N content of phloem and xylem exudates during the transition to flowering in Sinapis alba and Arabidopsis thaliana

The involvement of nitrogenous substances in the transition to flowering was investigated in Sinapis alba and Arabidopsis thaliana (Columbia). Both species grown in short days (SD) are induced to flower by one long day (LD). In S. alba, the phloem sap (leaf and apical exudates) and the xylem sap (root exudate) were analysed in LD versus SD. In A. thaliana, only the leaf exudate could be analysed but an alternative system for inducing flowering without day‐length extension was used: the displaced SD (DSD). Significant results are: (i) in both species, the leaf exudate was enriched in Gln during the inductive LD, at a time compatible with export of the floral stimulus; (ii) in S. alba, the root export of amino acids decreased in LD, whereas the nitrate remained unchanged – thus the extra‐Gln found in the leaf exudate should originate from the leaves; (iii) extra‐Gln was also found very early in the apical exudate of S. alba in LD, together with more Glu; (iv) in A. thaliana induced by one DSD, the leaf export of Asn increased sharply, instead of Gln in LD. This agrees with Asn prevalence in C‐limited plants. The putative role of amino acids in the transition to flowering is discussed.     

Effects of experimental manipulation of inflorescence size on pollination and pre-dispersal seed predation in the hummingbird-pollinated plant Ipomopsis aggregata

Large floral displays should theoretically provide advantages to plants through increased pollinator visitation and resulting fruit and seed set. However empirical tests of the response of pollinators to floral display size have been limited by a lack of direct experimentation, and the results of such studies have been equivocal. In addition, other selective agents such as pre-dispersal seed predators might modulate effects of floral display on pollination. By artificially altering flower number, we examined the direct effects of floral display in the monocarpic herb, Ipomopsis aggregata (Polemoniaceae), on visitation rates by broad-tailed and rufous hummingbird pollinators, as well destruction of fruits by a pre-dispersal seed predator (Hylemya: Anthomyiidae). In addition, we quantified the ultimate effects of flower number on female reproductive success. Plants with larger floral displays were most likely to be visited first in any given foraging bout (P < 0.01). As expected, plants with more flowers received more total flower visits. However, we found no gain in the proportion of flowers visited for many- versus few-flowered plants, or the total number of approaches/hour. In fact, a significantly greater percentage of flowers were visited on few-flowered plants. Plants did not compensate for our reduction in flowers by increasing investment in the number or proportion of flowers that set fruit, the number of seeds/fruit, or seed weight. Pre-dispersal seed predation was greater for many- than for few-flowered plants (P < 0.001), but this did not offset the potential fitness gains of producing large displays. Our data support the hypothesis that large floral displays function primarily in long-distance attraction of pollinators, and enhance maternal success. Received: 21 March 1996 / Accepted: 24 October 1996     

Wind-induced mechanical stimulation increases pest resistance in common bean

The activation of the phenylpropanoid pathway in plants by environmental stimuli is one of the most universal biochemical stress responses known. Induction of enzymes such as phenylalanine ammonia-lyase and peroxidase and the accumulation of such phenolics as lignin can occur in response to insect and pathogen attack, exposure to oxidizing pollutants, and mechanical stimulation, and are thought to function in the resistance of plants to damage by these stresses. I investigated whether induction of components of this generalized stress response by wind-induced mechanical stimulation could influence the resistance to pests of common bean. In greenhouse studies, exposure of 7- to 10-day-old bean seedlings to daily periods of fan-produced wind led to increased activities of peroxidase and cinnamyl alcohol-dehydrogenase and enhanced the accumulation of lignin in primary leaves of these plants. Egg production and population growth of two-spotted spider mites were reduced when offered leaves of mechanically-stimulated plants in leaf-disk and whole-plant bioassays. Infection by anthracnose after inoculation in a detached-leaf bioassay was also reduced in leaves of mechanically-stimulated plants. The consistent positive association between the enhanced activity of the lignin branch of the phenylpropanoid pathway and enhanced resistance to pests found in leaves of mechanically-stimulated plants illustrates one way in which exposure of plants to environmental stimuli that activate a generalized stress response (e.g., wind) can influence the interactions of those plants with other environmental stimuli (e.g., pests). Received: 1 October 1996 / Accepted: 23 January 1997     

The Arabidopsis sex1 mutant is defective in the R1 protein, a general regulator of starch degradation in plants, and not in the chloroplast hexose transporter

Starch is the major storage carbohydrate in higher plants and of considerable importance for the human diet and for numerous technical applications. In addition, starch can be accumulated transiently in chloroplasts as a temporary deposit of carbohydrates during ongoing photosynthesis. This transitory starch has to be mobilized during the subsequent dark period. Mutants defective in starch mobilization are characterized by high starch contents in leaves after prolonged periods of darkness and therefore are termed starch excess (sex) mutants. Here we describe the molecular characterization of the Arabidopsis sex1 mutant that has been proposed to be defective in the export of glucose resulting from hydrolytic starch breakdown. The mutated gene in sex1 was cloned using a map-based cloning approach. By complementation of the mutant, immunological analysis, and analysis of starch phosphorylation, we show that sex1 is defective in the Arabidopsis homolog of the R1 protein and not in the hexose transporter. We propose that the SEX1 protein (R1) functions as an overall regulator of starch mobilization by controlling the phosphate content of starch.     

Non-reversion of Impatiens in the absence of meristem commitment

Purple-flowered plants of Impatiens balsamina maintained floral development on transfer from inductive short days (SD) to long days (LD), a treatment in which red-flowered plants of Impatiens are known to revert to leaf production. An investigation into the non-reverting nature of purple-flowered plants was carried out to establish whether these plants achieved meristem commitment or whether their non-reverting state was controlled by the leaves. When the leaves that had unfolded during the inductive SD treatment were removed at the time of transfer to LD, the purple-flowered plant did revert. This result suggests that, as in red-flowered Impatiens, meristem commitment is absent, but that purple-flowered plants maintain flowering in LD conditions because of a more permanent supply of signal from their leaves than occurs in red-flowered plants. A working hypothesis is proposed to explain how a signal from the leaves can retain a controlling role during flower development.Key words: Floral commitment, Impatiens, floral reversion, floricaula.      

Growth, herbivory and disease in relation to gender in Salix viminalis L.

Many species of dioecious plants show sex-related differences in growth rate and rates of attack by various herbivores and diseases. The common pattern is for males to grow faster than females and to be less well defended against herbivores. In willows (Salix spp.), the predominance of female-biased sex ratios has been ascribed in part to differential feeding by herbivores. In this study of Salix viminalis, seven families grown on agricultural land showed no gender-related variation in shoot biomass or rates of herbivory by insects (lepidopterans and cecidomyiids). However, Melampsora rust disease was found to be more severe on females than on males when the plants were in a non-reproductive stage. After flowering and seed-set females tended to be more affected in some families but less affected in others. Although, on average, there was a female bias in the sex ratio of S. viminalis, sex ratios differed significantly between families. These ratios were not related to any of the recorded biotic agents, but rather to relationships between families. These results are interpreted in terms of resource allocation between reproduction, growth and defence, and causes for divergence from the expected patterns are discussed. The results may have implications for S. viminalis breeding strategies where the aim is to produce biofuel. For instance, these findings suggest that gender can be ignored when selecting for a high growth rate and resistance to Melampsora and certain insect pests. Received: 2 November 1996 / Accepted: 8 February 1997     

Sources of variation in floral nectar production rate in Epilobium canum (Onagraceae): implications for natural selection

Sources of variation in floral nectar production were investigated in a natural population of Epilobium canum (Onagraceae), a hummingbird-pollinated herbaceous shrub. Field measurements showed significant phenotypic variation among plants in floral nectar production rates. Average variance among flowers within plants was approximately one-third to one-half as great as variance among plants, with coefficients of variation among flowers ranging from 6.5% to 116.7%. A greenhouse experiment using clonally propagated ramets from field plants showed significant genetic variation for nectar production rates; broad sense heritability was estimated to have a maximum value of 0.64. In the greenhouse, plants grown under low water or low light conditions produced approximately 25% less nectar on average than those grown under control conditions. However, significant genotype-environment interactions indicated that genets differed in their responses to the changes in conditions. Rank correlations for genet mean nectar production rates across environmental conditions were low, and in two out of three comparisons were not different from zero. It is concluded that although the opportunity for natural selection on nectar production rates exits in this population, the response to selection will likely be slow, and the opportunity for selection of a narrow-optimum nectar production phenotype may be limited. Received: 9 January 1996 / Accepted: 18 December 1996     

Genetic evidence of a coupling role for the TraG protein family in bacterial conjugation

The ability of conjugative plasmids from six different incompatibility groups to mobilize a set of mobilizable plasmids was examined. The mobilization frequencies of plasmids RSF1010, ColE1, ColE3, and CloDF13 varied over seven orders of magnitude, depending on the helper conjugative plasmid used. Mobilization of CloDF13 was unique in that it did not require TrwB, TraG or TraD (all members of the TraG family) for mobilization by R388, RP4 or F, respectively. CloDF13 itself codes for an essential mobilization protein (MobB) which is also a TraG homolog, only requiring a source of the genes for pilus formation. Besides, CloDF13 was mobilized efficiently by all conjugative plasmids, suggesting that TraG homologs are the primary determinants of the mobilization efficiency of a plasmid, interacting differentialy with the various relaxosomes. Previous results indicated that TraG and TrwB were interchangeable for mobilization of RSF1010 and ColE1 by PIL_W (the pilus system of IncW plasmids) but TraG could not complement conjugation of trwB mutants, suggesting that additional interactions were taking place between TrwB and oriT(R388) that were not essential for mobilization. To further test this hypothesis, we analyzed the mobilization frequencies of ColE1 and RSF1010 by the P, W, and F pili in the presence of alternative TraG homologs. The results obtained indicated that the frequency of mobilization was determined both by the particular TraG-like protein used and by the pilus system. Thus, TraG-like proteins are not generally interchangeable for mobilization. Therefore we suggest that the factors that determine the frequencies of transfer of different MOB regions are the differential interactions of TrwB with pilus and relaxosome. Received: 9 September 1996 / Accepted: 17 December 1996     

Populus tremuloides photosynthesis and crown architecture in response to elevated CO2 and soil N availability

We tested the hypothesis that elevated CO₂ would stimulate proportionally higher photosynthesis in the lower crown of Populus trees due to less N retranslocation, compared to tree crowns in ambient CO₂. Such a response could increase belowground C allocation, particularly in trees with an indeterminate growth pattern such as Populus tremuloides. Rooted cuttings of P. tremuloides were grown in ambient and twice ambient (elevated) CO₂ and in low and high soil N availability (89 ± 7 and 333 ± 16 ng N g⁻¹ day⁻¹ net mineralization, respectively) for 95 days using open-top chambers and open-bottom root boxes. Elevated CO₂ resulted in significantly higher maximum leaf photosynthesis (A _max) at both soil N levels. A _max was higher at high N than at low N soil in elevated, but not ambient CO₂. Photosynthetic N use efficiency was higher at elevated than ambient CO₂ in both soil types. Elevated CO₂ resulted in proportionally higher whole leaf A in the lower three-quarters to one-half of the crown for both soil types. At elevated CO₂ and high N availability, lower crown leaves had significantly lower ratios of carboxylation capacity to electron transport capacity (V _cmax/J _max) than at ambient CO₂ and/or low N availability. From the top to the bottom of the tree crowns, V _cmax/J _max increased in ambient CO₂, but it decreased in elevated CO₂ indicating a greater relative investment of N into light harvesting for the lower crown. Only the mid-crown leaves at both N levels exhibited photosynthetic down regulation to elevated CO₂. Stem biomass segments (consisting of three nodes and internodes) were compared to the total A _leaf for each segment. This analysis indicated that increased A _leaf at elevated CO₂ did not result in a proportional increase in local stem segment mass, suggesting that C allocation to sinks other than the local stem segment increased disproportionally. Since C allocated to roots in young Populus trees is primarily assimilated by leaves in the lower crown, the results of this study suggest a mechanism by which C allocation to roots in young trees may increase in elevated CO₂. Received: 12 August 1996 / Accepted: 12 November 1996     

The exploitation of an ant-defended host plant by a shelter-building herbivore

Larvae of a Polyhymno species (Lepidoptera: Gelechiidae) feed on the ant-defended acacia, Acacia cornigera, in the tropical lowlands of Veracruz, Mexico. Polyhymno larvae construct sealed shelters by silking together the pinna or pinnules of acacia leaves. Although larval density and larval survival are higher on acacias not occupied by ants, shelters serve as a partial refuge from the ant Pseudomyrmex ferruginea (Hymenoptera: Formicidae), which defends A. cornigera plants; thus, shelters provide Polyhymno larvae access to an ant-defended host plant. P. ferruginea ants act as the primary antiherbivore defense of A. cornigera plants, which lack the chemical and mechanical defenses of non-ant-defended acacias. Thus, defeating the ant defense of A. cornigera provides Polyhymno larvae access to an otherwise poorly defended host plant. Damage caused by Polyhymno larval feeding reaches levels which can kill A. cornigera plants. Received: 6 June 1996 / Accepted: 6 September 1996     

Wound-inducible and organ-specific expression of ORF13 from Agrobacterium rhizogenes 8196 T-DNA in transgenic tobacco plants

Tissue-specific expression of the ORF13 promoter from Agrobacterium rhizogenes 8196 was assessed throughout the development of transgenic tobacco plants using a GUS reporter gene. ORF13 exhibited high activity in roots but with different patterns of expression. The activity of the ORF13 promoter in vascular tissues increased from the base to the tip of the stem. The ORF13 promoter is wound inducible in a limited area adjacent to the wound site. The time course of wound induction of ORF13 in transgenic tobacco containing an ORF13 promoter-GUS translational fusion was similar to that previously described for genes involved in plant defense responses. A series of 5′ deletions of the ORF13 promoter fused to the β-glucuronidase gene was examined for expression in roots and leaves of transgenic plants. Cis-acting elements that modulate quantitative expression of the transgene after wounding were detected. Received: 11 July 1996 / Accepted: 19 November 1996     

Leaf phenology, seasonal changes in leaf quality and herbivory pattern of Sanguisorba tenuifolia at different altitudes

Leaf demography, seasonal changes in leaf quality and leaf-beetle herbivory of a herbaceous perennial plant, Sanguisorba tenuifolia, were compared between low- and high-elevation sites. Leaf nitrogen concentration was higher and leaf mass per area (LMA) was lower at the higher site than at the lower one. At the lower site, with a long growth period, plants produced many leaf cohorts and leaves emerged throughout the growing season. At the higher site, with a short growth period, however, leaf emergence was concentrated early in the growing season. The improvement of leaf quality and acceleration of leaf emergence at higher altitude are seen as adaptations to a short growing season. Results of a feeding trial suggested that leaf quality for the leaf-beetle Galerucella grisescens was higher at the higher site, but plants at the higher site showed less damage. Oviposition of G. grisescens was seasonal and unimodal at both altitudes, but the period of oviposition was shorter and its density lower at the higher site. The low temperature and short growth period at the higher site appear to reduce the activity of the leaf-beetles, resulting in a decrease in damage by herbivory, despite better leaf quality. Received: 11 December 1997 / Accepted: 24 July 1998     

A ras homologue of Neurospora crassa regulates morphology

In order to study the role of signal transduction pathways in the regulation of morphology in Neurospora crassa, we cloned and characterized a ras homologue, termed NC-ras2. The predicted protein product of this gene is composed of 229 amino acid residues and contains all the consensus sequences shared by the ras protein family. The gene is located in linkage group V. An NC-ras2 disruptant showed morphological characteristics very similar to those of the smco7 mutant, which also maps to linkage group V. Nucleotide sequence analysis revealed that the smco7 mutant harbored a single base deletion in the NC-ras2 gene, which is predicted to result in the truncation of the protein product. Introduction into the smco7 mutant of an NC-ras2 clone yielded stable transformants with a wild-type phenotype. The smco7 mutant exhibited very slow hyphal growth and the rate of conidial formation was approximately one two-hundredth of wild type. The smco7 mutation causes both the changes in the pattern of hyphal growth and the defects in cell wall synthesis. Both the diameter and the length of the apical compartment were shorter in the hyphae of the smco7 mutant. These results suggest that NC-ras2 is identical to smco7, and that the signal transduction pathway mediated by the NC-ras2 protein regulates the apical growth of hyphae, cell wall synthesis, and conidial formation in N. crassa. Received: 1 October 1996 / Accepted: 9 December 1996     

Sexual dimorphism and species differences in the neurophysiology and morphology of the acoustic communication system of two neotropical hylids

We examined auditory tuning and the morphology of the anatomical structures underlying acoustic communication in female Hyla microcephala and H. ebraccata and compared our findings to data from a previous study (Wilczynski et al. 1993) in which we showed species differences in the traits that in males relate to differences in the species-typical calls. Female species differences in the best excitatory frequency (BEF) of the basilar papilla (BP) were similar to the differences seen in males, and females had a significantly lower BEF in H. ebraccata, but not H. microcephala. In both species, females had lower BP thresholds. Snout-vent length, head width, and tympanic membrane diameters were sexually dimorphic in both species and larger in females, whereas laryngeal components were sexually dimorphic and larger in males. Middle and inner ear volumes were not sexually dimorphic. Despite the significant species differences in laryngeal morphology seen in males, female larynges are not significantly different. Furthermore, the interaction of species and sex differences resulted in significantly different degrees of sex dimorphism in the species, particularly for the larynx, which is more sexually dimorphic in H. microcephala, and measures of body size, which are more dimorphic in H. ebraccata. Accepted: 6 December 1996     

A comparison of two-dimensional electrophoresis data with phenotypical traits in Arabidopsis leads to the identification of a mutant (cri1) that accumulates cytokinins

Total proteins extracted from developmental mutants of Arabidopsis thaliana (L.) Heyhn. and from wild-type plants cultivated in the presence of various hormones were analyzed by two-dimensional (2-D) gel electrophoresis. Computer analysis of 2-D gels followed by a statistical treatment of data allowed us to build a phenogram that describes the biochemical distances between the different genotypes. Analysis of the 2-D electrophoresis data allowed us to discriminate mutants in agreement with phenotypical and physiological traits. This biochemical analysis helped us to develop a working hypothesis which led us to show that one developmental mutant (cri1 ) overaccumulates cytokinins. Received: 5 August 1996 / Accepted: 11 December 1996     

Analysis of eryBI , eryBIII and eryBVII from the erythromycin biosynthetic gene cluster in Saccharopolyspora erythraea

The gene cluster (ery) governing the biosynthesis of the macrolide antibiotic erythromycin A by Saccharopolyspora erythraea contains, in addition to the eryA genes encoding the polyketide synthase, two regions containing genes for later steps in the pathway. The region 5′ of eryA that lies between the known genes ermE (encoding the erythromycin resistance methyltransferase) and eryBIII (encoding a putative S-adenosylmethionine-dependent methyltransferase), and that contains the gene eryBI (orf2), has now been sequenced. The inferred product of the eryBI gene shows striking sequence similarity to authentic β-glucosidases. Specific mutants were created in eryBI, and the resulting strains were found to synthesise erythromycin A, showing that this gene, despite its position in the biosynthetic gene cluster, is not essential for erythromycin biosynthesis. A␣mutant in eryBIII and a double mutant in eryBI and eryBIII were obtained and the analysis of novel erythromycins produced by these strains confirmed the proposed function of EryBIII as a C-methyltransferase. Also, a chromosomal mutant was constructed for the previously sequenced ORF19 and shown to accumulate erythronolide B, as expected for an eryB mutant and consistent with its proposed role as an epimerase in dTDP-mycarose biosynthesis. Received: 13 August 1997 / Accepted: 27 November 1997