Didier Combes

Existence of four acetylcholinesterase genes in the nematodes Caenorhabditis elegans and Caenorhabditis briggsae

Three genes, ace‐1, ace‐2 and ace‐3, respectively located on chromosomes X, I and II, were reported to encode acetylcholinesterases (AChEs) of classes A, B and C in the nematode Caenorhabditis elegans. We have previously cloned and sequenced ace‐1 in the two related species C. elegans and C. briggsae. We report here partial sequences of ace‐2 (encoding class B) and of two other ace sequences located in close proximity on chromosome II in C. elegans and C. briggsae. These two sequences are provisionally named ace‐x and ace‐y, because it is not possible at the moment to establish which of these two genes corresponds to ace‐3. Ace‐x and ace‐y are transcribed in vivo as shown by RT‐PCR and they are likely to be included in a single operon.

A study of ryegrass architecture as a self-regulated system, using functional–structural plant modelling

The canopy structure of grasslands is a major determinant of their use-value, as it affects the quantity and quality of the forage removed when mowed or grazed. The structure of this canopy is determined by individual plant architecture, which is highly sensitive to both environmental variations and management practices such as cutting regimes. In the case of perennial ryegrass (Lolium perenne L.), this architectural plasticity may partially be mediated by a self-regulation process, i.e. the actual state of the architecture (e.g. length of the pseudostem) may indirectly control some morphogenetic processes. To test the robustness of this hypothesis, we designed an exploratory model of ryegrass morphogenesis exhibiting this cybernetic behaviour. This functional-structural model is based on the L-system formalism. It was able to capture satisfactorily the major quantitative architectural traits of ryegrass under non-limiting growing conditions and under a cutting constraint. From these simulation results it appears that (i) self-regulation rules could be of practical use to ryegrass modelling, and (ii) when activated in an integrated model, they are not markedly incompatible with observations.

Light controls shoot meristem organogenic activity and leaf primordia growth during bud burst in Rosa sp.

Light controls bud burst in many plants, which subsequently affects their architecture. Nevertheless, very little is known about this photomorphogenic process. This study ascertains the effects of light on bud burst and on two of its components, i.e. growth of preformed leaves and meristem organogenesis in six cultivars from three Rosa species (R. hybrida L., R. chinensis L., R. wichurana L.). Defoliated plants were severed above the third basal bud and exposed, either to darkness or to different intensities of white light, to blue, red or to FR, at constant temperature. Bud bursting was inhibited in darkness in the six cultivars of Rosa, but not in Arabidopsis, tomato and poplar plants under the same condition. In all Rosa cultivars, bud burst, growth of preformed leaves and meristem organogenesis were triggered by blue and red lights, and extended by increasing light intensities. FR was inhibitory of bud burst. Partial shading experiments demonstrated that bud and not stem was the active site for light perception in bud burst.

Multiple ace genes encoding acetylcholinesterases of Caenorhabditis elegans have distinct tissue expression

ace‐1 and ace‐2 genes encoding acetylcholinesterase in the nematode Caenorhabditis elegans present 35% identity in coding sequences but no homology in noncoding regions (introns, 5′‐ and 3′‐untranslated regions). A 5′‐region of ace‐2 was defined by rescue of ace‐1;ace‐2 mutants. When green fluorescent protein (GFP) expression was driven by this regulatory region, the resulting pattern was distinct from that of ace‐1. This latter gene is expressed in all body‐wall and vulval muscle cells ( Culetto et al., 1999 ), whereas ace‐2 is expressed almost exclusively in neurons. ace‐3 and ace‐4 genes are located in close proximity on chromosome II ( Combes et al., 2000 ). These two genes were first transcribed in vivo as a bicistronic messenger and thus constitute an ace‐3;ace‐4 operon. However, there was a very low level of monocistronic mRNA of ace‐4 (the upstream gene) in vivo, and no ACE‐4 enzymatic activity was ever detected. GFP expression driven by a 5′ upstream region of the ace‐3;ace‐4 operon was detected in several muscle cells of the pharynx (pm3, pm4, pm5 and pm7) and in the two canal associated neurons (CAN cells). A dorsal row of body‐wall muscle cells was intensively labelled in larval stages but no longer detected in adults. The distinct tissue‐specific expression of ace‐1, ace‐2 and ace‐3 (coexpressed only in pm5 cells) indicates that ace genes are not redundant.

Acetylcholinesterase genes in the nematode Caenorhabditis elegans

Acetylcholinesterase (AChE, EC 3.1.1.7) is responsible for the termination of cholinergic nerve transmission. It is the target of organophosphates and carbamates, two types of chemical pesticides being used extensively in agriculture and veterinary medicine against insects and nematodes. Whereas there is usually one single gene encoding AChE in insects, nematodes are one of the rare phyla where multiple ace genes have been unambiguously identified. We have taken advantage of the nematode Caenorhabditis elegans model to identify the four genes encoding AChE in this species. Two genes, ace-1 and ace-2, encode two major AChEs with different pharmacological properties and tissue repartition: ace-1 is expressed in muscle cells and a few neurons, whereas ace-2 is mainly expressed in motoneurons. ace-3 represents a minor proportion of the total AChE activity and is expressed only in a few cells, but it is able to sustain double null mutants ace-1; ace-2. It is resistant to usual cholinesterase inhibitors. ace-4 was transcribed but the corresponding enzyme was not detected in vivo.

How good is the turbid medium-based approach for accounting for light partitioning in contrasted grass–legume intercropping systems?

BACKGROUND AND AIMS Most studies dealing with light partitioning in intercropping systems have used statistical models based on the turbid medium approach, thus assuming homogeneous canopies. However, these models could not be directly validated although spatial heterogeneities could arise in such canopies. The aim of the present study was to assess the ability of the turbid medium approach to accurately estimate light partitioning within grass-legume mixed canopies. METHODS Three contrasted mixtures of wheat-pea, tall fescue-alfalfa and tall fescue-clover were sown according to various patterns and densities. Three-dimensional plant mock-ups were derived from magnetic digitizations carried out at different stages of development. The benchmarks for light interception efficiency (LIE) estimates were provided by the combination of a light projective model and plant mock-ups, which also provided the inputs of a turbid medium model (SIRASCA), i.e. leaf area index and inclination. SIRASCA was set to gradually account for vertical heterogeneity of the foliage, i.e. the canopy was described as one, two or ten horizontal layers of leaves. KEY RESULTS Mixtures exhibited various and heterogeneous profiles of foliar distribution, leaf inclination and component species height. Nevertheless, most of the LIE was satisfactorily predicted by SIRASCA. Biased estimations were, however, observed for (1) grass species and (2) tall fescue-alfalfa mixtures grown at high density. Most of the discrepancies were due to vertical heterogeneities and were corrected by increasing the vertical description of canopies although, in practice, this would require time-consuming measurements. CONCLUSIONS The turbid medium analogy could be successfully used in a wide range of canopies. However, a more detailed description of the canopy is required for mixtures exhibiting vertical stratifications and inter-/intra-species foliage overlapping. Architectural models remain a relevant tool for studying light partitioning in intercropping systems that exhibit strong vertical heterogeneities. Moreover, these models offer the possibility to integrate the effects of microclimate variations on plant growth.

Light-mediated K leaf induction and contribution of both the PIP1s and PIP2s aquaporins in five tree species: walnut ( Juglans regia) case study

Understanding the response of leaf hydraulic conductance (K(leaf)) to light is a challenge in elucidating plant-water relationships. Recent data have shown that the effect of light on K(leaf) is not systematically related to aquaporin regulation, leading to conflicting conclusions. Here we investigated the relationship between light, K(leaf), and aquaporin transcript levels in five tree species (Juglans regia L., Fagus sylvatica L., Quercus robur L., Salix alba L. and Populus tremula L.) grown in the same environmental conditions, but differing in their K(leaf) responses to light. Moreover, the K(leaf) was measured by two independent methods (high-pressure flow metre (HPFM) and evaporative flux method (EFM)) in the most (J. regia) and least (S. alba) responsive species and the transcript levels of aquaporins were analyzed in perfused and unperfused leaves. Here, we found that the light-induced K(leaf) value was closely related to stronger expression of both the PIP1 and PIP2 aquaporin genes in walnut (J. regia), but to stimulation of PIP1 aquaporins alone in F. sylvatica and Q. robur. In walnut, all newly identified aquaporins were found to be upregulated in the light and downregulated in the dark, further supporting the relationship between the light-mediated induction of K(leaf) and aquaporin expression in walnut. We also demonstrated that the K(leaf) response to light was quality-dependent, K(leaf) being 60% lower in the absence of blue light. This decrease in K(leaf) was correlated with strong downregulation of three PIP2 aquaporins and of all the PIP1 aquaporins tested. These data support a relationship between light-mediated K(leaf) regulation and the abundance of aquaporin transcripts in the walnut tree.

Enzymatic Synthesis of Thioesters in Non-conventional Solvents

The feasibility of enzymatic thioesterification between oleic acid and butanethiol in n-hexane, with the immobilised lipase (Lipozyme) from Mucor miehei, has been demonstrated. The immobilised enzyme quantity (100 mg), temperature (40°C), pH range (6–9) and water content (10%) were studied and their optimum values were determined. A preliminary kinetic study indicated a low butanethiol affinity for the enzyme (Km = 1·85 mol dm−3). Even when butanethiol was used without solvent, no substrate inhibition was observed. The possibility of carrying out this reaction in a natural solvent, supercritical carbon dioxide (SCCO2), was successfully verified. After 8 h reaction, a conversion yield of oleic acid of about 33% was obtained.

Four Acetylcholinesterase Genes in the Nematode Caenorhabditis Elegans

Whereas a single gene encodes acetylcholinesterase (AChE) in vertebrates and most insect species, four distinct genes have been cloned and characterized in the nematode Caenorhabditis elegans. We found that ace-1 (mapped to chromosome X) is prominently expressed in muscle cells whereas ace-2 (located on chromosome I) is mainly expressed in neurons. Ace-x and ace-y genes are located in close proximity on chromosome II where they are separated by only a few hundred base pairs. The role of these two genes is still unknown.

How does pea architecture influence light sharing in virtual wheat-pea mixtures? A simulation study based on pea genotypes with contrasting architectures

Light sharing within virtual wheat-pea mixtures was influenced by the variability of pea’s architectural parameters affecting LAI and height. Light capture was affected by the development of leaflets, number of branches and phytomers and internode length.