Les mécanismes de perception du phosphate chez les plantes ainsi que la mise en place des réponses à une carence demeurent relativement méconnus. On explique le nombre restreint d'éléments identifiés jusqu'ici par la complexité des mécanismes mis en jeu, impliquant probablement une redondance fonctionnelle. Ce travail de thèse avait pour but de contourner cet obstacle pour tenter d'apporter de nouveaux éléments de compréhension de la réponse à la carence en phosphate chez la plante modèle Arabidopsis thaliana. Une approche de génétique chimique a donc été utilisée pour isoler une famille de molécules (les Phosphatins) inhibant de nombreuses réponses induites par la carence phosphatée. Elles agissent aussi bien au niveau de la réponse morphologique que métabolique. Ceci révèle l'existence de blocages génétiques limitants la croissance des plantes en condition de carence en phosphate. Cette étude suggère aussi l'existence de « cross talks » potentiels ou de mécanismes redondants ignorés jusqu'ici entre les réponses morphologiques et biochimiques. / The plant mechanism of phosphate perception and responses pathways to a starvation are still poorly understood. We explain that only few elements have been identified so far by the complexity of the mechanisms involved, probably coupled with functional redundancy. The aim of this work was to bypass this obstacle in order to deep our knowledge on the understanding of the phosphate starvation response in the plant model Arabidopsis thaliana. A chemical genetic approach was used to isolate a drug family (the Phosphatins) inhibiting several responses induced by a phosphate starvation. They act both on morphological and metabolic response. This reveals the existence of genetic blocking limiting plant growth under conditions of phosphate deficiency. This study also suggests the existence of potential "cross talks" or redundant mechanisms hitherto unknown between morphological and biochemical responses.

Domestic goats were used to modify the growth form of blackbrush, a spinescent shrub occurring in nearly monospecific stands on several million hectares of rangeland in the southwestern United States. The objective of this research was to evaluate goat browsing as a means of improving these rangelands for cattle. Winter goat browsing stimulated spring twig growth from basal and axillary buds which resulted in increased production. Twig production by heavily browsed plants (>95 percent removal of current season's twigs) was a function of precipitation, soil depth, branch location on the plant, and period of rest after browsing. As precipitation doubled, production increased by a factor of 1.9. Twig production by plants growing on deep soils (71 cm) was 1.9 times that by plants growing on shallow soils (39 cm). Older branches growing on the outer edges of blackbrush plants (terminal branches) produced 4.6 times more current season's twigs than sprouts and young branches (basal branches) growing within the shrub canopy. Heavily browsed plants increased twig production by a factor of 3.6 relative to control plants, and production remained at this level, even after four consecutive years of browsing. Stocking intensities of 2.4 animal-unit-months·hectare-1 were required to achieve utilization levels of 80 percent in blackbrush pastures. Annual twig production declined with rest from browsing. However, plants which were browsed and subsequently rested for two years yielded an aggregate 1.6 times more available forage than plants which were browsed on a yearly basis. This was due to an accumulation of twigs ranging in age from one to three years. Browsing also improved the apparent nutritional quality of blackbrush twigs. Current season's twigs contained more crude protein (6.5 versus 4.6 percent), phosphorus (0.10 versus 0.08 percent), and in vitro digestible dry matter (48 versus 38 percent) than older twigs. Current season's twigs from basal branches contained more crude protein (6.1 versus 5.7 percent) and in vitro digestible dry matter (44 versus 41 percent) than those from terminal branches. The palatability of current season's twigs to goats and cattle was lower, however, than that of older twigs, presumably due to their higher tannin levels. Within individual blackbrush plants, current season's twigs from terminal branches were higher in tannins than those from basal branches. Rest from browsing resulted in decreased tannin levels due to a decrease in the proportion of current season's to older twigs. Goats and cattle tended to prefer older twigs to current season's twigs, and current season's twigs from basal branches to those from terminal branches. The occurrence and allocation of tannins within blackbrush support hypotheses dealing with the elaboration and allocation of phyto-chemicals as defense mechanisms countering herbivory. Esophageally fistulated goats (does and kids) browsing in pastures where forage consisted primarily of current season's twigs consumed diets with more crude protein, in vitro digestible dry matter, and tannins than goats browsing in pastures where forage consisted primarily of older twigs. They also lost less weight. Does initially consumed diets higher, but later consumed diets lower in crude protein than those consumed by kids. Kids consumed diets with more in vitro digestible dry matter, but lost more weight than does. No statistically significant differences in weight response were recorded for cattle browsing in pastures which were, and were not, previously browsed by goats. However, the average heifer in previously unbrowsed pastures consumed 1.9 times more protein supplement than her counterpart in previously browsed pastures.