Professor Andrew Smith

Research Interests

Cellular and molecular basis of plant responses to drought, salinity and heavy metals

The distribution and accumulation of solutes in plants depends on the activity of precisely regulated transport processes at the cellular level. These can play an important role in plant responses to environmental stress. We are investigating the molecular basis of plant adaptations to drought, salinity and metals. Much of our work focuses on solute accumulation in the cell vacuole, since this provides the most important storage compartment for osmotically active and potentially cytotoxic ions.

One system studied as a paradigm for regulation of vacuolar solute transport is malic-acid accumulation in plants showing crassulacean acid metabolism (CAM). This process is energized by a vacuolar H+-ATPase (V-ATPase), which drives the influx of malate ions across the tonoplast through a malate-selective ion channel. A combination of approaches has been used to study the mechanism of both malate influx and efflux, as well as the metabolic characteristics of CAM induction during leaf development. In halophytic CAM plants, such as Mesembryanthemum crystallinum, CAM is environmentally regulated, and cell vacuoles can accumulate sodium chloride to concentrations in excess of 1.0 M. We have been studying the transport of both Na+ and Cl- using ion-sensitive fluorescent probes and the patch-clamp technique, as well as the transcriptional regulation of genes encoding the V-ATPase.

Our other major research effort is in the study of "metal-hyperaccumulator plants". These are extremely metal-tolerant plants that accumulate exceptional concentrations of transition metals in their leaves (for example, more than 3 % by dry biomass of nickel or zinc). We are investigating the cellular and molecular basis of this phenomenon to improve our understanding of metal-ion homeostasis in plant cells. Further, it is possible that such plants may aid in the clean-up of contaminated soils through the process of phytoremediation. However, many soils are polluted with mixtures of both organic and inorganic contaminants, and the implications of this for the bioremediation process are being investigated.

Publications

  • Low-level CAM photosynthesis in a succulent-leaved member of the Urticaceae, Pilea peperomioides

    December 2020
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    Journal article
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    Functional Plant Biology

  • Anaerobic digestion of Crassulacean Acid Metabolism plants: Exploring alternative feedstocks for semi-arid lands

    October 2019
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    Journal article
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    Bioresource Technology
    In this work, five Crassulacean Acid Metabolism (CAM) species from the five different genera (Agave, Ananas, Euphorbia, Kalanchoe, and Opuntia) were selected as alternative feedstocks and their biochemical methane potentials (BMP) were investigated. Batch assays were performed using sludge and rumen fluid as inocula under uncontrolled pH and at mesophilic temperature (39 °C). Mean methane yields from the CAM plants inoculated with AD sludge ranged from 281 to 382 ml/gVS. These values were not significantly different from the methane yield obtained from maize, a feedstock for biomethane and volatile fatty acid (VFA), suggesting that CAM plants may be viable as bioenergy crops on poor-quality soils in areas with low rainfall that are unsuitable for cultivation of food crops.
    FFR, biomethane, rumen fluid, sludge, CAM plants, anaerobic digestion

  • Plant-pathogen interactions in the halophytic crop species Chenopodium quinoa

    October 2019
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    Conference paper
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    MOLECULAR PLANT-MICROBE INTERACTIONS

  • Variation in defence strategies in the metal hyperaccumulator plant Noccaea caerulescens is indicative of synergies and trade-offs between forms of defence.

    January 2019
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    Journal article
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    Royal Society open science
    In the metal hyperaccumulator plant Noccaea caerulescens, zinc may provide a defence against pathogens. However, zinc accumulation is a variable trait in this species. We hypothesize that this variability affects the outcome of interactions between metal accumulation and the various constitutive and inducible defences that N. caerulescens shares with non-accumulator plants. We compare zinc concentrations, glucosinolate concentrations and inducible stress responses, including reactive oxygen species (ROS) and cell death, in four N. caerulescens populations, and relate these to the growth of the plant pathogen Pseudomonas syringae, its zinc tolerance mutants and Pseudomonas pathogens isolated from a natural population of N. caerulescens. The populations display strikingly different combinations of defences. Where defences are successful, pathogens are limited primarily by metals, cell death or organic defences; there is evidence of population-dependent trade-offs or synergies between these. In addition, we find evidence that Pseudomonas pathogens have the capacity to overcome any of these defences, indicating that the arms race continues. These data indicate that defensive enhancement, joint effects and trade-offs between different forms of defence are all plausible explanations for the variation we observe between populations, with factors including metal availability and metal-tolerant pathogen load probably shaping the response of each population to infection.

  • Ammonium intensifies CAM photosynthesis and counteracts drought effects by increasing malate transport and antioxidant capacity in Guzmania monostachia

    February 2018
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    Journal article
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    Journal of Experimental Botany
    Guzmania monostachia (Bromeliaceae) is a tropical epiphyte capable of up-regulating crassulacean acid metabolism (CAM) in its photosynthetic tissues in response to changing nutrient and water availability. Previous studies have shown that under drought there is a gradient of increasing CAM expression from the basal (youngest) to the apical (oldest) portion of the leaves, and additionally that nitrogen deficiency can further increase CAM intensity in the leaf apex of this bromeliad. The present study investigated the inter-relationships between nitrogen source (nitrate and/or ammonium) and water deficit in regulating CAM expression in G. monostachia leaves. The highest CAM activity was observed under ammonium nutrition in combination with water deficit. This was associated with enhanced activity of the key enzyme phosphoenolpyruvate carboxylase, elevated rates of ATP- and PPi-dependent proton transport at the vacuolar membrane in the presence of malate, and increased transcript levels of the vacuolar malate channel-encoding gene, ALMT. Water deficit was consistently associated with higher levels of total soluble sugars, which were maximal under ammonium nutrition, as were the activities of several antioxidant enzymes (superoxide dismutase, catalase, ascorbate peroxidase, and glutathione reductase). Thus, ammonium nutrition, whilst associated with the highest degree of CAM induction in G. monostachia, also mitigates the effects of water deficit by osmotic adjustment and can limit oxidative damage in the leaves of this bromeliad under conditions that may be typical of its epiphytic habitat.
    FFR

  • The Kalanchoë genome provides insights into convergent evolution and building blocks of crassulacean acid metabolism

    December 2017
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    Journal article
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    Nature Communications
    Crassulacean acid metabolism (CAM) is a water-use efficient adaptation of photosynthesis that has evolved independently many times in diverse lineages of flowering plants. We hypothesize that convergent evolution of protein sequence and temporal gene expression underpins the independent emergences of CAM from C3 photosynthesis. To test this hypothesis, we generate a de novo genome assembly and genome-wide transcript expression data for Kalanchoë fedtschenkoi, an obligate CAM species within the core eudicots with a relatively small genome (~260 Mb). Our comparative analyses identify signatures of convergence in protein sequence and re-scheduling of diel transcript expression of genes involved in nocturnal CO2 fixation, stomatal movement, heat tolerance, circadian clock, and carbohydrate metabolism in K. fedtschenkoi and other CAM species in comparison with nonCAM species. These findings provide new insights into molecular convergence and building blocks of CAM and will facilitate CAM-into-C3 photosynthesis engineering to enhance wateruse efficiency in crops.
    kalanchoe, genome, plant, plants, FFR, acids, water, carbon dioxide, photosynthesis, phylogeny, evolution, molecular, gene duplication

  • Temporal and spatial transcriptomic and microRNA dynamics of CAM photosynthesis in pineapple.

    October 2017
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    Journal article
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    The Plant journal : for cell and molecular biology
    The altered carbon assimilation pathway of crassulacean acid metabolism (CAM) photosynthesis results in an up to 80% higher water-use efficiency than C3 photosynthesis in plants making it a potentially useful pathway for engineering crop plants with improved drought tolerance. Here we surveyed detailed temporal (diel time course) and spatial (across a leaf gradient) gene and microRNA (miRNA) expression patterns in the obligate CAM plant pineapple [Ananas comosus (L.) Merr.]. The high-resolution transcriptome atlas allowed us to distinguish between CAM-related and non-CAM gene copies. A differential gene co-expression network across green and white leaf diel datasets identified genes with circadian oscillation, CAM-related functions, and source-sink relations. Gene co-expression clusters containing CAM pathway genes are enriched with clock-associated cis-elements, suggesting circadian regulation of CAM. About 20% of pineapple microRNAs have diel expression patterns, with several that target key CAM-related genes. Expression and physiology data provide a model for CAM-specific carbohydrate flux and long-distance hexose transport. Together these resources provide a list of candidate genes for targeted engineering of CAM into C3 photosynthesis crop species.
    Ananas, Carbon, Water, Plant Proteins, MicroRNAs, RNA, Plant, Photosynthesis, Gene Expression Regulation, Plant, Plant Stomata, Circadian Clocks, Transcriptome

  • Nitrate enhancement of CAM activity in two Kalanchoë species is associated with increased vacuolar proton transport capacity.

    August 2017
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    Journal article
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    Physiologia plantarum
    Among species that perform CAM photosynthesis, members of the genus Kalanchoë have been studied frequently to investigate the effect of environmental factors on the magnitude of CAM activity. In particular, different nitrogen sources have been shown to influence the rate of nocturnal CO2 fixation and organic-acid accumulation in several species of Kalanchoë. However, there has been little investigation of the interrelationship between nitrogen source (nitrate versus ammonium), concentration and the activity of the vacuolar proton pumps responsible for driving nocturnal organic-acid accumulation in these species. In the present study with Kalanchoë laxiflora and Kalanchoë delagoensis cultivated on different nitrogen sources, both species were found to show highest total nocturnal organic-acid accumulation and highest rates of ATP- and PPi-dependent vacuolar proton transport on 2.5 mM nitrate, whereas plants cultivated on 5.0 mM ammonium showed the lowest values. In both species malate was the principal organic-acid accumulated during the night, but the second-most accumulated organic-acid was fumarate for K. laxiflora and citrate for K. delagoensis. Higher ATP- and PPi-dependent vacuolar proton transport rates and greater nocturnal acid accumulation were observed in K. delagoensis compared with K. laxiflora. These results show that the effect of nitrogen source on CAM activity in Kalanchoë species is reflected in corresponding differences in activity of the tonoplast proton pumps responsible for driving sequestration of these acids in the vacuole of CAM-performing cells.
    Vacuoles, Kalanchoe, Nitrates, Protons, Nitrogen, Proton Pumps, Plant Proteins, Photosynthesis, Ion Transport
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Graduate Students
Contact Details
E:

andrew.smith@plants.ox.ac.uk
T:

+44 (0) 1865 275009