La résistance induite : une nouvelle stratégie de défense des plantes contre les agents pathogènes

Tout au long de leur co-évolution, les plantes et les microorganismes pathogènes ont développé des relations complexes résultant d'un échange constant d'informations moléculaires. Les agents pathogènes ont élaboré toute une gamme de stratégies offensives pour parasiter les plantes et en contrepartie, les plantes ont déployé un arsenal défensif similaire à bien des égards aux défenses immunitaires animales. Les percées récentes en biologie moléculaire et en transformation des végétaux ont démontré que sensibiliser une plante à répondre plus rapidement à l'infection pouvait lui conférer une protection accrue contre des microorganismes virulents. Un aspect important dans la mise en évidence du rôle joué par les molécules de défense au niveau de l'expression de la résistance est une connaissance exacte de leur localisation spatio-temporelle dans les tissus en état de stress. Afin de cerner le processus associé à l'induction de résistance chez les plantes, l'effet d'éliciteurs biologiques, microbiens et chimiques sur la réponse cellulaire des plantes envers une attaque pathogène a fait l'objet d'investigations et les mécanismes impliqués dans le phénomène ont été étudiés. Dans tous les cas, il a été montré qu'une corrélation existait entre la réponse globale de la plante et des changements dans la biochimie et la physiologie des cellules, lesquels étaient accompagnés de modifications structurales incluant la formation d'appositions pariétales riches en callose et l'infiltration de composés phénoliques aux sites de pénétration potentielle par l'agent pathogène. L'activation du sentier des phénylpropanoïdes est un phénomème crucial dans la restriction de la croissance de l'agent pathogène et dans la survie des cellules-hôtes en conditions de stress. Bien qu'il n'existe que peu d'exemples d'application pratique de la résistance induite en tant que méthode de lutte contre les maladies des plantes, les résultats obtenus à partir de quelques expériences menées en plein champ et en serre sont encourageants et indiquent que cette approche a le potentiel de devenir une stratégie de lutte efficace et durable contre toute une gamme d'agents pathogènes.During the course of their coevolution, plants and pathogens have evolved an intricate relationship resulting from a continuous exchange of molecular information. Pathogens have developed an array of offensive strategies to parasitize plants and, in turn, plants have deployed a wide range of defense mechanisms similar in some respects to the immune defenses produced in animais. The recent advances in molecular biology and plant transformation have provided evidence that sensitizing a plant to respond more rapidly to infection could confer increased protection against virulent pathogens. One important facet in ascertaining the significance of defense molecules in plant disease resistance isthe exact knowledgeof their spatio-temporal distribution in stressed plant tissues. In an effort to understand the process associated with the induction of plant disease resistance, the effect of biological, microbial and chemical elicitors on the plant cell response during attack by fungal pathogens was investigated and the mechanisms underlying the expression of resistance studied. Evidence was provided that, in all cases, disease-resistance reactions correlated with changes in cell biochemistry and physiology that were accompanied by structural modifications including the formation of callose-enriched wall appositions and the infiltration of phenolic compounds at sites of potential pathogen penetration. Activation of the phenylpropanoid pathway appeared to be a crucial phenomenon involved in pathogen growth restriction and host cell survival under stress conditions. Although examples of practical use of induced resistance as a method of plant disease control are few, a number of field and greenhouse experiments are encouraging and indicate that this approach has the potential to become a powerful strategy against an array of pathogens in a persistent manner

Coping with spatial heterogeneity and temporal variability in resources and risks: Adaptive movement behaviour by a large grazing herbivore.

Movement is a key mean for mobile species to cope with heterogeneous environments. While in herbivorous mammals large-scale migration has been widely investigated, finescale movement responses to local variations in resources and predation risk remain much less studied, especially in savannah environments. We developed a novel approach based on complementary movement metrics (residence time, frequency of visits and regularity of visits) to relate movement patterns of a savannah grazer, the blue wildebeest Connochaetes taurinus , to fine-scale variations in food availability, predation risk and water availability in the Kruger National Park, South Africa. Wildebeests spent more time in grazing lawns where the grass is of higher quality but shorter than in seep zones, where the grass is of lower quality but more abundant. Although the daily distances moved were longer during the wet season compared to the dry season, the daily net displacement was lower, and the residence time higher, indicating a more frequent occurrence of area-concentred searching. In contrast, during the late dry season the foraging sessions were more fragmented and wildebeests moved more frequently between foraging areas. Surprisingly, predation risk appeared to be the second factor, after water availability, influencing movement during the dry season, when resources are limiting and thus expected to influence movement more. Our approach, using complementary analyses of different movement metrics, provided an integrated view of changes in individual movement with varying environmental conditions and predation risk. It makes it possible to highlight the adaptive behavioral decisions made by wildebeest to cope with unpredictable environmental variations and provides insights for population conservation

Certainty Closure: Reliable Constraint Reasoning with Incomplete or Erroneous Data

Constraint Programming (CP) has proved an effective paradigm to model and solve difficult combinatorial satisfaction and optimisation problems from disparate domains. Many such problems arising from the commercial world are permeated by data uncertainty. Existing CP approaches that accommodate uncertainty are less suited to uncertainty arising due to incomplete and erroneous data, because they do not build reliable models and solutions guaranteed to address the user's genuine problem as she perceives it. Other fields such as reliable computation offer combinations of models and associated methods to handle these types of uncertain data, but lack an expressive framework characterising the resolution methodology independently of the model. We present a unifying framework that extends the CP formalism in both model and solutions, to tackle ill-defined combinatorial problems with incomplete or erroneous data. The certainty closure framework brings together modelling and solving methodologies from different fields into the CP paradigm to provide reliable and efficient approches for uncertain constraint problems. We demonstrate the applicability of the framework on a case study in network diagnosis. We define resolution forms that give generic templates, and their associated operational semantics, to derive practical solution methods for reliable solutions.Comment: Revised versio

Simultaneous Heat and Mass Transfer in Inclined Channel with Asymmetrical Conditions

The present work is a numerical study of simultaneous heat and mass transfer with phase change in an inclined channel formed by two parallel plates. The lower one is covered by a thin liquid water film and the upper one is considered impermeable. The plates are maintained at a constant temperature. The liquid film is assumed to be extremely thin and its temperature is uniform and equal to that of the wall. Thermo-physical properties are considered constant and the Boussinesq assumption is adopted. Results show that the effects of the buoyancy forces on the hydrodynamic, thermal and mass fraction fields are important. These effects depend on the channel inclination and may result on flow reversal when the channel approaches the vertical position. This phenomenon is addressed and a flow reversal chart, as well as the corresponding correlations, for different channel inclinations is given. These correlations give the values of Grashof numbers, which induce flow reversal for a given Reynolds number and inclination angle

Melody processing characterizes functional neuroanatomy in the aging brain

The functional neuroanatomical mechanisms underpinning cognition in the normal older brain remain poorly defined, but have important implications for understanding the neurobiology of aging and the impact of neurodegenerative diseases. Auditory processing is an attractive model system for addressing these issues. Here, we used fMRI of melody processing to investigate auditory pattern processing in normal older individuals. We manipulated the temporal (rhythmic) structure and familiarity of melodies in a passive listening, ‘sparse’ fMRI protocol. A distributed cortico-subcortical network was activated by auditory stimulation compared with silence; and within this network, we identified separable signatures of anisochrony processing in bilateral posterior superior temporal lobes; melodic familiarity in bilateral anterior temporal and inferior frontal cortices; and melodic novelty in bilateral temporal and left parietal cortices. Left planum temporale emerged as a ‘hub’ region functionally partitioned for processing different melody dimensions. Activation of Heschl’s gyrus by auditory stimulation correlated with the integrity of underlying cortical tissue architecture, measured using multi-parameter mapping. Our findings delineate neural substrates for analyzing perceptual and semantic properties of melodies in normal aging. Melody (auditory pattern) processing may be a useful candidate paradigm for assessing cerebral networks in the older brain and potentially, in neurodegenerative diseases of later life