Larvae of Baris coerulescens Scop. (Coleoptera: Curculionidæ) exhibit a complex array of gut proteinase activities comprising cysteine and serine proteinases. The major cysteine proteinase activity, showing an optimum at pH 6.0, corresponds to at least 4 different proteinases. On the contrary, the minor serine proteinase activity, with an optimum at pH 9.0, seems to be due essentially to a single proteinase. The cysteine proteinase inhibitor oryzacystatin I (OC-I) inhibits completely the cysteine proteinase activity in vitro. However, larval growth and survival were not significantly different on control and transgenic oilseed rape plants expressing high levels of active OC-I. In larvae grown on transgenic plants, cysteine proteinase activity was dramatically decreased, whereas serine proteinase activity was increased by more than 2-fold, when compared to larvae raised on control plants. For both activities, no new proteinase was detected in insects fed plants expressing OC-I. These results suggest that partial compensation of the inhibition of cysteine proteinase activity by the increase in serine proteinase activity allowed the larvae to overcome the effects of OC-I consumption. This case illustrates problems that could arise when trying to achieve high levels of protection for plants against Coleopteran pests possessing a complex digestive proteinase pool.

The resistance of a transgenic line of oilseed rape expressing constitutively the cysteine proteinase inhibitor oryzacystatin I (OCI) was assessed against Psylliodes chrysocephala L. (Coleoptera: Chrysomelidae). The levels of OCI expression in the transformed line averaged 0.2% and 0.05% of total soluble protein in leaves and petioles respectively. In vitro analyses showed that P. chrysocephala larvae use both cysteine and serine proteinases for protein digestion, and that all the cysteine proteolytic activity is OCI-sensitive. However, bioassays showed that adults fed identically on leaf discs from control or transformed plants. When larvae were reared on transgenic plants expressing OCI, they showed an increase in weight gain compared to those reared on control plants. Furthermore, those larvae from transgenic plants exhibited a 2-fold increase in both cysteine and serine proteolytic activity as a reponse to the presence of OCI. The plasticity of insect digestive physiology and feeding behaviour are discussed, as well as the relevance of engineering a genotype expressing both types of proteinase inhibitors.

Si nombre de gènes végétaux de résistance spécifiques d'agent pathogène sont connus et utilisés depuis long-temps dans les programmes d'amélioration des plantes, il aura fallu attendre 1993 pour que l'un d'entre eux, un gène de résistance à une bactérie chez la tomate soit enfin cloné. Depuis, quatre autres gènes de résistance - à une bactérie, à un virus et à deux champignons - ont été clonés. Leur fonction dans la détermination des mécanismes de défense des plantes se précise, des applications se dessinent.

Si nombre de gènes végétaux de résistance spécifiques d'agent pathogène sont connus et utilisés depuis long-temps dans les programmes d'amélioration des plantes, il aura fallu attendre 1993 pour que l'un d'entre eux, un gène de résistance à une bactérie chez la tomate soit enfin cloné. Depuis, quatre autres gènes de résistance - à une bactérie, à un virus et à deux champignons - ont été clonés. Leur fonction dans la détermination des mécanismes de défense des plantes se précise, des applications se dessinent.