Water potentials for developing cladodes and fruits of a succulent plant, including xylem-versus-phloem implications for water movement

Developing cladodes had lower water potentials and developingfruits had higher water potentials than the underlying cladodesof the widely cultivated prickly pear cactus, Opuntia ficus-indica.The 0.06 MPa lower value in 4-week-old daughter cladodes indicateda typical water potential gradient from the underlying clad-odealong the xylem of –0.2 MPa m^–1; the 0.17 MPa highervalue in 4-week-old fruits, which decreased to 0.07 MPa by 10weeks, implicated the phloem as their supplier of water. Thephloem sap of the underlying cladodes had an osmotic pressureof only 0.90 to 0.98 MPa, so the phloem could supply a relativelydilute solution to the photosynthetically dependent fruits (daughtercladodes of O. ficus-indica are photosynthetically independentat 4 weeks). Although the water potentials were similar foradjacent tissues, the osmotic pressures were lower for the water-storagecompared with the photosynthetic tissue; the osmotic pressureswere higher for xylem sap from fruits, for which xylary flowapparently occurred toward the underlying cladodes, than fordaughter cladodes. The relative capacitance (change in relativewater content divided by change in tissue water potential) wasapproximately 0.71 MPa^–1 for the water-storage tissueand the photosynthetic tissue of both daughter cladodes andfruits at 4 weeks of age. When these organs approached maturityat 10 weeks, the relative capacitance increased about 40% fortheir water-storage tissue, but decreased 30% for their photosynthetictissue. As the plant water content decreases during drought,about twice as much water will thus be lost per unit volumeof the water-storage tissue compared with the photosynthetictissue of maturing fruits and cladodes. Key words: Opuntia ficus-indica, phloem, relative water content, water capacitance, water potential     

Redox-active bis-cysteinyl peptides. II. Comparative study on the sequence-dependent tendency for disulfide loop formation

Bis (cysteinyl) octapeptides related to the active sites of the oxidoreductases protein disulfide isomerase (PDI), thioredoxin reductase (trr), glutaredoxin (grx), and thioredoxin (trx) were analyzed for their propensity to form the intramolecular 14-membered disulfide ring in oxidation experiments. The rank order of percentage of cyclic monomer formed in aqueous buffer (pH 7.0) at 10^?3 M concentration was found to be very similar, but opposite to that of the K_ox and, correspondingly, of the redox potentials of the native enzymes. Attempts to induce intrinsic conformational preferences of the peptides by addition of trifluoroethanol led to enhancements of β-turn structures as reflected by the CD and Fourier transform ir spectra. The induced secondary structure, instead of aligning the tendencies of the excised fragments for loop formation with those of the intact proteins, was found to suppress the differences by significantly increasing the preference for cyclic monomers (≈ 90%). Similarly, operating under denaturing conditions, i.e., in 6M guanidinium hydrochloride, only for the trx peptide was the statistical product distribution obtained. For the remaining peptides, again a strong increase of cyclic monomer contents was observed that could not be correlated with dissolution of β-sheet type aggregates. The CD spectra are more consistent with the presence of ordered structure to some extent, possibly resulting from an hydrophobic collapse of the sparingly soluble peptides. The results of the oxidation experiments further support previous findings from thiol disulfide interchange equilibria, which clearly revealed a decisive role of the characteristic thioredoxin structural motif in dictating the redox properties of the enzymes. Point mutations in the active sites of the oxidoreductases allowed us to affect their redox potentials strongly, but apparently only in the constraint form of the three-dimensional structure as similar exchanges in the excised fragments did not produce the expected effect. This observation contrasts with numerous reports that the conformation of short disulfide loops is mainly dictated by the amino acid sequence. © 1994 John Wiley & Sons, Inc. © 1994 John Wiley & Sons, Inc.     

Probability of mast cells to mediate anaphylaxis in skeletal muscle

Are there enough mast cells in denervated skeletal muscle to account for autopharmacological mediation of the antigen potentials (APs) elicited by microtaps? Through rough qualitative estimations, some authors have suggested a positive answer to this question. However, in view of measurements performed in this investigation of both the density of mast cells and the diffusion coefficient of antigens, the probability of such mediated effects was found to be relatively low:P=0.016 for egg albumin andP=0.004 for ferritin. Therefore, most APs induced by microtaps should be attributed to the direct effect of antigen over the sensitized muscle fibers. Yet, both the density of mast cells found in this work and the known amount of histamine they are capable of releasing when challenged with antigen, support the hypothesis regarding the involvement of these cells when antigen is massively superfused so as to induce Schultz-Dale reactions in muscle strips. Under this circumstance, the direct and mediated mechanisms may coexist.     

Experimental and theoretical definition of geminivirus origin of replication

Geminiviruses are plant pathogens that replicate by a rolling-circle mechanism, analogous to that used by several prokaryotic ssDNA replicons. Recent reports provide important progress in understanding the structure and functioning of replication origin from these viruses. We have used these data to propose models for the initiation of replication in dicot- and monocot-infecting geminiviruses.     

Morphological evolution and changes in foraging behaviour of island and mainland populations of Blue Tit (Parus caeruleus) — a test of convergence and ecomorphological hypotheses

Summary We study the leg morphology and feeding postures of two subspecies of the Blue Tit (Parus caeruleus; Tenerife island and the Iberian Peninsula) and the Coal Tit (Parus ater; Iberian Peninsula). We search for evidence supporting the hypothesis of convergent evolution in morphological and ecological traits and we discuss the role of ecomorphological hypotheses as predictors of foraging differences at the intraspecific level. To overcome the problems introduced by environmental characteristics not related to locomotion and competition, we make observations under controlled situations to manage food quality and food access. We determine that the island Blue Tit has a longer tarsometatarsus, larger foot span and a more proximal insertion of the tibialis cranialis muscle (flexor of the tarsometatarsus) than the mainland Blue Tit. These morphological differences are consistent with the more frequent use of hanging and clinging head-up postures by the Iberian Blue Tit. Several ecomorphological hypotheses obtained at the interspecific level with other taxa, have proved to be of high predictive value for explaining ecological differences considering morphological evolution. The Tenerife Blue Tit and the Iberian Coal Tit clearly show close convergence in both feeding postures and leg structure, although some differences in morphology were found between these two species. Convergence in foraging methods between the island Blue Tit and the mainland Coal Tit can be explained without considering current interspecific competition as a determinant of niche space.     

Clonal analysis in hybrids between Drosophila melanogaster and Drosophila simulans

We have analysed the viability of cellular clones induced by mitotic recombination in Drosophila melanogaster/D. simulans hybrid females during larval growth. These clones contain a portion of either melanogaster or simulans genomes in homozygosity. Analysis has been carried out for the X and the second chromosomes, as well as for the 3L chromosome arm. Clones were not found in certain structures, and in others they appeared in a very low frequency. Only in abdominal tergites was a significant number of clones observed, although their frequency was lower than in melanogaster abdomens. The bigger the portion of the genome that is homozygous, the less viable is the recombinant melano-gaster/simulans hybrid clone. The few clones that appeared may represent cases in which mitotic recombination took place in distal chromosome intervals, so that the clones contained a small portion of either melanogaster or simulans chromosomes in homozygosity. Moreover, Lhr, a gene of D. simulans that suppresses the lethality of male and female melanogaster/simulans hybrids, does not suppress the lethality of the recombinant melanogaster/simulans clones. Thus, it appears that there is not just a single gene, but at least one per tested chromosome arm (and maybe more) that cause hybrid lethality. Therefore, the two species, D. melanogaster and D. simulans, have diverged to such a degree that the absence of part of the genome of one species cannot be substituted by the corresponding part of the genome of the other, probably due to the formation of co-adapted gene complexes in both species following their divergent evolution after speciation. The disruption of those coadapted gene complexes would cause the lethality of the recombinant hybrid clones.     

Prior experience affects the visual ability ofRhagoletis pomonella flies (Diptera: Tephritidae) to find host fruit

Females of the apple maggot fly,Rhagoletis pomonella, were allowed for 3 days to alight upon and oviposit in green or red 18- to 20-mm hawthorn host fruit (Crategus mollis) or green or red 45- to 55-mm apple host fruit (Malus pumila) hung from branches of potted host trees in field enclosures. Subsequently, when females were released individually on potted host trees harboring fruit of one of these types, their ability to find fruit of unfamiliar size proved unaffected by prior experience with fruit but their ability to find fruit of unfamiliar color was significantly affected. Specifically, females exposed to red hawthorns or red apples were less able to find green hawthorns or green apples than were females experienced with either of the latter fruit types. Fruit odor was found to have no effect on female ability to find familiar compared with unfamiliar green fruit. In contrast, a difference in size (or surface chemistry) between familiar and unfamiliar fruit but not a difference in fruit color had a significant negative influence on the propensity of alighting females to bore into unfamiliar fruit. Three bouts of experience with alighting upon and ovipositing into fruit over a period of about 1 h had no detectable effect on female ability to find unfamiliar fruit but did reduce propensity to bore into unfamiliar fruit. Our findings are discussed in relation to insect ability to learn visual and chemical stimuli of resources and insect propensity to form host races. We also discuss the potential impact of our findings on nonpesticidal, behavioral methods of managingR. pomonella in commercial apple orchards.     

Two related,low-temperature-induced genes from Brassica napus are homologous to the human tumour bbc1 (breast basic conserved) gene

In order to identify genes involved in cold acclimation, we have constructed a cDNA library from Brassica napus (cv. Samouraï) cold-acclimated etiolated seedlings. By differential screening, a cDNA clone named pBnC24 (Brassica napus Cold), corresponding to a new cold-inducible plant gene, was isolated. Northern blot hybridizations using total RNA from acclimated and unacclimated seedlings confirmed that BnC24 represents a cold-regulated gene. In contrast with a number of cold-inducible plant genes, BnC24 does not seem to be responsive to abscisic acid (ABA). In addition, further screening of the cold-acclimated cDNA library using pBnC24 cDNA as a probe, allowed the isolation of a second type of homologous cDNA. Sequence analysis showed that the two BnC24 genes encode basic 24 kDa proteins, which are highly hydrophilic and rich in alanine, lysine and arginine. The nucleotide and deduced amino acid sequences of these clones do not show any homology with other previously described cold-induced plants genes. However they have strong homology with a recently discovered human tumour gene, bbc1 (breast basic conserved), which seems to be highly conserved in eukaryotes.     

Comparative analysis of C3 and botulinal neurotoxin genes and their environment in Clostridium botulinum types C and D.

The C3 exoenzyme gene is located on a bacteriophage in Clostridium botulinum types C and D (M. R. Popoff, D. Hauser, P. Boquet, M. W. Eklund, and D. M. Gill, Infect. Immun. 59:3673-3679, 1991). A derivative CN phage from phage C of C. botulinum Stockholm (C-St) (K. Oguma, H. Iida, and K. Inoue, Jpn. J. Microbiol. 19:167-172, 1975), isolated as neurotoxin negative, also does not produce exoenzyme C3. The botulinal neurotoxin C1 gene is present on the CN phage but contains a stop mutation in the DNA region encoding the N-terminal part of the heavy chain (codon 553). The putative truncated botulinal neurotoxin C1 protein was not recovered in a C. botulinum strain harboring the CN phage. We found that the C3 gene is localized on a 21.5-kbp DNA fragment flanked by the core motif 5'-AAGGAG-3' in DNAs of phage C of C. botulinum 468 (C-468), C-St phage, and phage D of C. botulinum 1873 (D-1873). The 21.5-kbp DNA fragment is deleted in CN phage DNA, and the motif 5'-AAGGAG-3' is present only in one copy at the deletion junction, but the deletion in the CN phage could be nonspecific, since this phage was obtained by nitrosoguanidine treatment. These findings could indicate that the C3 gene is localized on a 21.5-kbp mobile element. C. botulinum type C strain 003-9 produces a C3 exoenzyme (Y. Nemoto, T. Namba, S. Kozaki, and S. Narumiya, J. Biol. Chem. 266:19312-19319, 1991), and Staphylococcus aureus E1 produces a related C3 enzyme which is named epidernmal cell differentiation inhibitor (S. Inoue, M. Sugai, Y. Murooka, S. Y. Paik, Y. M. Hong, H. Oghai, and H. Suginaka, Biochem. Biophys. Res. Comm. 174:459-464, 1991) and which shares 80.6 and 56.6% similarity, respectively with the C3 enzymes from C-468 or C-St and D-1873 phages athe amino acid level. The features of the putative 21.5-kbp transposon were not found in C. botulinum 003-9 and S. aureus E1, as determined by analysis of the C3 and epidermal cell differentiation inhibitor gene-flanking DNA regions. These data suggest a common ancestral origin and divergent evolution of the C3 genes in these three groups of bacterial strains and dissemination of a 21.5-kbp element carrying the C3 gene C-468, C-St, and D-1873 phages.