Acclimation of Arabidopsis thaliana to the light environment: Changes in composition of the photosynthetic apparatus

Acclimation to changes in the light environment was investigated in Arabidopsis thaliana (L.) Heynh. cv. Landsberg erecta. Plants grown under four light regimes showed differences in their development, morphology, photosynthetic performance and in the composition of the photosynthetic apparatus. Plants grown under high light showed higher maximum rates of oxygen evolution and lower levels of light-harvesting complexes than their low light-grown counterparts; plants transferred to low light showed rapid changes in maximum photosynthetic rate and chlorophyll-a/b ratio as they became acclimated to the new environment. In contrast, plants grown under lights of differing spectral quality showed significant differences in the ratio of photosystem II to photosystem I. These changes are consistent with a model in which photosynthetic metabolism provides signals which regulate the composition of the thylakoid membrane.Abbreviations Aac1 gene encoding actin - Chl chlorophyll - F far-red-enriched light (R:FR = 0.72) - FR far-red light - H high light (400 mol · m^–2 · s^–1) - L low light (100 ml · m^–2 · s^–1) - LHCII light-harvesting complex of PSII - Lhcb genes encoding the proteins of LHCII - R red light - Rbcs genes encoding the small subunit of Rubisco - Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase - W white light (R:FR = 1.40) This work was supported by Natural Environment Research Council Grant No. GR3/7571A. We would like to thank H. Smith (Botany Department, University of Leicester) and E. Murchie (University of Sheffield) for helpful discussions.     

Avian daily foraging patterns: Effects of digestive constraints and variability

Summary Birds show a typical daily pattern of heavy morning and secondary afternoon feeding. We investigate the pattern of foraging by a bird that results in the lowest long-term rate of mortality. We assume the following: mortality is the sum of starvation and predation. The bird is characterized by two state variables, its energy reserves and the amount of food in its stomach. Starvation occurs during the day if the bird's reserves fall to zero. The bird starves during the night if the total energy stored in reserves and the stomach is less than a critical amount. The probability that the bird is killed by a predator is higher if the bird is foraging than if it is resting. Furthermore, the predation risk while foraging increases with the bird's mass. From these assumptions, we use dynamic programming techniques to find the daily foraging routine that minimizes mortality. The principal results are (1) Variability in food finding leads to routines with feeding concentrated early in the day, (2) digestive constraints cause feeding to be spread more evenly through the day, (3) even under fairly severe digestive constraints, the stomach is generally not full and (4) optimal fat reserve levels are higher in more variable environments and under digestive constraints. This model suggests that the characteristic daily feeding pattern of small birds is not due to digestive constraints but is greatly influenced by environmental variability.     

Evidence that residues −15 to −46 of the haemolysin secretion signal are involved in early steps in secretion, leading to recognition of the translocator

We previously identified three well-dispersed mutations, E⁹⁷⁸-K, F⁹⁸⁹-L and D¹⁰⁰⁹-R within the haemolysin A signal region, located at positions –46, –35 and –15, with respect to the C-terminus, respectively. Each mutation reduces the efficiency of secretion two- to threefold leaving 30–45% of the wild-type activity. We have constructed by in vitro manipulations double mutants of HlyA carrying all combinations of these mutations and a triple mutant carrying all three mutations. The effects on secretion were determined and the results, including residual levels of secretion with the triple mutant of only 0.6%, compared with the wild type, indicated that these residues may interact to form a single function in the wild-type signal. To test this further, we developed a secretion competition assay in order to classify signal mutations. We demonstrated that a CIZ-HlyA fusion protein, containing the C-terminal 81 kDa of HlyA fused to virtually the whole LacZ protein, strongly inhibits the secretion of the wild-type HlyA co-expressed In the same cell. The properties of the fusion indicate that it blocks the translocator. The three mutations singly and in combinations were recombined in vitro into the 3′-end of the hybrid gene. In every case, the presence of a mutation in the secretion signal of the hybrid protein alleviated the inhibition of secretion of the co-expressed HiyA. All the mutations are therefore essentially recessive and we propose that they all affect an early function, probably recognition of the translocator, rather than a subsequent step involved in translocation or final release of the toxin to the medium. This would indicate that residues involved in recognition for steps     

Simple sequences

Simple sequences (or microsatellites) are stretches of monotonous repetitions of short (1–5 bp) nucleotide motifs that are distributed across the whole genome in eukaryotes. They are probably generated by slippage during replication and their primary mutation rate seems to be controlled predominantly by the efficiency of the mismatch repair system. Although most mutations in simple sequence loci appear to be neutral, some mutations in particular stretches have been implicated as having a role in human genetic diseases.     

Polygalacturonase-inhibiting protein accumulates in Phaseolus vulgaris L. in response to wounding, elicitors and fungal infection

Polygalacturonase-inhibiting protein (PGIP) is a cell wall-associated protein that specifically binds to and inhibits the activity of fungal endopolygalacturonases. The Phaseolus vulgaris gene encoding PGIP has been cloned and characterized. Using a fragment of the cloned pgip gene as a probe in Northern blot experiments, it is demonstrated that the pgip mRNA accumulates in suspension-cultured bean cells following addition of elicitor-active oligogalacturonides or fungal glucan to the medium. Rabbit polyclonal antibodies specific for PGIP were generated against a synthetic peptide designed from the N-terminal region of PGIP; the antigenicity of the peptide was enhanced by coupling to KLH. Using the antibodies and the cloned pgip gene fragment as probes in Western and Northern blot experiments, respectively, it is shown that the levels of PGIP and its mRNA are increased in P. vulgaris hypocotyls in response to wounding or treatment with salicylic acid. Using gold-labeled goat-anti-rabbit secondary antibodies in EM studies, it has also been demonstrated that, in bean hypocotyls infected with Colletotrichum lindemuthianum, the level of PGIP preferentially increases in those cells immediately surrounding the infection site. The data support the hypothesis that synthesis of PGIP constitutes an active defense mechanism of plants that is elicited by signal molecules known to induce plant defense genes.     

Association of a 76 kDa polypeptide with soluble starch synthase I activity in maize (cv B73) endosperm

Soluble starch synthase (SSS) I was purified 361-fold from hand-dissected endosperm tissue of inbred maize (Zea mays, cv. B73) to specific activities ranging between 5 and 9 µmol min⁻¹ mg⁻¹. A key to this purification protocol was the introduction of a size-exclusion chromatography step, a size-based fractionation which provided abundant levels of desalted SSS forms I and II. The native molecular masses calculated for SSS forms I and II were 75.5 kDa and 180 kDa, respectively. SSSI was then further purified by hydrophobic interaction chromatography on Phenyl-Superose and by FPLC on Mono Q. Analysis of column peaks by SDS—PAGE and scanning densitometry revealed that a 76 kDa polypeptide is strongly correlated with SSSI activity. Antibodies were then generated against a 76 kDa polypeptide extracted from starch granules. These antibodies, which were monospecific for the soluble 76 kDa polypeptide, neutralized greater than 90% of SSSI activity, and precipitated the 76 kDa protein. These results establish the 76 kDa protein as an SSSI in the B73 line of inbred maize. An immunologically similar 76 kDa protein also appears to be tightly associated with the starch granule.     

A (1→3,1→4)-β-glucan-specific monoclonal antibody and its use in the quantitation and immunocytochemical location of (1→3,1→4)-β-glucans

Monoclonal antibodies were raised against a (1→3,1→4)-β-glucan-bovine serum albumin (BSA) conjugate. One antibody (BG1) selected for further characterization, was specific for (1→3,1→4)-β-glucan, displaying no binding activity against a (1→3)-β-glucan-BSA conjugate and minimal binding against a cellopentaose-BSA conjugate. A range of oligosaccharides was prepared by enzymatic digestion of (1→3,1→4)-β-glucan, purified by size exclusion chromatography and characterized by ¹H-NMR and anion exchange chromatography. These (1→3,1→4)-β-oligoglucosides, together with (1→3)-β- and (1→4)-β-oligoglucosides were used to characterize the binding site of the monoclonal antibody (BG1) by competitive inhibition. The monoclonal antibody showed maximal binding to a heptasaccharide with the structure Glc(1→3) Glc(1→4) Glc(1→4) Glc(1→3) Glc(1→4) Glc(1→4) Glc and was determined to have an affinity constant of 3.8 × 10⁴ M⁻¹ for this oligoglucoside. The monoclonal antibody (BG1) has been used to develop a sensitive sandwich ELISA for the specific quantitation of (1→3,1→4)-β-glucans. The assay operates in the range 1–10 ng ml⁻¹ and shows no significant cross-reaction with tamarind xyloglucan, wheat endosperm arabinoxylan or carboxymethyl-pachyman ((1→3)-β-glucan). When used with a second-stage, rabbit anti-mouse gold conjugate and viewed under the electron microscope, the monoclonal antibody probe was found to bind strongly to the walls of the aleurone in thin sections of immature wheat (Triticum aestivum) cv. Millewa grains but not to the middle lamella region. A previously described specific anti-(1→3)-β-glucan antibody (Meikle et al., 1991) bound to discrete patches on the aleurone walls, believed to be plasmodesmata.