The plant hormone cytokinin plays essential roles in many aspects of growth and development. The cytokinin signal is transmitted by a multistep phosphorelay to the members of two functionally antagonistic classes of Arabidopsis response regulators (ARRs): the type-B ARRs (response activators) and type-A ARRs (negative-feedback regulators). Previous studies have shown that mutations in AXR1, encoding a subunit of the E1 enzyme in the related to ubiquitin (RUB) modification pathway, leads to decreased cytokinin sensitivity. This research shows that the cytokinin resistance of axr1 seedlings is suppressed by loss-of-function of type-A ARRs and that the cytokinin resistance caused by ectopic expression of ARR5, a type-A ARR family member, is enhanced in axr1 background. Based on the established role of the RUB pathway in ubiquitin-dependent proteolysis, these data suggested that AXR1 promotes the cytokinin response by facilitating type-A ARR degradation. Indeed, both genetic (axr1 mutants) and chemical (MLN4924) suppression of RUB E1 increased ARR5 stability, suggesting that the ubiquitin ligase that promotes ARR5 proteolysis requires RUB modification for optimal activity. In addition, ARR1, a type-B ARR family member, also accumulated in the axr1 mutant background, suggesting that AXR1 regulates primary cytokinin signaling at multiple levels.

Catharanthus roseus est une plante tropicale qui produit spécifiquement des alcaloïdes indoliques monoterpéniques (AIM) d’intérêt thérapeutique. Chez C. roseus, la branche terpénique incluant la voie du méthylérythritol phosphate (MEP) est considérée comme limitante et présente une régulation transcriptionnelle coordonnée en réponse aux hormones inductrices de l’accumulation alcaloïdique. Lors de ce travail, suite à des analyses bioinformatiques et à la caractérisation de promoteurs de gènes de la voie MEP, nous avons identifié de nouvelles familles de facteurs de transcription impliquées dans la régulation de la biosynthèse des AIM. Des membres de la famille des ZCT, des WRKY et des RR type B interagissent avec le promoteur du gène hds de la voie MEP et régulent son activité. Ces travaux ont permis d’approfondir les connaissances sur les réseaux transcriptionnels régulateurs de la biosynthèse des AIM. L’utilisation de ces nouveaux facteurs de transcription activateurs peut désormais être envisagée dans le cadre d’expériences d’ingénierie métabolique afin d’augmenter l’accumulation d’alcaloïdes d’intérêt pharmaceutique chez C. roseus. / Catharanthus roseus is a tropical plant producing specifically monoterpene indole alkaloids (MIA) of high interest due to their therapeutical values. In C. roseus cells, the terpenoid branch including the methyl erythritol phosphate pathway (MEP) provides the MIA terpenoid moiety and is regarded as limited for MIA biosynthesis. This branch presents a coordinated transcriptional regulation in response to hormonal signals leading to MIA production. In this context, bioinformatic analysises and functional characterization of MEP pathway gene promoters allowed the identification of new transcription factor families involved in the MIA pathway regulation. Members of ZCT proteins, WRKY and type B RR families specifically interact with the hds promoter from the MEP pathway and regulate its activity. This work permits to gain into insight the transcriptional network controlling the MIA biosynthesis. It is possible now to consider using transcription factor that act as activators and target genes from the terpenoid branch to increase the accumulation of alkaloids of pharmaceutical interest in C. roseus by metabolic engineering approaches.