Predicting mechanisms across scales: amplified effects of abiotic constraints on the recruitment of yew Taxus baccata

Efforts to disentangle the mechanisms underlying large‐scale spatial patterns need to rely on multi‐scale approaches. We illustrate this key issue by analyzing the spatial consistency across scales of the effects of abiotic constraints on the regeneration of English yew Taxus baccata in Europe. We hypothesized that the recruitment rates in a given population would be strongly affected by water availability, which should result in a predictable pattern of regeneration success at regional and continental scales. Accordingly, we predicted: 1) at the regional scale water availability should be higher in sites occupied by yew populations than in random locations; 2) at the regional scale regeneration success should decrease when water availability is lower; and 3) at the continental scale, regeneration success should also decrease where water availability is lower, resulting in decreasing regeneration southwards. To test these predictions we first monitored seedling emergence and survival in two central Spanish populations over two years, and confirmed that yew recruitment is limited by water availability. Additionally, our analysis supported predictions 1 and 2: water availability strongly affected yew presence and regeneration success. At the continental scale (prediction 3), our results confirmed lower regeneration in southern European populations. Assessing the effect of climatic constraints across scales in key population parameters can help to improve large‐scale assessments of impacts of climate change on biodiversity.     

Post‐dispersal seed depletion by rodents in marginal populations of yew (Taxus baccata): consequences at geographical and local scales

Post‐dispersal seed predation is a crucial phenomenon for plant recruitment, and its incidence can be hypothesized to increase in ecologically and geographically marginal populations of threatened species, such as yew (Taxus baccata). Here we examine the among‐ and within‐population patterns of seed consumption by rodents and evaluate to what extent they are linked to marginality in Mediterranean low‐density yew stands. Among populations we tested: (i) whether the rates of seed predation found in our marginal sites were consistently higher than in populations from core regions; (ii) within populations we evaluated whether rodents preferred microhabitats with greater seed availability (beneath female yew trees) or with lower predation risk (shrubs) in two seeding seasons (fall–winter 2005 and 2006). Predation rates were extremely high (92.5%) and they were well above values reported for core populations (65.4%), to the extent that rodents almost completely depleted the experimental seeds in all populations and years. Our expectation of lower predation rates with decreasing vegetation cover was also confirmed for all years and populations, suggesting that rodent foraging was risk‐sensitive. This microhabitat effect outweighed the effect of seed availability under female yew trees, implying also that rodents selectively consumed the most valuable seeds in terms of their recruitment prospects. Overall, our results suggest that the mechanisms underlying seed depletion and its demographic consequences are linked to the effects of reduced yew performance in ecologically marginal habitats.     

Pollen limitation and fruit abortion in a declining rare tree,the Eurasian yew (Taxus baccata L.): A reproductive cost of ecological marginality

Populations of rare tree species such as the dioecious and anemophilous yew (Taxus baccata) are especially prone to extinction if they occupy marginal habitats. This is the case of yew populations growing in Mediterranean mountains under dry climate and a severe anthropogenic disturbance regime, which show insufficient regeneration. We examined two of the likely mechanisms driving this population decline. The study was conducted in a central Spanish yew population in 2005 and 2006. On one hand, we tested for the effect of the amount of pollen received by female flowers by means of a pollen addition experiment. On the other hand, we tested for the effect of initial fruit set on abortion and mature fruit production. Finally, we evaluated the effect of the spatial arrangement of male and female yew trees on realized fecundity. The success of fertilization and fruiting in the experimental population was pollen-limited. Female trees aborted ca. 70% of the fruit set in both years and treatments. Fruit set was significantly influenced by the females' neighbourhood, with fecundity decreasing exponentially with distance to the nearest male in both years. Overall, our results indicate that factors associated with ecological marginality have a strong influence on yew performance.     

Microsporogenesis in Taxus baccata L.: The Development of the archaesporium

An ultrastructural and cytochemical investigation of the developmentof the mierosporangium of Taxus baccata reveals many resemblancesto comparable events in other land plants, but also severaldistinctive features. During early development the archaesporiumcontains groups of degenerate cells which ultimately disappear,presumably by resorption. Scattered elsewhere in the cells ofthe archaesporium are highly ordered bodies, possibly crystal-likeformations of tubulin. The tapetum differentiates from the outerlayer of the archaesporium. Mitoses coincidentally cease tobe detectable within the sporangium and the sporogenous cellsbecome recognizable about three weeks before their nuclei entermeiosis. Although, as in other gymnosperms investigated, theearly development of the archaesporial cells is not synchronized,there is a tendency for synchrony to be acquired by the sporogenouscells and diad formation takes place more or less simultaneouslythroughout the sporangium. Measurements by microfluorometry of RNA levels during developmentof the archaesporium show a progressive rise in the RNA contentof the cells up to the beginning of meiosis. Microsporogenesis, Taxus baccata, archesporium     

Microsporogenesis in Taxus baccata L.: The Formation of the Tetrad and Development of the Microspores

Following meiosis II in Taxus microsporangia a small proportionof the tetrads regularly degenerated. Despite frequent inequalityin the frequency of ribosomes between the spores of a tetrad,partial degeneration within a tetrad was never observed. Theinitial wall of the young spores was found to resemble the wallof the mother cell in containing a fibrillar layer, and thetwo walls may possess similar isolating properties. The symmetryof the tetrad was regularly iso-bilateral. The formation ofthe sporoderm began as the spores were released into the loculusby the rapid dissolution of the wall of the mother cell. Osmiophilicdroplets emerged from the spore protoplast and entered the wall.The fibrillar layer ceased to be recognizable and the dropletscoalesced to form an outer layer on which up to six sporopolleninlamellae, probably of tapetal origin, were deposited. The accretionof a single layer of sporopollenin droplets, in no recognizablepattern, gave rise to the outer verrucose part of the exine.Cytochemical tests showed that the tapetum was rich in acidphosphatases from the beginning of meiosis. Towards the endof its degeneration the tapetum intruded into the loculus andcould therefore be regarded as partly invasive. Taxus baccata, microsporogenesis, tetrad symmetry, sporoderm     

Manipulation by culture mixing and elicitation of paclitaxel and baccatin III production in Taxus baccata suspension cultures

Summary Experiments were carried out with Taxus baccata cell lines showing different paclitaxel-producing capacities (between 1.74 and 19.91 mgl^−1) when growing in a selected product-formation medium that specifically stimulated the production of taxane to the detriment of cell growth. Through mixing low-, medial- and high-producing lines, it could be observed that paclitaxel productivity in the resulting mixed lines was clearly higher than the mean productivity of the individual lines before mixing. This suggests that culture components generated by high-producing individual lines within the population might induce paclitaxel production. Although the accumulation of paclitaxel and baccatin III was higher when 100 μM methyl jasmonate was added to the subcultures of the mixed lines, the results indicate that exogenously applied methyl jasmonate was not the first factor to stimulate taxane production. The possible effects of methyl jasmonate elicitation and paclitaxel accumulation on cell viability are also considered.     

Inhibition of paclitaxel and baccatin III accumulation by mevinolin and fosmidomycin in suspension cultures of Taxus baccata

To achieve a better understanding of the metabolism and accumulation of paclitaxel and baccatin III in cell cultures of Taxus, inhibitors of the early steps in the terpenoid pathway were applied to a cell suspension culture of Taxus baccata: fosmidomycin as an inhibitor of the non-mevalonate branch of the pathway, and mevinolin as an inhibitor of the mevalonate branch. Synthesis of both taxanes in the cell suspension was first increased when cultured in the product formation medium supplemented with methyljasmonate (100 microM). The product formation medium was selected after assaying 24 different culture media. When fosmidomycin (200 microM) was added to the product formation medium together with the elicitor, the accumulation of paclitaxel and baccatin III was reduced by up to 3.0 and 1.5 times, respectively, whereas the inhibitory effect of mevinolin (1 microM) was only clearly exerted in the case of paclitaxel. Under the conditions of our experiment, we conclude that in the synthesis of both taxanes, the non-mevalonate pathway is the main source of the universal terpenoid precursor isopentenyl diphosphate (IPP).     

Paclitaxel and baccatin III production induced by methyl jasmonate in free and immobilized cells of Taxus baccata

The effects of 100 and 200 μM methyl jasmonate (MJA) on cell proliferation and paclitaxel and baccatin III production were investigated in free and alginate immobilized cells of Taxus baccata growing in a selected product formation culture medium. The greatest accumulation of paclitaxel (13.20 mg dm⁻³) and baccatin III (4.62 mg dm⁻³) occurred when 100 μM MJA was added to the culture medium of cells entrapped using a 1.5 and 2.5 % alginate solution. The effects of different treatments on the viability of cultured cells and their capacity to excrete both taxanes into the surrounding medium were considered.     

斑块生境中啮齿类和蚁类动物对南方红豆杉地面种子的搬运

南方红豆杉(Taxus chinensis)在我国南方地区多分布在村落附近的斑块生境中,成熟后的种子直接落在母树下或被鸟类等动物搬运至其他斑块中。林下地面种子会被地面活动的动物搬运,继而影响种子命运及种群更新。于2016年、2017年南方红豆杉果期,在浙江天目山一个红豆杉种群分布地(临安市桐坑村),采用野外种子摆放实验的方法研究了动物对地面种子的搬运情况。结果表明:啮齿类动物取食是南方红豆杉地面种子消失的主要原因,但种子消失率在斑块间及边缘生境中的差异较大。母树林和竹林是啮齿类动物的主要觅食生境,两种生境的动物取食率明显高于山核桃种植园。边缘生境成为啮齿类斑块间移动的通道,而非觅食场所。4种啮齿类动物中,淡腹松鼠(Callosciurus pygerythrus)在母树林斑块以外的生境中出现频率最高,而其他地面搬运者回避利用山核桃种植园,仅在母树林和竹林斑块中搬运南方红豆杉地面种子。可见,斑块生境中动物改变了南方红豆杉地面种子的空间沉积格局,进而影响植物种群的更新。     

Facilitation by herbivore-mediated nurse plants in a threatened tree, Taxus baccata: local effects and landscape level consistency

We investigated the regeneration of a threatened tree, the yew Taxus baccata, in relation to the presence of fleshy‐fruited woody plants acting as seed dispersal foci as well as protecting yew recruits against ungulate herbivores. We seek to determine if local facilitative effects are consistent across landscape in the Cantabrian range (NW Spain). Yew seed rain by birds mostly concentrated under yew trees and beneath hollies Ilex aquifolium. Seedling emergence distributed similarly to seed rain, but first‐year seedling survival was higher beneath hollies. In one site where woody vegetation was structured as nucleation centres (multispecific patches of fleshy‐fruited plants acting as foci for seed rain) yew recruits mostly occurred in yew‐dominated centres, suggesting dispersers‐mediated facilitation. However, holly was the main nurse plant for most of these recruits, considering the nurse as the species whose canopy covered directly the yew recruit. Living beneath nurse plants reduced herbivore damage on saplings and enhanced seedling survival. A planting experiment with yew rooted‐cuttings beneath different spiny shrubs corroborated this effect. Additional evidence on yew recruitment limitation by herbivory emerged from one population where ungulates were fence‐excluded. Our results suggest that nurse plants mitigate the negative effect of herbivores on yew regeneration, by providing defence against browsing and trampling. Shelter ability related to nurse structure, cone‐shaped shrubs with branches at their bases acting better as a barrier. Paradoxically, this structure resulted from heavy browsing on nurse plants. The study of yew regeneration and habitat structure in seven sites provided evidence for the consistency of facilitation by holly at the landscape level, since local values of yew recruitment positively related to nurse ground cover. Range‐scale yew management must consider the local functioning of the interaction among avian seed‐dispersers, nurse fleshy‐fruited plants and ungulate herbivores, in combination with regional measures, targeting the habitats where facilitation emerges.     

Leaf Production and Morphology in the Artichoke Gall of Yew (Taxus baccata L.)

Comparative leaf production rates and leaf morphology studiesfor galled and normal shoots of yew trees have been obtainedthroughout the life cycle of the causative agent, the gall midge,Taxomyia taxi. Normal and galled shoot leaf numbers have beenrelated to those of their parent shoots. It was found that whereasthe annual leaf production of normal shoots was positively relatedto that of the parent shoots, galled shoot leaf production remainedconstant regardless of parent shoot vigour showing leaf stimulationby the midge to be a special case. The midge larva appears tobe determining the rate of leaf production in galled shoots.In galls leaf production continues throughout the winter monthswith no dormant period. From morphological evidence, alternationof leaves and cataphylls is continued in galled buds.     

Taxol production in bioreactors: Kinetics of biomass accumulation, nutrient uptake, and taxol production by cell suspensions of Taxus baccata

The kinetics of biomass accumulation, nutrient uptake and taxol production of Taxus baccata cell suspensions were examined in three bioreactor configurations, viz. 250-mL Erienmeyerflasks, 1-L working volume pneumatically mixed (PMB), and stirred tank (STB) bioreactors. Qualitatively similar kinetics were observed in all three bioreactor types. Biomass accumulation and specific nutrient uptake rates exhibited biphasic characteristics. Carbohydrate uptake and biomass accumulation substantially ceased when phosphate was depleted from the medium. Phosphate was identified as a possible growth-limiting nutrient. Taxol accumulated exclusively in the second phase of growth. A maximum taxol concentration of 1.5 mg/L was obtained in the PMB which was fivefold greater than that obtained in the Erienmeyer flasks and the STB, but the relative kinetics of taxol production was the same in all three reactor types. Biomass yields were calculated from the kinetic data and a stoichiometry for biomass formation was evaluated. The similarity of kinetics in the three bioreactor configurations suggests that taxol production by T. baccata cell suspensions is amenable to scateup. (c) 1995 John Wiley & Sons, Inc.     

Fruit consumption and seed dispersal by birds in native vs. ex situ individuals of the endangered Chinese yew,Taxus chinensis

The conservation success of endangered trees may depend on re-establishing or replacing the mutualisms that were important in their native habitats. In this study, we quantified avian frugivore diversity on individuals of the endangered Chinese yew (Taxus chinensis) in a botanical garden and at a natural site. We found that frugivore species diversity was lower in the botanical garden than in the natural site. In spite of the relatively low frugivore diversity, however, the ex-situ population of Chinese yew attracted similar disperser species to those in the natural population, and trees in the ex-situ population were visited more frequently than those in the natural population. Furthermore, the different perching behavior of dispersers resulted in different dispersal efficiencies. Although the Chinese bulbul (Pycnonotus sinensis) was the most common forager in the botanical garden, its dispersal efficiency was the lowest, and thus it could not like the role played by the mountain bulbul (Hypsipetes maclellandii) in the natural site. Only the red-billed blue magpie (Urocissa erythrorhyncha) provided a high-quality dispersal service in both sites. Our results highlight the ability of the Chinese yew to recruit seed dispersal agents in new habitats. However, if the newly recruited species is a low-quality disperser, the plants will depend more heavily on other avian vectors for regeneration.     

Expression of recombinant alpha and beta tubulins from the yew Taxus cuspidata and analysis of the microtubule assembly in the presence of taxol

Taxol was originally isolated from the yew Taxus brevifolia. Because taxol inhibits the depolymerization of microtubules, the presence of a self-resistance mechanism in Taxus spp. was hypothesized. The cloning of the cDNA for alpha and beta tubulins from Taxus cuspidata and those from the human embryonic kidney cell line HEK293T revealed that the ²⁶Asp, ³⁵⁹Arg, and ³⁶¹Leu residues in the human beta tubulin, which are important for taxol binding, were replaced with Glu, Trp, and Met in the beta tubulin of T. cuspidata, respectively. The microtubule assembly of the recombinant alpha and beta tubulins was monitored turbidimetrically, and the results clearly demonstrated that the microtubule from T. cuspidata is less sensitive to taxol than that from HEK293T cells. The Taxus microtubule composed of the wild-type alpha tubulin and the beta tubulin with the E26D mutation restored the sensitivity to taxol. We thus postulated that the mutation identified in the beta tubulin of T. cuspidata plays a role in the self-resistance of this species against taxol.     

Improved effects of polyethylene glycol on the growth,antioxidative enzymes activity and taxanes production in a Taxus baccata L. callus culture

Plant Cell, Tissue and Organ Culture (PCTOC) - Application of abiotic stress-inducing elicitors is one of the effective strategies in order to increase the production of secondary metabolites in...