasites has been investigated only for a few genera. We investigated heterotrophic carbon gain by two root hemiparasites, Rhinanthus minor L. and Euphrasia rostkoviana Hayne (Orobanchaceae), using natural abundance stable isotope (delta(13)C) profiles of both parasites attached to C(3) (wheat) and C(4) (maize) hosts coupled to a linear two-source isotope-mixing model to estimate the percentage of carbon in the parasite that was derived from the host. Both R. minor and E. rostkoviana attached to maize hosts were significantly more enriched in (13)C than those attached to wheat hosts with R. minor becoming more enriched in (13)C than E. rostkoviana. The natural abundance (13)C profiles of both parasites were not significantly different from their wheat hosts, but were less enriched in (13)C than maize hosts. Using a linear two-source isotope-mixing model, we estimated that R. minor and E. rostkoviana adult plants derive c. 50 and 25% of their carbon from their hosts, respectively. In light of these results, we hypothesise that repeatedly observed negative effect of competition for light on hemiparasites acts predominantly in early ontogenetic stages when parasites grow unattached or the abstraction of host nutrients is less effective.

The root parasite Rhinanthus minor feeds on the xylem of a diverse range of species. Grasses and legumes are the best hosts, while on forbs R. minor typically shows poorer growth. It has been hypothesized that host quality is linked to the expression of defences against the parasite seen in forb roots, but never in grasses. The efficacy of these defence mechanisms in preventing resource loss has not, however, been measured directly. Here we combine histological characterization of haustoria formed on Cynosurus cristatus (a grass), Leucanthemum vulgare and Plantago lanceolata (forbs) with (15)N tracers supplied to the host to quantify the efficacy of these defence responses. Rhinanthus minor penetrated only the xylem of C. cristatus, abstracting an average of 17% of the (15)N tracer taken up, but only 2.5 and 0.2%, respectively, when attached to L. vulgare and P. lanceolata. For the first time, this study has established that the resistance mechanisms of the forbs are effective in preventing the parasite from directly accessing their xylem solutes.

Background and Aims: Rhinanthus minor is a root hemiparasitic plant that attacks a wide range of host species which are severely damaged by the parasite. Rhinanthus minor also attempts unsuccessfully to form connections to a range of non-hosts which in contrast are not damaged by the parasite; however, the underlying physiological basis of these differences is not fully understood., Methods: Biomass of host-parasite combinations was studied, and histology, electron microscopy and FT-IR microspectroscopy were used to determine the cellular-level interactions between Rhinanthus haustoria (the parasite's connective structure) and the roots of a range of potential host species., Results: Two distinct defence responses were observed in the non-host forbs Plantago lanceolata and Leucanthemum vulgare. Firstly, L. vulgare was able to encapsulate the parasite's invading structures preventing it from gaining access to the stele. This was supported by FT-IR microspectroscopy, used to monitor lignification in response to Rhinanthus haustoria. Secondly, host cell fragmentation was observed at the interface between the parasite and P. lanceolata. Growth data confirmed the non-host status of the two forbs whilst, in contrast, grasses and a legume which were good hosts showed no evidence of defence at the host/parasite interface., Conclusions: Variable resistance to Rhinanthus is shown for the first time to be controlled by cellular-level resistance to haustoria by either cell fragmentation or lignification at the host/parasite interface.

Highlights: [•] We propose a novel four-phase model of mycorrhiza-induced resistance (MIR). [•] Phase I: root exudation of mycorrhiza-recruiting chemicals. [•] Phase II: the plant immune system responding to infection by abuscular mycorrhizal fungi (AMF). [•] Phase III: immune suppression by AMF and recruitment of mycorrhizosphere bacteria. [•] Phase IV: establishment of the mycorrhizosphere and induction of systemic resistance by mycorrhizosphere bacteria. [Copyright &y& Elsevier]