Sweet orange genetic transformation with the attacin A gene under the control of phloem-specific promoters and inoculation with Candidatus Liberibacter asiaticus

An alternative approach to control Huanglongbing disease in Citrus is the development of transgenic plants expressing genes that may influence pathogen development. We report herein the production of sweet orange transgenic plants bearing gene constructs containing the antimicrobial gene attacin A (attA)  under the control of phloem-specific promoters. Transcripts of the attA gene were accumulated in the transgenic lines bearing the three gene constructs. However, phloem protein 2 promoters drive higher levels of attA mRNA than sucrose transporter 2 promoter. Candidatus Liberibacter asiaticus (CLas) were transmitted to transgenic plants via infected Asian citrus psyllid or by grafting infected budwood. The efficiency of CLas transmission via infected psyllids was very low and therefore did not allow evaluating the influence of attA expression on bacteria multiplication. On the other hand, CLas transmission via grafting infected budwood was very efficient. Huanglongbing (HLB) symptoms were detected five months after inoculation in both transgenic and non-transgenic inoculated plants. CLas titers, determined 12 months after inoculation, were similar in both transgenic and non-transgenic inoculated plants. However, non-transgenic inoculated plants showed a significant reduction in shoot development comparing to transgenic inoculated plants indicating that the expression of attA gene may influence the plant tolerance to HLB disease.