Is there leaderless protein secretion in plants?

The sugar alcohol mannitol and it’s catabolic enzyme mannitol dehydrogenase (MTD), in addition to welldocumented roles in metabolism and osmoprotection, may play roles in hostpathogen interactions. Research suggests that in response to the mannitol that pathogenic fungi secrete to suppress reactive oxygen-mediated host defenses, plants make MTD to catabolize fungal mannitol. Yet previous work suggested that pathogen-secreted mannitol is extracellular, while in healthy plants MTD is cytoplasmic. We have presented results showing that the normally cytoplasmic MTD is exported into the cell wall or extracellular space in response to the endogenous inducer of plant defense responses salicylic acid (SA). This SA-induced secretion is insensitive to brefeldin A, an inhibitor of Golgimediated protein transport. Together with the absence of MTD in Golgi stacks and the lack of a documented extracellular targeting sequence in the MTD protein, this suggests MTD is secreted by a non-Golgi, pathogen-activated secretion mechanism in plants. Here we discuss the potential significance of non-Golgi secretion in response to stress.Key words: protein secretion, mannitol metabolism, plant-pathogen interaction, extracellular space, apoplast     

Is increased male flower production a strategy for avoidance of predispersal seed predation in andromonoecious plants?

Floral gender in angiosperms often varies within and among populations. We conducted a field survey to test how predispersal seed predation affects sex allocation in an andromonoecious alpine herb Peucedanum multivittatum. We compared plant size, male and perfect flower production, fruit set, and seed predation rate over three years among nine populations inhabiting diverse snowmelt conditions in alpine meadows. Flowering period of individual populations varied from mid‐July to late August reflecting the snowmelt time. Although perfect flower and fruit productions increased with plant size, size dependency of male flower production was less clear. The number of male flowers was larger in the early‐flowering populations, while the number of perfect flowers increased in the late‐flowering populations. Thus, male‐biased sex allocation was common in the early‐flowering populations. Fruit‐set rates varied among populations and between years, irrespective of flowering period. Fruit‐set success of individual plants increased with perfect flower number, but independent of male flower number. Seed predation by lepidopteran larvae was intense in the early‐flowering populations, whereas predation damage was absent in the late‐flowering populations, reflecting the extent of phenological matching between flowering time of host plants and oviposition period of predator moths. Seed predation rate was independent of male and perfect flower numbers of individual plants. Thus, seed predation is a stochastic event in each population. There was a clear correlation between the proportion of male flowers and the intensity of seed predation among populations. These results suggest that male‐biased sex allocation could be a strategy to reduce seed predation damage but maintain the effort as a pollen donor under intensive seed predation.     

Why are adaptations for long-range seed dispersal rare in desert plants?

Summary The rarity of long-range seed dispersal (telechory) and commonness of antitelechory in desert plants are examined in light of contemporary mathematical theories of the evolution of dispersal and germination behaviors. Analysis of dispersal 3-habitat relationships in the flora of Israel supports the general trend towards atelechory in deserts; in particular epizoochory and tumbleweeds are practically absent from the desert and heterocarpy is centered in the Mediterranean region. In contradiction to the accepted mother-site theory, we find that (a) there is a high turnover in microscale spatial pattern among antitelechoric species; (b) antitelechoric (especially basicarpic) species are widespread and dominant in the desert vegetation of Israel; (c) amphicary and geocary are rare in the desert flora of Israel.We argue that the openness of desert vegetation and the patterns of climatic variation favor atelechory while antitelechory is generally a side-effect of mechanisms whose adaptive value is not directly related to dispersal. Thus for example the desert plants of Israel have evolved a variety of dispersal-restricting seed-containers that protect the seed from predation and flooding, regulate the within-season timing of germination, and spread dispersal and germination over several years.Dedicated to Professor Dr. Michael Evenari     

Is hexokinase really a sugar sensor in plants?

The molecular mechanisms by which plant cells sense sugar levels are not understood, but current models (adapted from models for sugar sensing in yeast) favour hexokinase as the primary sugar sensor. However, the hypothesis that yeast hexokinase has a signalling function has not been supported by more recent studies and the idea that hexokinase is involved in sugar sensing in plants has yet to be proven.     

Is cell polarity under mechanical control in plants?

Plant cells experience a tremendous amount of mechanical stress caused by turgor pressure. Because cells are glued to their neighbors by the middle lamella, supracellular patterns of physical forces are emerging during growth, usually leading to tension in the epidermis. Cortical microtubules have been shown to reorient in response to these mechanical stresses, and to resist them, indirectly via their impact on the anisotropic structure of the cell wall. In a recent study, we show that the polar localization of the auxin efflux carrier PIN1 can also be under the control of physical forces, thus linking cell growth rate and anisotropy by a common mechanical signal. Because of the known impact of auxin on the stiffness of the cell wall, this suggests that the mechanical properties of the extracellular matrix play a crucial signaling role in morphogenesis, notably controlling the polarity of the cell, as observed in animal systems.Key words: development, growth, auxin, microtubule, PIN1, stiffness, cell wall, biophysics, meristemThe current development of high throughput analyses of gene regulatory networks is feeding a very complex view of growth control and shape changes. To go beyond the accumulation of data, the identification of universal and parsimonious mechanisms explaining the robustness of morphogenesis becomes a central issue in today''s developmental biology.^1–3 Among them, the coupling between molecular and mechanical signals has the strong advantage of providing a simple way to coordinate cell behavior synchronously and over long distances. The role of such signals has been investigated in different systems and the contribution of mechanical forces to animal development is now widely accepted, as the expression of key genes (e.g., TWIST⁴) and key cellular events (e.g., mitotic spindle orientation⁵) have been shown to depend on the mechanical environment of the tissue. Several mechanosensors have also been identified.⁶In an earlier study, we showed that the orientation of the cortical microtubular cytoskeleton in plant shoot meristems depends on the principal direction of mechanical stress. Cortical microtubules are known to guide the deposition of the cellulose microfibrils in the cell wall and thus to control the main direction of growth, and consequently, shape. Evidence indicates that the epidermis is under tension, and therefore the shape of the tissue can influence the pattern of mechanical stress. In this framework, multicellular shape is transposed into a map of stress directions in the epidermis that can act as a supracellular instructional signal. By applying mechanical constraints on a meristem with GFP-marked microtubules, we were able to close the feedback loop: microtubule orientation became parallel to the externally applied stress, supporting a view in which mechanical stress controls cell behavior.⁷In a more recent study we showed that in addition to the cortical microtubules, the polar localization of the auxin efflux carrier PIN1 can also be controlled by its mechanical environment. In particular, we observed that, when viewed from the top, PIN1 is usually concentrated on the membrane that is parallel to the microtubule orientation. Furthermore, a single cell ablation, which induces both a circumferential pattern of stress around the wound and a circumferential orientation of microtubules, also induced a relocalization of PIN1 away from the wound on the circumferential membrane, consistent with the hypothesis that PIN1 would be preferentially recruited on the membrane undergoing the most tensile stress.⁸ Mechanistically, it is unclear how this could be achieved, but the PIN1 vesicle recycling machinery is likely to play a major role, since it is now well established that membrane tension inhibits endocytosis and favors exocytosis.⁹ In such a scenario, PIN1 would be trapped in a membrane as long as the tension of the membrane is higher than that of its neighbours.To further test the response of PIN1 to mechanical forces, we used a pharmacological approach. Figure 1 highlights the correlation between the predicted opposable impacts of isoxaben and oryzalin on stress and the response of PIN1. In the presence of isoxaben, a well known inhibitor of cellulose synthesis, the thickness of the cell wall is supposed to decrease. Knowing that mechanical stress is here defined as a force divided by the area of a section of the wall, stress is expected to increase after isoxaben treatment. When we treated PIN1-GFP meristems with isoxaben, we observed a “hyper” localization of PIN1, with in most cases a preferential localization of PIN1 along the supracellular stress patterns, and within the cell, a concentration of the signal at cell corners, predicted sites of stress maxima. In contrast, in the presence of oryzalin, which by depolymerising the microtubules leads to isotropic growth and thus isotropic stresses, PIN1 localization became more homogeneous.Open in a separate windowFigure 1Impact of isoxaben and oryzalin on the localization of PIN1 in meristematic cells. The PIN1-GFP signal (in black) is very heterogenous in the control meristematic cells, consistent with the preferential localization of PIN1 to one side of the cells. Sometimes the signal is even restricted to one cell corner. After microtubule depolymerization with oryzalin, cell growth becomes more isotropic, and while PIN1 localization remains heterogenous, the signal becomes more widespread on each plasma membranes and thus tends to homogeneity. In contrast, after isoxaben treatment (which inhibits cellulose synthesis and thus is predicted to increase stress levels), the PIN1-GFP protein concentrates at the corners of the cells.⁸It seems therefore plausible that mechanical stress acts as a common instructional signal for both microtubule-dependent cell anisotropy and PIN1/auxin-dependent growth rate. Mathematical modeling further supported this proposal. Several successful models for the generation of organ patterns in the meristem assume an ability of individual cells to sense auxin concentration in their neighbours.^10–16 However to date no mechanism had been proposed to explain how one cell could measure the concentration of auxin in its vicinity. One of the main implications of our study is that, if PIN1 can respond to the mechanical status of the wall, then it also integrates auxin concentration of the neighboring cells, indirectly, as auxin loosens the cell wall, allowing cell expansion. Using such a hypothesis, computer simulations were able to reproduce the stereotyped pattern of organogenesis in the shoot further confirming the plausibility of the model.It must be noted however that our work does not exclude other hypotheses. In particular, it has recently been proposed that the ROP2 and ROP6 proteins, well known effectors of cell polarity, could respond differently to ABP1-dependent auxin signaling, thus providing a model in which cell-cell communication via ROP could “measure” local differences in auxin between neighbors.¹⁷ These different scenarios could actually be reconciled some day, especially knowing that Rho proteins in animals have been involved in the responses to mechanical forces.¹⁸ Last, the control of PIN1 polar localization by its mechanical environment could actually reveal a more universal response of cells to the stiffness and tension of the extracellular matrix. Similarly, animal motile (and polar) cells can sense the rigidity of their substrate^19–21 and respond by reinforcing the cytoskeleton at the cell cortex.^22–25     

Fertilization in plants: Is calcium a key player?

For many years, the physiological significance of Ca(2+) oscillations has been a matter of debate, but the potential to encode and transduce information in the pattern of an oscillation is obvious. In this review, we only consider transients and oscillations observed during fertilization in plants with the major focus on flowering plants. After presenting data related to algae, fertilization mechanisms in flowering plants are defined as a multi-step phenomenon, starting with pollination during which calcium plays a key role, especially during pollen-stigma interactions (compatible and incompatible reactions). The pollen tube serves as a guide and a pathway for the sperm cells on their course towards their female target cells. For many years, the pollen tube has also been studied as an easily accessible in vitro model to elucidate the role of calcium on tip growth. Finally, in flowering plants, a unique double fertilization system is present. Interesting data obtained from an in vitro fertilization system in maize are presented and discussed. In addition, the new approaches made possible by Arabidopsis and Torenia and their potential limitations are covered.     

Are correlations among foliar traits in ferns consistent with those in the seed plants?

Broad-based studies of gymnosperms and angiosperms reveal consistent and functionally significant correlations among foliar traits such as leaf mass per area (LMA), maximum photosynthetic rate (A(area)), foliar nitrogen (N(area)), foliar chlorophyll (Chl) and leaf longevity. To assess the generality of these relationships, we studied 20 fern species growing in the understorey of a temperate deciduous forest. We found that foliar N(area) increases with LMA, and that foliar N(area) and A(area) are positively correlated with one another, as are foliar N(area) and Chl. The ferns in general have very low LMA compared with most seed plants; A(area), N(area) and Chl are below median values for seed plants but are not extreme. Species with overwintering fronds have significantly higher LMA than species with fronds that senesce at the end of the growing season, as well as a significantly higher C : N ratio in frond tissue and relatively high foliar N on an areal basis. Correlations among foliar traits associated with gas exchange in these forest understorey ferns are in accordance with patterns reported for seed plants, suggesting a high degree of functional constraint on the interrelationships among key elements in foliar design.     

Acoustic and magnetic communication in plants: Is it possible?

Over the last two decades, important insights into our understanding of plant ecology and the communicative nature of plants have not only confirmed the existence of a wide range of communication means used by plants, but most excitingly have indicated that more modalities remain to be discovered. In fact, we have recently found that seeds and seedlings of the chili plant, Capsicum annuum, are able to sense neighbors and identify relatives using alternative mechanisms beyond previously studied channels of plant communication. In this addendum, we offer a hypothetical mechanistic explanation as to how plants may do this by quantum-assisted magnetic and/or acoustic sensing and signaling. If proven correct, this hypothesis prompts for a re-interpretation of our current understanding of plasticity in germination and growth of plants and more generally, calls for developing a new perspective of these biological phenomena.     

Assembly rules operate only in equilibrium communities: Is it true?

Very little is known of how disturbance affects community assembly rules. We examine this in three disturbance states in each of two ski areas on southern New Zealand mountains. Theory suggests that a community will become progressively more spatially organized during recovery from disturbance. Firstly, different patches of the community should become more similar through time, but this was seen in only one of the two areas and even then only examining species presence/absence. Secondly, it has been suggested that spatial autocorrelation will be stronger in less‐disturbed conditions, that is, there will be a stronger pattern of more distant patches being more dissimilar in species composition. This was generally borne out. However, the method indicated more point randomness in less‐disturbed sites. Assembly rules might be seen in species abundances. Previous work has found maximum evenness of abundances in later successional communities, but the pattern here was the opposite: high evenness in the most disturbed communities. The literature suggests that in undisturbed communities the distribution of species abundances (relative abundance distribution) will be general lognormal, and we further argue that the identity of the species across occupying rank positions in that distribution should be more consistent (rank consistency). Both predictions were borne out in one area, but neither in the other. Many workers suggest that niche‐based assembly rules will be stronger in undisturbed communities. However, there was only weak evidence of constancy in species richness. Local species assemblages tended to contain a relatively constant representation from different morphological/taxonomic guilds (guild proportionality) and this was significant in some tests, but contrary to theory this effect occurred mainly in the most disturbed sites. It is concluded that there is only limited truth in the frequent assumption that community structure is stronger in undisturbed, equilibrium communities.     

Is fire a selective force of seed size in pine species?

Fire is selectively shaping most of the traits of plants growing in fire-prone environments. However, seed size and other features related to seed production have not been studied in the light of the evolutionary role of fire. Our research tests the hypothesis that larger seeds have a higher chance of surviving wildfires and produce more vigorous seedlings with a lower death rate. To test this hypothesis the germination and early seedling growth of five Spanish pine species were studied. Weight, length and width of all seeds were measured. The biomass (fresh and dry weight) and length (root and total) of subsequent seedlings were also measured after 30 d from emergence. Seeds were submitted to elevated temperatures for periods in which the chance of survival was 50 % (calculated by means of a logistic model for each pine species). The differences observed among species suggests that fire may be adaptively shaping seed size in pines with larger seeds (Pinus canariensis and P. pinaster), because larger seeds are more likely to survive after heat shocks. Furthermore, in P. canariensis, seedlings after heat treatment are even larger than those submitted to control. In P. halepensis, despite being well adapted to fire, our results indicated no relationships between fire and seed characteristics. Finally, although heat treatment has a general adverse effect on seedling growth in the case of the two subalpine pines, we have detected a positive relationship between seed size and seedling growth but only in the largest seeds. This might also suggest the relevance of fire as a selective force for these pines which is outperformed by the relevance of dispersal and emergence time as adaptive traits in the post-fire scenario.     

Is trehalose-6-phosphate a regulator of sugar metabolism in plants?

It has recently emerged that many higher plants can synthesize trace amounts of trehalose. In arabidopsis disruption of the first step of trehalose synthesis, catalysed by trehalose-6-phosphate synthase (TPS), has lethal consequences, demonstrating an important physiological role. It is not yet clear what the precise function of trehalose synthesis is, but there is mounting evidence that trehalose-6-phosphate is implicated in the regulation of sugar metabolism. Further work is necessary to confirm this hypothesis and determine the underlying mechanism.