Barbara G. Pickard - Queen of Plant Electrophysiology.

Barbara Gillespie Pickard (1936-2019) studied plant electrophysiology and mechanosensory biology for more than 50 y. Her first papers on the roles of auxin in plant tropisms were coauthored with Kenneth V. Thimann. Later, she studied plant electrophysiology. She made it clear that plant action potentials are not a peculiar feature of so-called sensitive plants, but that all plants exhibit these fast electric signals. Barbara Gillespie Pickard proposed a neuronal model for the spreading of electric signals induced by mechanical stimuli across plant tissues. In later years, she studied the stretch-activated plasma membrane channels of plants and formulated the plasma-membrane control center model. Barbara Pickard summarized all her findings in a new model of phyllotaxis involving waves of auxin fluxes and mechano-sensory signaling.

Sink Strength Maintenance Underlies Drought Tolerance in Common Bean.

Drought is a major limiter of yield in common bean, decreasing food security for those who rely on it as an important source of protein. While drought can have large impacts on yield by reducing photosynthesis and therefore resources availability, source strength is not a reliable indicator of yield. One reason resource availability does not always translate to yield in common bean is because of a trait inherited from wild ancestors. Wild common bean halts growth and seed filling under drought and awaits better conditions to resume its developmental program. This trait has been carried into domesticated lines, where it can result in strong losses of yield in plants already producing pods and seeds, especially since many domesticated lines were bred to have a determinate growth habit. This limits the plants ability to produce another flush of flowers, even if the first set is aborted. However, some bred lines are able to maintain higher yields under drought through maintaining growth and seed filling rates even under water limitations, unlike their wild predecessors. We believe that maintenance of sink strength underlies this ability, since plants which fill seeds under drought maintain growth of sinks generally, and growth of sinks correlates strongly with yield. Sink strength is determined by a tissue's ability to acquire resources, which in turn relies on resource uptake and metabolism in that tissue. Lines which achieve higher yields maintain higher resource uptake rates into seeds and overall higher partitioning efficiencies of total biomass to yield. Drought limits metabolism and resource uptake through the signaling molecule abscisic acid (ABA) and its downstream affects. Perhaps lines which maintain higher sink strength and therefore higher yields do so through decreased sensitivity to or production of ABA.

Understanding Plant Behavior: A Student Perspective.

Biology students need special incentive to learn plant physiology. Framing plant function as 'behavior' analogous to animal neurobiology and behavior and integrating active learning methods is a successful way to generate an inclusive space for a wide range of learning styles, cultural backgrounds, and scientific contributions.

Sensitivity of leaflet growth rate to drought predicts yield in common bean (Phaseolus vulgaris).

Although drought limits yield by decreasing photosynthesis and therefore biomass accumulation, biomass is not the strongest predictor of yield under drought in common beans (Phaseolus vulgaris L.). Instead, resource partitioning from pod walls into seeds is a stronger correlate. Our aim was to determine whether growth rates of developing leaflets and pods, as independent indicators of sink strength, predict resource partitioning into seeds. Using 20 field-grown genotypes, we paired biomass, yield, and resource partitioning data with leaflet and pod growth rates under well-watered and droughted conditions. We hypothesised that genotypes with faster growing leaflets and pods under drought would fill seeds better. However, we found that leaflet and pod growth rates did not predict partitioning to seeds; rather, sensitivity of leaflet growth rate to drought was a good predictor of yield reduction. Further, plants with rapidly growing leaves under well-watered conditions were most vulnerable to decreases in leaflet growth rate under drought. This suggests that lines that inherited a conservative growth strategy were better able to maintain yield by allocating resources to seeds. Our findings indicate that inherent sensitivity of leaflet growth rate to drought may be used as a predictor of partitioning and yield in common beans.

To Stimulate or Inhibit? That Is the Question for the Function of Abscisic Acid.

Physiologically, abscisic acid (ABA) is believed to be a general inhibitor of plant growth, including during the crucial early development of seedlings. However, this view contradicts many reports of stimulatory effects of ABA that, so far, have not been considered in the debate concerning ABA's function in plant development. To address this apparent contradiction, we propose a hypothetical mechanism to explain how ABA might contribute to the promotion of cell expansion. We wish to overturn conventional views on ABA's role during juvenile plant development and put forward the idea that, as for other phytohormones, the role of ABA is determined by dose and sensitivity and ranges from stimulatory to inhibitory effects.

Comparative leaf growth strategies in response to low-water and low-light availability: variation in leaf physiology underlies variation in leaf mass per area in Populus tremuloides.

Developmental phenotypic plasticity can allow plants to buffer the effects of abiotic and biotic environmental stressors. Therefore, it is vital to improve our understanding of how phenotypic plasticity in ecological functional traits is coordinated with variation in physiological performance in plants. To identify coordinated leaf responses to low-water (LW) versus low-light (LL) availability, we measured leaf mass per area (LMA), leaf anatomical characteristics and leaf gas exchange of juvenile Populus tremuloides Michx. trees. Spongy mesophyll tissue surface area (Asmes/A) was correlated with intrinsic water-use efficiency (WUEi: photosynthesis, (Aarea)/stomatal conductance (gs)). Under LW availability, these changes occurred at the cost of greater leaf tissue density and reduced expansive growth, as leaves were denser but were only 20% the final area of control leaves, resulting in elevated LMA and elevated WUEi. Low light resulted in reduced palisade mesophyll surface area (Apmes/A) while spongy mesophyll surface area was maintained (Asmes/A), with no changes to WUEi. These leaf morphological changes may be a plastic strategy to increase laminar light capture while maintaining WUEi. With reduced density and thickness, however, leaves were 50% the area of control leaves, ultimately resulting in reduced LMA. Our results illustrate that P. tremuloides saplings partially maintain physiological function in response to water and light limitation by inducing developmental plasticity in LMA with underlying anatomical changes. We discuss additional implications of these results in the context of developmental plasticity, growth trade-offs and the ecological impacts of climate change.

Serpentine tolerance in Mimulus guttatus does not rely on exclusion of magnesium.

The effect of serpentine soil-like low Ca:Mg ratios on growth was investigated in serpentine-adapted and nonadapted populations of Mimulus guttatus Fischer ex DC through soil and hydroponic reciprocal transplants. Adaptation to Ca:Mg ratios in M. guttatus was measured as differences in biomass accumulation, uptake of Ca and Mg, and photosynthetic rates. Serpentine-adapted plants persisted on both serpentine and nonserpentine soils, but nonadapted plants survived only on nonserpentine soil. When grown hydroponically, a low Ca:Mg ratio decreased the biomass of nonadapted plants but serpentine-adapted plants increased in biomass relative to their growth on high Ca:Mg. Internal concentrations of Ca and Mg mirrored those of the growth solution in both populations; however, serpentine-adapted M. guttatus had a higher shoot:root ratio of Mg when grown in low Ca:Mg solutions. Elevated Mg reduced photosynthetic rates in nonadapted plants without changes in chlorophyll concentration or photosystem efficiency. Hydroponic culture isolated the Ca:Mg ratio from other soil characteristics as the dominant factor affecting growth. Differences in the growth of plants from these populations in reciprocal transplant experiments indicate a genetic basis for a tolerance mechanism to low Ca:Mg, but one that is not based on the exclusion of Mg.

Why not beans?

Changes in climate and urbanisation rapidly affecting human livelihood are particularly threatening to developing nations in tropical regions. Food production crises have focused the global development agenda on agricultural research, a proven approach for increasing crop yield. A few crops benefit from private investment, but improvement of most crops will rely on limited public funding that must be deployed strategically, pushing forward both proven approaches and new ideas. Why not invest in beans? More than 300 million people rely on this crop, considered to be the most important grain legume for human consumption. Yet the yield of beans, especially in poor regions or marginal soils, is reduced by abiotic stresses such as phosphorus deficiency, aluminum toxicity and especially drought. Is it possible to assemble resources, including genetic diversity in beans, breeding expertise, genomic information and tools, and physiological insight to generate rapid progress in developing new lines of beans more tolerant to abiotic stress? A workshop to address this question was held in November 2010 at the International Center for Tropical Agriculture (CIAT) in Colombia. The resulting 'call to action' is presented in this issue which also includes research papers focused on tolerance of beans to stress.

Non-invasive approaches for phenotyping of enhanced performance traits in bean.

Plant phenotyping is an emerging discipline in plant biology. Quantitative measurements of functional and structural traits help to better understand gene-environment interactions and support breeding for improved resource use efficiency of important crops such as bean (Phaseolus vulgaris L.). Here we provide an overview of state-of-the-art phenotyping approaches addressing three aspects of resource use efficiency in plants: belowground roots, aboveground shoots and transport/allocation processes. We demonstrate the capacity of high-precision methods to measure plant function or structural traits non-invasively, stating examples wherever possible. Ideally, high-precision methods are complemented by fast and high-throughput technologies. High-throughput phenotyping can be applied in the laboratory using automated data acquisition, as well as in the field, where imaging spectroscopy opens a new path to understand plant function non-invasively. For example, we demonstrate how magnetic resonance imaging (MRI) can resolve root structure and separate root systems under resource competition, how automated fluorescence imaging (PAM fluorometry) in combination with automated shape detection allows for high-throughput screening of photosynthetic traits and how imaging spectrometers can be used to quantify pigment concentration, sun-induced fluorescence and potentially photosynthetic quantum yield. We propose that these phenotyping techniques, combined with mechanistic knowledge on plant structure-function relationships, will open new research directions in whole-plant ecophysiology and may assist breeding for varieties with enhanced resource use efficiency varieties.

Stress tolerance in plants via habitat-adapted symbiosis.

We demonstrate that native grass species from coastal and geothermal habitats require symbiotic fungal endophytes for salt and heat tolerance, respectively. Symbiotically conferred stress tolerance is a habitat-specific phenomenon with geothermal endophytes conferring heat but not salt tolerance, and coastal endophytes conferring salt but not heat tolerance. The same fungal species isolated from plants in habitats devoid of salt or heat stress did not confer these stress tolerances. Moreover, fungal endophytes from agricultural crops conferred disease resistance and not salt or heat tolerance. We define habitat-specific, symbiotically-conferred stress tolerance as habitat-adapted symbiosis and hypothesize that it is responsible for the establishment of plants in high-stress habitats. The agricultural, coastal and geothermal plant endophytes also colonized tomato (a model eudicot) and conferred disease, salt and heat tolerance, respectively. In addition, the coastal plant endophyte colonized rice (a model monocot) and conferred salt tolerance. These endophytes have a broad host range encompassing both monocots and eudicots. Interestingly, the endophytes also conferred drought tolerance to plants regardless of the habitat of origin. Abiotic stress tolerance correlated either with a decrease in water consumption or reactive oxygen sensitivity/generation but not to increased osmolyte production. The ability of fungal endophytes to confer stress tolerance to plants may provide a novel strategy for mitigating the impacts of global climate change on agricultural and native plant communities.

Plant neurobiology: an integrated view of plant signaling.

Plant neurobiology is a newly focused field of plant biology research that aims to understand how plants process the information they obtain from their environment to develop, prosper and reproduce optimally. The behavior plants exhibit is coordinated across the whole organism by some form of integrated signaling, communication and response system. This system includes long-distance electrical signals, vesicle-mediated transport of auxin in specialized vascular tissues, and production of chemicals known to be neuronal in animals. Here we review how plant neurobiology is being directed toward discovering the mechanisms of signaling in whole plants, as well as among plants and their neighbors.

Shade-Induced Action Potentials in Helianthus annuus L. Originate Primarily from the Epicotyl.

Repeated observations that shading (a drastic reduction in illumination rate) increased the generation of spikes (rapidly reversed depolarizations) in leaves and stems of many cucumber and sunflower plants suggests a phenomenon widespread among plant organs and species. Although shaded leaves occasionally generate spikes and have been suggested to trigger systemic action potentials (APs) in sunflower stems, we never found leaf-generated spikes to propagate out of the leaf and into the stem. On the contrary, our data consistently implicate the epicotyl as the location where most spikes and APs (propagating spikes) originate. Microelectrode studies of light and shading responses in mesophyll cells of leaf strips and in epidermis/cortex cells of epicotyl segments confirm this conclusion and show that spike induction is not confined to intact plants. 90% of the epicotyl-generated APs undergo basipetal propagation to the lower epicotyl, hypocotyl and root. They propagate with an average rate of 2 +/- 0.3 mm s(-1) and always undergo a large decrement from the hypocotyl to the root. The few epicotyl-derived APs that can be tracked to leaf blades (< 10%) undergo either a large decrement or fail to be transmitted at all. Occasionally (5% of the observations) spikes were be generated in hypocotyl and lower epicotyl that moved towards the upper epicotyl unaltered, decremented, or amplified. This study confirms that plant APs arise to natural, nontraumatic changes. In simultaneous recordings with epicotyl growth, AP generation was found to parallel the acceleration of stem growth under shade. The possible relatedness of both processes must be further investigated.

Development of Erect Leaves in a Modern Maize Hybrid is Associated with Reduced Responsiveness to Auxin and Light of Young Seedlings In Vitro.

Modern corn (Zea mays L.) varieties have been selected for their ability to maintain productivity in dense plantings. We have tested the possibility that the physiological consequence of the selection involves changes in responsiveness to light and auxin.Etiolated seedlings of two older corn hybrids 307 and 3306 elongated significantly more than seedlings of a modern corn hybrid 3394. The level of endogenous auxin and activity of PAT in 307 and 3394 were similar. Hybrid 3394 shows resistance to auxin- and light-induced responses at the seedling, cell and molecular levels. Intact 3394 plants exhibited less responsiveness to the inhibitory effect of R, FR and W, auxin, anti-auxin and inhibitors of PAT. In excised mesocotyl tissue 3394 seedlings also showed essentially low responsiveness to NAA. Cells of 3394 were insensitive to auxin- and light-induced hyperpolarization of the plasma membrane. Expression of ABP4 was much less in 3394 than in 307, and in contrast to 307, it was not upregulated by NAA, R and FR. Preliminary analysis of abp mutants suggests that ABPs may be involved in development of leaf angle in corn.Our results confirm the understanding that auxin interacts with light in the regulation of growth and development of young seedlings and suggest that in corn ABPs may be involved in growth of maize seedlings and development of leaf angle. We hypothesize that ABP4 plays an important role in the auxin- and/or light-induced growth responses. We also hypothesize that in the modern corn hybrid 3394, ABP4 is "mutated," which may result in the observed 3394 phenotypes, including upright leaves.

Effect of divalent cations on ion fluxes and leaf photochemistry in salinized barley leaves.

Photosynthetic characteristics, leaf ionic content, and net fluxes of Na(+), K(+), and Cl(-) were studied in barley (Hordeum vulgare L) plants grown hydroponically at various Na/Ca ratios. Five weeks of moderate (50 mM) or high (100 mM) NaCl stress caused a significant decline in chlorophyll content, chlorophyll fluorescence characteristics, and stomatal conductance (g(s)) in plant leaves grown at low calcium level. Supplemental Ca(2+) enabled normal photochemical efficiency of PSII (F(v)/F(m) around 0.83), restored chlorophyll content to 80-90% of control, but had a much smaller (50% of control) effect on g(s). In experiments on excised leaves, not only Ca(2+), but also other divalent cations (in particular, Ba(2+) and Mg(2+)), significantly ameliorated the otherwise toxic effect of NaCl on leaf photochemistry, thus attributing potential targets for such amelioration to leaf tissues. To study the underlying ionic mechanisms of this process, the MIFE technique was used to measure the kinetics of net Na(+), K(+), and Cl(-) fluxes from salinized barley leaf mesophyll in response to physiological concentrations of Ca(2+), Ba(2+), Mg(2+), and Zn(2+). Addition of 20 mM Na(+) as NaCl or Na(2)SO(4) to the bath caused significant uptake of Na(+) and efflux of K(+). These effects were reversed by adding 1 mM divalent cations to the bath solution, with the relative efficiency Ba(2+)>Zn(2+)=Ca(2+)>Mg(2+). Effect of divalent cations on Na(+) efflux was transient, while their application caused a prolonged shift towards K(+) uptake. This suggests that, in addition to their known ability to block non-selective cation channels (NSCC) responsible for Na(+) entry, divalent cations also control the activity or gating properties of K(+) transporters at the mesophyll cell plasma membrane, thereby assisting in maintaining the high K/Na ratio required for optimal leaf photosynthesis.

Decrement and amplification of slow wave potentials during their propagation in Helianthus annuus L. shoots.

Slow wave potentials (SWPs) are transitory depolarizations occurring in response to treatments that result in a pressure increase in the xylem conduits (P(x)). Here SWPs are induced by excision of the root under water in 40- to 50-cm-tall light-grown sunflower plants in order to determine the effective signal range to a naturally sized pressure signal. The induced slow wave depolarization appears to move up the stem while it is progressively decremented (i.e. the amplitude decreases with increasing distance from the point of excision) with a rate that appears to rise acropetally from 2.5 to 5.5% cm(-1). The decline of the SWP signal, in both amplitude and range, could be experimentally increased (i) when root excision was carried out in air and (ii) when the transpiration of the sunflower shoot was minimized by a preceding removal or coating of the leaves. A further decline of the SW signal was expected to occur when leaves were included in the measured path. However, when the most distant apical electrode was attached to an upper leaf, it showed a considerably larger depolarization than a neighboring stem position. This apparent amplification of the SWP signal is not confined to the leaf blade but includes the petiole as well. The amplification disappeared (i) when the illumination level was lowered to room light, (ii) when the blade was excised either completely or along the remaining midvein and (iii) when the intact leaf blade was submersed in water. These treatments reduce the SWP at the petiole to a small fraction of the signal in the opposite control leaf and specify bright illumination and blade-mediated transpiration as prerequisites of a signal increase that is confined to young, expanding leaves.

A developmental gradient in the mechanism of K+ uptake during light-stimulated leaf growth in Nicotiana tabacum L.

Light causes growth of dicotyledonous leaves by stimulating proton efflux, cell wall acidification and loosening, and solute accumulation for turgor maintenance. For cells still undergoing cell division at the base of expanding tobacco ( Nicotiana tabacum L. cv. Xanthi) leaves, light-stimulated growth depends on K+ uptake, and is inhibited by the potassium channel blocker tetraethylammonium (TEA). The generality of this mechanism has been tested by comparing the effect of light on the growth-associated physiology of dividing and expanding cells in the base with cells at the tip growing by cell expansion only. The magnitude of the light-induced growth response of excised leaf discs is greatest at the leaf base and declines as cells mature. Basal tissue is more sensitive to exogenous potassium, which enhances light-stimulated growth at <1 mM, whereas tip tissue requires higher levels (>10 mM). Growth is inhibited by TEA similarly in tip and base. However, light-stimulated K+ uptake and proton efflux respond differently to TEA in tip and basal tissue. In basal tissue, TEA reduces light-stimulated K+ uptake by 60% and inhibits light-stimulated proton efflux. These results agree with those presented by M. Claussen et al. (1997, Planta 201:227-234) showing that auxin-stimulated H+ pump activity and growth in coleoptiles require K+ uptake as an electrical counterbalance to H+ efflux. In contrast, in tip tissue, TEA inhibits light-stimulated K+ uptake by only 17% and does not inhibit proton efflux. Our results suggest that the basipetal gradient in the effect of TEA on light-regulated growth physiology can be explained by TEA effects on K+ uptake: TEA inhibits light-stimulated H+ pump activity, wall acidification and membrane hyperpolarization only in cells dependent on TEA-sensitive channels for light-stimulated K+ uptake. Further, our data suggest that younger, basal tissue is dependent on TEA-sensitive, sucrose-stimulatable channels for light-stimulated K+ uptake whereas older, tip tissue is able to use an additional, TEA-insensitive K+ transporter to mediate light-stimulated K+ uptake.

Light interacts with auxin during leaf elongation and leaf angle development in young corn seedlings.

Modern corn ( Zea mays L.) varieties have been selected for their ability to maintain productivity in dense plantings. We have tested the possibility that the physiological consequence of the selection of the modern hybrid, 3394, for increased crop yield includes changes in responsiveness to auxin and light. Etiolated seedlings in the modern line are shorter than in an older hybrid, 307, since they produce shorter coleoptile, mesocotyl, and leaves (blade as well as sheath). Etiolated 3394 seedlings, as well as isolated mesocotyl and sheath segments, were less responsive to auxin and an inhibitor of polar auxin transport, N-1-naphthylphthalamic acid (NPA). Reduced response of 3394 to auxin was associated with less reduction of elongation growth by light (white, red, far-red, blue) than in 307, whereas the activity of polar auxin transport (PAT) and its reduction by red or far-red light was similar in both genotypes. NPA reduced PAT in etiolated 3394 seedlings much less than in 307. A characteristic feature of 3394 plants is more erect leaves. In both hybrids, light (white, red, blue) increases leaf declination from the vertical, whereas NPA reduces leaf declination in 307, but not in 3394. Our results support findings that auxin and PAT are involved in elongation growth of corn seedlings, and we show that light interacts with auxin or PAT in regulation of leaf declination. We hypothesize that, relative to 307, more erect leaves in the modern hybrid may be primarily a consequence of a reduced amount of auxin receptor(s) and reduced responsiveness to light in etiolated 3394 plants. The more erect leaves in 3394 may contribute to the tolerance of the modern corn hybrid to dense planting.

Light-regulated leaf expansion in two Populus species: dependence on developmentally controlled ion transport.

Leaf growth responses to light have been compared in two species of Populus, P. deltoides and P. trichocarpa. These species differ markedly in morphology, anatomy, and dependence on light during leaf expansion. Light stimulates the growth rate and acidification of cell walls in P. trichocarpa but not in P. deltoides, whereas leaves of P. deltoides maintain growth in the dark. Light-induced growth is promoted in P. deltoides when cells are provided 50-100 mM KCl. In both species, light initially depolarizes, then hyperpolarizes mesophyll plasma membranes. However, in the dark, the resting E(m) of mesophyll cells in P. deltoides, but not in P. trichocarpa, is relatively insensitive to decade changes in external [K+]. Results suggest that light-stimulated leaf growth depends on developmentally regulated cellular mechanisms controlling ion fluxes across the plasma membrane. These developmental differences underlie species-level differences in growth and physiological responses to the photoenvironment.