How to drive phloem gene expression? A case study with preferentially expressed citrus gene promoters

Abstract

Abstract New approaches for developing disease-resistant genetically modified organisms have included specific targets for gene expression to enhance the chances for pathogen control. Gene expression driven by phloem-derived Citrus sinensis gene promoters could be evaluated and compared with the expression induced by a strong constitutive promoter in the same tissue, leading to the production of transgenic sweet oranges potentially more resistant to diseases caused by phloem-limited bacteria. ‘Carrizo’ citrange [ (Poncirus trifoliataL.) Raf. x Citrus sinensis (L.) Osbeck] was transformed, via Agrobacterium tumefaciens, with the binary vector pCAMBIA2301 bearing the uidA gene (ß-glucuronidase) driven by the CaMV35S constitutive promoter (CaMV35S::uidA) or by the CsPP2.B1 (CsPP2.B1::uidA) or by the CsVTE2 (CsVTE2::uidA) citrus promoters. In vitro regenerated shoots were grafted onto ‘Rangpur’ lime (C. limonia Osbeck). The genetic transformation was confirmed by Southern blot analyses. uidA gene expression was evaluated by RT-qPCR, and gene histolocalization controlled by these three promoters was accessed by X-GLUC treated stem sections. uidA gene expression exhibited by tissue-specific promoters was overall lower than from the constitutive promoter CaMV35; however, constructs driven by tissue-specific promoters may lead to expression in restricted tissues. CsPP2.B1 and CsVTE2 promoters can be considered adequate for the utilization in gene constructs aiming disease resistance.