Overcompensation and adaptive plasticity of apical dominance in Erysimum strictum (Brassicaceae) in response to simulated browsing and resource availability

In the cases where overcompensation has been observed in monocarpic herbs, overcompensation is associated with an apically dominant shoot architecture of intact plants, increased lateral branching following herbivory, and increased reproductive success as a consequence of damage. The compensatory continuum hypothesis expects overcompensation to be more prevalent in resource rich environments compared to poor environments. This is paradoxical since in resource rich conditions the intact plants should branch most vigorously and hence any further increase in branch number should lead to lower seed yield. An explanation could be that apical dominance is rather insensitive to changes in resource availability, and that overcompensation is possible in conditions where plants experience meristem limitation (due to apical dominance) in relation to available resources. We explored the branching patterns and fitness responses of tall wormseed mustard (Erysimum strictum) to simulated browsing, soil nutrients, and competition in common garden. Competition increased apical dominance and reduced plant fitness whereas fertilization had the reverse effects. Simulated browsing increased lateral branching and had little impact on plant fitness. Fitness overcompensation was observed only among plants grown in competition and in the absence of fertilization – the most resource poor treatment combination in the experiment. The results contradict both with the compensation continuum and the assumption that apical dominance shows no or very little plasticity in relation to growing conditions. Because directional selection gradients on branch number were invariantly positive irrespective of growing conditions, we propose that, in spite of phenotypic plasticity of apical dominance, the plants appear to be meristem rather than resource limited, and that meristem limitation is strongest in conditions where intact plants produce fewest lateral branches. Our results deviate from the compensation continuum because resource availability affected compensation ability more strongly through phenotypic plasticity of shoot architecture rather than via changes in resource availability per se.     

Overcompensation through the paternal component of fitness in Ipomopsis arizonica

Estimates of the effects of herbivory on plant fitness based on female fitness alone may be misleading if plants experience either reduced or increased male fitness. Because there are many plants that produce more flowers following herbivory where seed set is unaffected or reduced, total fitness may be enhanced through the paternal component alone. Here we show that herbivory results in an increase in reproductive success due solely to an increase in paternal fitness in the monocarpic biennial Ipomopsis arizonica. These results suggest that overcompensation may be more common than presently thought, requiring a reexamination of the fitness consequences of herbivory for many plant species.     

Regrowth following ungulate herbivory in Ipomopsis aggregata: geographic evidence for overcompensation

Studies were conducted on eight populations of scarlet gilia, Ipomopsis aggregata, across Colorado and in northern Arizona, to assess the fitness consequences of natural and simulated herbivory. To date, geographic studies have failed to incorporate treatment groups that included naturally browsed plants along with clipping treatments. The results presented here clearly demonstrate the importance of assessing, a priori, whether or not clipping experiments accurately reflect natural patterns of herbivory. Although because of the timing of the clip no evidence of overcompensation was found in any of the Colorado populations when ungulate herbivory was simulated in experimental clipping treatments, evidence for overcompensation was found in more than half the populations when plants that were naturally browsed and matched for size using root diameters were included early in the season. Matching plants for size based on root diameter late in the season would be problematic because root diameters significantly increased in size in all eight populations following high levels of ungulate herbivory and/or experimental clipping. Results from this study, and other recent studies on another biennial herb, the field gentian, clearly demonstrate that overcompensation is not only a real phenomenon but also is more widespread, both taxonomically and geographically, than previously thought. Received: 15 July 1998 / Accepted: 9 November 1998     

Comparative analysis of quantitative trait loci controlling glucosinolates,myrosinase and insect resistance in Arabidopsis thaliana

Evolutionary interactions among insect herbivores and plant chemical defenses have generated systems where plant compounds have opposing fitness consequences for host plants, depending on attack by various insect herbivores. This interplay complicates understanding of fitness costs and benefits of plant chemical defenses. We are studying the role of the glucosinolate-myrosinase chemical defense system in protecting Arabidopsis thaliana from specialist and generalist insect herbivory. We used two Arabidopsis recombinant inbred populations in which we had previously mapped QTL controlling variation in the glucosinolate-myrosinase system. In this study we mapped QTL controlling resistance to specialist (Plutella xylostella) and generalist (Trichoplusia ni) herbivores. We identified a number of QTL that are specific to one herbivore or the other, as well as a single QTL that controls resistance to both insects. Comparison of QTL for herbivory, glucosinolates, and myrosinase showed that T. ni herbivory is strongly deterred by higher glucosinolate levels, faster breakdown rates, and specific chemical structures. In contrast, P. xylostella herbivory is uncorrelated with variation in the glucosinolate-myrosinase system. This agrees with evolutionary theory stating that specialist insects may overcome host plant chemical defenses, whereas generalists will be sensitive to these same defenses.     

Overcompensation by plants: Herbivore optimization or red herring?

Summary The increased growth rates, higher total biomass, and increased seed production occasionally found in grazed or clipped plants are more accurately interpreted as the results of growth at one end of a spectrum of normal plant regrowth patterns, rather than as overcompensation, herbivore-stimulated growth, plantherbivore mutualisms, or herbivore enhanced fitness. Plants experience injury from a wide variety of sources besides herbivory, including fire, wind, freezing, heat, and trampling; rapid regrowth may have been selected for by any one of the many types of physical disturbance or extreme conditions that damage plant tissues, or by a combination of all of them. Rapid plant regrowth is more likely to have evolved as a strategy to reduce the negative impacts of all types of damage than as a strategy to increase fitness following herbivory above ungrazed levels. There is no evolutionary justification and little evidence to support the idea that plant-herbivore mutualisms are likely to evolve. Neither life history theory nor recent theoretical models provide plausible explanations for the benefits of herbivory.Several assumptions underlie all discussions of the benefits of herbivory: that plant species are able to evolve a strategy of depending on herbivores to increase their productivity and fitness; that herbivores do not preferentially regraze the overcompensating plants; that resources will be sufficient for regrowth; and that being larger is always better than being smaller. None of these assumptions is necessarily correct.     

The effects of herbivory on paternal fitness in scarlet gilia: better moms also make better pops

Abstract Whether plants can benefit from the direct effects of herbivory has been contentious among ecologists and evolutionary biologists. Although previous studies have provided experimental evidence of enhanced maternal fitness following herbivory in a natural system, an accurate depiction of plant-herbivore interactions must include the effects of herbivory on male as well as female fitness. Here we show that ungulate herbivory on scarlet gilia results in an increase in paternal as well as maternal fitness. This study represents the first evidence of overcompensation in a natural system where both paternal and maternal components of fitness have been assessed.     

Expression pattern of glucose metabolism genes in relation to development rate of buffalo (Bubalus bubalis) oocytes and in vitro–produced embryos

The expression pattern of glucose metabolism genes (hexokinase, phosphofructokinase, glucose-6-phosphate dehydrogenase [G6PDH], lactate dehydrogenase [LDH], and pyruvate dehydrogenase [PDH]) were studied in buffalo in vitro–matured oocytes and in vitro–produced embryos cultured under different glucose concentrations (0 mM, 1.5 mM, 5.6 mM, and 10 mM) during in vitro maturation of oocytes and culture of IVF produced embryos. The expression of the genes varied significantly over the cleavage stages under different glucose concentrations. Developmental rate of embryos was highest under a constant glucose level (5.6 mM) throughout during maturation of oocytes and embryo culture. Expression pattern of glucose metabolism genes under optimum glucose level (5.6 mM) indicated that glycolysis is the major pathway of glucose metabolism during oocyte maturation and early embryonic stages (pre-maternal to zygotic transition [MZT]) and shifts to oxidative phosphorylation during post-MZT stages in buffalo embryos. Higher glucose level (10 mM) caused abrupt changes in gene expression and resulted in shifting toward anaerobic metabolism of glucose during post-MZT stages. This resulted in decreased development rate of embryos during post-MZT stages. High expression of LDH and PDH in the control groups (0 mM glucose) indicated that in absence of glucose, embryos try to use available pyruvate and lactate sources, but succumb to handle the post-MZT energy requirement, resulting to poor development rate. Expression pattern of G6PDH during oocyte maturation as well early embryonic development was found predictive of quality and development competence of oocytes/ embryos.     

Intraspecific competition facilitates the evolution of tolerance to insect damage in the perennial plant Solanum carolinense

Tolerance to herbivory (the degree to which plants maintain fitness after damage) is a key component of plant defense, so understanding how natural selection and evolutionary constraints act on tolerance traits is important to general theories of plant–herbivore interactions. These factors may be affected by plant competition, which often interacts with damage to influence trait expression and fitness. However, few studies have manipulated competitor density to examine the evolutionary effects of competition on tolerance. In this study, we tested whether intraspecific competition affects four aspects of the evolution of tolerance to herbivory in the perennial plant Solanum carolinense: phenotypic expression, expression of genetic variation, the adaptive value of tolerance, and costs of tolerance. We manipulated insect damage and intraspecific competition for clonal lines of S. carolinense in a greenhouse experiment, and measured tolerance in terms of sexual and asexual fitness components. Compared to plants growing at low density, plants growing at high density had greater expression of and genetic variation in tolerance, and experienced greater fitness benefits from tolerance when damaged. Tolerance was not costly for plants growing at either density, and only plants growing at low density benefited from tolerance when undamaged, perhaps due to greater intrinsic growth rates of more tolerant genotypes. These results suggest that competition is likely to facilitate the evolution of tolerance in S. carolinense, and perhaps in other plants that regularly experience competition, while spatio-temporal variation in density may maintain genetic variation in tolerance.     

The effects of condensed tannins derived from senescing <Emphasis Type="Italic">Rhizophora mangle</Emphasis> leaves on carbon,nitrogen and phosphorus mineralization in a <Emphasis Type="Italic">Distichlis spicata</Emphasis> salt marsh soil

Background and aims

Low nitrogen negatively affects soil fertility and plant productivity. Glucose-6-phosphate dehydrogenase (G6PDH) and Epichloë gansuensis endophytes are two factors that are associated with tolerance of Achnatherum inebrians to abiotic stress. However, the possibility that E. gansuensis interacts with G6PDH in enhancing low nitrogen tolerance of host grasses has not been examined.

Methods

A. inebrians plants with (E+) and without E. gansuensis (E?) were subjected to different nitrogen concentration treatments (0.1, 1, and 7.5 mM). After 90 days, physiological studies were carried out to investigate the participation of G6PDH in the adaption of host plants to low nitrogen availability.

Results

Low nitrogen retarded the growth of A. inebrians. E+ plants had higher total dry weight, chlorophyll a and b contents, net photosynthesis rate, G6PDH activity, and GSH content, while having lower plasma membrane (PM) NADPH oxidase activity, NADPH/NADP⁺ ratios, and MDA and H₂O₂ than in E? A. inebrians plants under low nitrogen concentration.

Conclusions

The presence of E. gansuensis played a key role in maintaining the growth of the A. inebrians plants under low nitrogen concentration by regulating G6PDH activity and the NADPH/NADP⁺ ratio and improving net photosynthesis rate.

     

高等植物葡萄糖-6-磷酸脱氢酶与6-磷酸葡萄糖酸脱氢酶基因的不同进化起源

葡萄糖-6-磷酸脱氢酶与6-磷酸葡萄糖酸脱氢酶是植物戊糖磷酸途径中的两个酶.在克隆了水稻质体葡萄糖-6-磷酸脱氢酶基因OsG6PDH2和质体6-磷酸葡萄糖脱氢酶基因Os6PGDH2基础上,分析比较了水稻胞质和质体葡萄糖-6-磷酸脱氢酶基因和6-磷酸葡萄糖酸脱氢酶基因的基因结构、表达特性和进化地位.结合双子叶模式植物拟南芥两种酶基因的分析结果,认为高等植物葡萄糖-6-磷酸脱氢酶基因和6-磷酸葡萄糖酸脱氢酶基因在进化方式上截然不同,葡萄糖-6-磷酸脱氢酶的胞质基因与动物和真菌等真核生物具有共同的祖先;6-磷酸葡萄糖酸脱氢酶的胞质酶和质体酶基因都起源于原核生物的内共生.讨论了植物葡萄糖-6-磷酸脱氢酶与6-磷酸葡萄糖酸脱氢酶基因可能的进化模式,为高等植物及质体的进化起源提供了新的资料.     

高等植物葡萄糖-6- 磷酸脱氢酶与6- 磷酸葡萄糖酸脱氢酶基因的不同进化起源

葡萄糖-6-磷酸脱氢酶与6-磷酸葡萄糖酸脱氢酶是植物戊糖磷酸途径中的两个关键酶。在克隆了水稻质体葡萄糖-6-磷酸脱氢酶基因OsG6PDH2和质体6-磷酸葡萄糖脱氢酶基因Os6PGDH2基础上,分析比较了水稻胞质和质体葡萄糖-6-磷酸脱氢酶基因和6-磷酸葡萄糖酸脱氢酶基因的基因结构、表达特性和进化地位。结合双子叶模式植物拟南芥两种酶基因的分析结果,认为高等植物葡萄糖-6-磷酸脱氢酶基因和6-磷酸葡萄糖酸脱氢酶基因在进化方式上截然不同,葡萄糖-6-磷酸脱氢酶的胞质基因与动物和真菌等真核生物具有共同的祖先;6-磷酸葡萄糖酸脱氢酶的胞质酶和质体酶基因都起源于原核生物的内共生。讨论了植物葡萄糖-6-磷酸脱氢酶与6-磷酸葡萄糖酸脱氢酶基因可能的进化模式,为高等植物及质体的进化起源提供了新的资料。     

Improved oxytetracycline production in Streptomyces rimosus M4018 by metabolic engineering of the G6PDH gene in the pentose phosphate pathway

The aromatic polyketide antibiotic, oxytetracycline (OTC), is produced by Streptomyces rimosus as an important secondary metabolite. High level production of antibiotics in Streptomycetes requires precursors and cofactors which are derived from primary metabolism; therefore it is exigent to engineer the primary metabolism. This has been demonstrated by targeting a key enzyme in the oxidative pentose phosphate pathway (PPP) and nicotinamide adenine dinucleotide phosphate (NADPH) generation, glucose-6-phosphate dehydrogenase (G6PDH), which is encoded by zwf1 and zwf2. Disruption of zwf1 or zwf2 resulted in a higher production of OTC. The disrupted strain had an increased carbon flux through glycolysis and a decreased carbon flux through PPP, as measured by the enzyme activities of G6PDH and phosphoglucose isomerase (PGI), and by the levels of ATP, which establishes G6PDH as a key player in determining carbon flux distribution. The increased production of OTC appeared to be largely due to the generation of more malonyl-CoA, one of the OTC precursors, as observed in the disrupted mutants. We have studied the effect of zwf modification on metabolite levels, gene expression, and secondary metabolite production to gain greater insight into flux distribution and the link between the fluxes in the primary and secondary metabolisms.     

Herbivory Differentially Affects Plant Fitness in Three Populations of the Perennial Herb Lythrum salicaria along a Latitudinal Gradient

Herbivory can negatively and selectively affect plant fitness by reducing growth, survival and reproductive output, thereby influencing plant population dynamics and evolution. Latitudinal variation in intensity of herbivory is common, but the extent to which it translates into corresponding variation in effects on plant performance is still poorly known. We tested the hypothesis that variation in the fitness-consequences of herbivory mirror differences in intensity of herbivory among three natural populations of the perennial herb Lythrum salicaria along a latitudinal gradient from southern to northernmost Sweden. We documented intensity of herbivory and examined its effect on survival, growth and reproductive output over two years by experimentally removing herbivores with insecticide. The intensity of herbivory and the effects of herbivory on plant fitness were strongest in the southern population, intermediate in the central population and weakest in the northern population. The mean proportion of the leaf area removed ranged from 11% in the southern to 3% in the northern population. Herbivore removal increased plant height 1.5-fold in the southern and 1.2-fold in the central population, the proportion plants flowering 4-fold in the southern and 2-fold in the central population, and seed production per flower 1.6-fold in the southern and 1.2-fold in the central population, but did not affect plant fitness in the northern population. Herbivore removal thus affected the relative fecundity of plants in the three populations: In the control, seed output per plant was 8.6 times higher in the northern population compared to the southern population, whereas after herbivore removal it was 2.5 times higher in the southern population. The results demonstrate that native herbivores may strongly affect the demographic structure of L. salicaria populations and thereby shape geographic patterns of seed production. They further suggest that the strength of herbivore-mediated selection varies among populations and decreases towards the north.     

How plants may benefit from their consumers: leaf-cutting ants indirectly improve anti-herbivore defenses in <Emphasis Type="Italic">Carduus nutans</Emphasis> L

Although herbivores often have a negative impact on plant fitness, sometimes plants may benefit from their consumers. However, these positive interactions usually occur as a result of plant damage (e.g., overcompensation, defense induction). I present evidence of a novel way by which plants may benefit from their consumers without being eaten. Plants of Carduus nutans increased their physical defenses when grown in external refuse dumps of the leaf-cutting ant Acromyrmex lobicornis. Seedlings planted in refuse exhibited longer spines and tougher leaves than those planted in control soils. Pick-up assays with entire leaves and leaf discs demonstrated that these enhanced physical defenses prevented leaf-cutting ant harvest. Additionally, plants established in refuse dumps showed fewer insect herbivory than those in non-nest soils. The nutrient-rich refuse dump appeared to reduce the stage at which leaves are tender and thus more vulnerable to herbivory. This is the first case where plants may benefit from insect herbivores via waste products without the cost of being eaten. This illustrates how plants may plastically respond to reliable cues of the risk of herbivory.