Location: Subtropical Plant Pathology ResearchTitle: Las 5315 effector induces extreme starch accumulation and chlorosis as Ca. Liberibacter asiaticus infection in Nicotiana benthamiana
Submitted to: Frontiers in Plant Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/22/2018
Publication Date: 2/7/2018
Citation: Pitino, M., Allen, V.W., Duan, Y. 2018. Las 5315 effector induces extreme starch accumulation and chlorosis as Ca. Liberibacter asiaticus infection in Nicotiana benthamiana. Frontiers in Plant Science. 9:113.
Interpretive Summary: Huanglongbing (HLB) is one of the most complex and severe diseases, which causes billions of dollars’ loss yearly in the citrus industry worldwide. HLB associated with Candidatus Liberibacter asiaticus (Las) often causes yellow shoots and blotchy mottle on leaves in infected citrus plants. These HLB symptoms were hypothetically attributed to starch accumulation in infected leaves, which resulted from callose deposition and disruption of phloem cells that interfered carbohydrate transportation. In this study, we demonstrate that starch accumulation and chlorosis (yellowing), the main symptoms of HLB, were induced by a virulence protein of Las bacterium in the experimental host plant, Nicotiana benthamiana. This unusual starch accumulation was the outcome by increasing starch synthesis during the day while decreasing starch degradation at night. These pathogenic effects are the opposite of normal sugar metabolism in non-infected citrus plants. Meanwhile, we demonstrated N. benthamiana plants can be infected by Las bacterium with the help of dodder plants. This is an important finding for HLB research because N. benthamiana grows much faster than citrus, and many useful genes of this plant could be further characterized with Las infection, and adapted for the control of citrus HLB.
Technical Abstract: Huanglongbing (HLB), a destructive plant bacterial disease, severely impedes worldwide citrus production. HLB is associated with an unculturable phloem-limited a-proteobacterium, Candidatus Liberibacter asiaticus (Las). Las infection causes yellow shoots and blotchy mottle on leaves that are associated with excessive starch accumulation. However, the mechanism(s) underlying starch accumulation remains unknown. We previously showed that Las5315mp effector induced callose deposition and cell death in Nicotiana benthamiana. In this study, we demonstrated that N. benthamiana can be experimentally infected with Las via dodder transmission. Furthermore, we revealed another key function of Las5315 effector by demonstrating that transient expression of Las'5315 induced excessive starch accumulation (6 fold after 8 days post inoculation) in N. benthamiana after removal of the chloroplast transit peptide from the Las5315mp. The induction mechanisms of Las'5315 in N. benthamiana were attributed to the up-regulation of AGPase/ADP-glucose pyrophosphorylase, granule-bound starch synthase, soluble starch synthase, and starch branching enzyme for increasing starch production, and to the significant down-regulation of the starch degradation enzymes: alpha-glucosidase, alpha-amylase, and glycosyl hydrolase for decreasing starch degradation. This is the first report that Las can infect the model plant, N. benthamiana, and using this model plant, we demonstrated that Las'5315 effector caused one of the most important HLB symptoms, starch accumulation and chlorosis phenotype, suggesting it is a critical target for interference, and therefore, control of HLB.