PHLOEM PROMOTERS IN TRANSGENIC SWEET ORANGE ARE DIFFERENTIALLY TRIGGERED BY Candidatus Liberibacter asiaticus

ABSTRACT The use of promoters preferentially expressed in specific plant tissues is a desirable strategy to search for resistance for pathogens that colonize these tissues. The bacterium Candidatus Liberibacter asiaticus (Las), associated with huanglongbing disease (HLB) of citrus, colonizes phloem vessels. Some promoters, besides conferring tissue-specific expression, can also respond to the presence of the pathogen. The objective of the present study was to verify if the presence of Las could modulate the activation of the phloem-specific promoters AtPP2 (Arabidopsis thaliana phloem protein 2), AtSUC2 (A. thaliana sucrose transporter 2) and CsPP2 ( pCitrus phloemrotein 2), known to be expressed in Citrus sinensis phloem. ‘Hamlin’ sweet orange plants (Citrus sinensis L. Osbeck) transformed with the uidA (GUS) reporter gene under the control of AtPP2, AtSUC2 and CsPP2 promoters were infected to evaluate the interdependence between transgene expression and the concentration of Las. Plants were inoculated with Las by Diaphorina citri and eighteen months later, bacterial concentration and uidA expression were determined by qPCR and RT-qPCR, respectively. Reporter gene expression driven by AtSUC2 promoter was strongly and positively correlated with Las concentration. Therefore, this promoter combines desirable features of both tissue-specificity and pathogen-inducibility for the production of transgenic plants tolerant to Las.

Transient uidA gene expression in electroporated cotyledonary protoplasts of Pinus nigra ssp. salzmannii and in bombarded cotyledons

We have developed a method for the routine isolation of Pinus nigra Arn. ssp. salzmannii (Dunal) Franco protoplasts. The optimized electroporation conditions for uidA gene (gene for beta-glucuronidase) expression in protoplasts from cotyledons excised from 8-day-old seedlings were determined using an exponential pulse wave generator for gene transfer. The protoplasts were electroporated with plasmids containing the chimeric uidA gene under the control of several promoters, and in parallel, cotyledons were bombarded with the same constructs using a biolistic gun. Both techniques confirmed that gene expression was higher when controlled by the sunflower polyubiquitin gene promoter than by the cauliflower mosaic virus CaMV35S promoter, whereas the rice actin and maize alcohol dehydrogenase promoters resulted in lower uidA expression levels, as determined fluorometrically. In this study, the electroporation procedure has been more effective than the particle bombardment procedure to determine the promoter strength on transient gene expression in P. nigra.