Submitted to: PLoS One
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/29/2013
Publication Date: 9/25/2013
Citation: Martinelli, F., Reagan, R.L., Uratsu, S.L., Phu, M.L., Albrecht, U., Zhao, W., Davis, C.E., Bowman, K.D., Dandekar, A.M. 2013. Gene regulatory networks elucidating Huanglongbing disease mechanisms. PLoS One. 8(9):e74256. doi:10.1371/journal.pone.0074256. Interpretive Summary: Huanglongbing (HLB – formerly known as “citrus greening”) is one of the most destructive diseases of citrus in Florida and other citrus-producing countries worldwide. In Florida the disease is associated with the bacterium Candidatus Liberibacter asiaticus (CaLas). Studies were conducted to examine the metabolic changes associated with infection by CaLas with a goal of better understanding the plant response to infection. The study compared three tissues (immature fruit, young and mature leaves) and four categories of orchard trees (symptomatic, asymptomatic, apparently healthy, and healthy). Sucrose and starch metabolism was found to play a major role in disease development in all tissues. An HLB-induced change in source-sink movement of carbohydrates was measured by upregulation and downregulation of specific genes in leaves and fruit. Systemic acquired responses that often allow plants to defeat bacterial infection, were observed to be inadequately activated in young leaves from CaLas-infected trees.
Technical Abstract: Next-generation sequencing was exploited to gain deeper insight into the response to infection by Candidatus liberibacter asiaticus (CaLas), especially the immune disregulation and metabolic dysfunction caused by source-sink disruption. Previous fruit transcriptome data were compared with additional Ribonucleic acid (RNA)-Seq data in three tissues: immature fruit, young and mature leaves. Four categories of orchard trees were studied: symptomatic, asymptomatic, apparently healthy, and healthy. Principal component analysis found distinct expression patterns between immature and mature fruits and leaf samples for all four categories of trees. A predicted protein-protein interaction network identified huanglongbing (HLB)-regulated genes for sugar transporters playing key roles in the overall plant responses. Gene set and pathway enrichment analyses highlight the role of sucrose and starch metabolism in disease symptom development in all tissues. HLB-regulated genes (glucose-phosphate-transporter, invertase, starch-related genes) would likely determine the source-sink relationship disruption. In infected leaves, transcriptomic changes were observed for light reactions genes (downregulation), sucrose metabolism (upregulation), and starch biosynthesis (upregulation). In parallel, symptomatic fruits over-expressed genes involved in photosynthesis, sucrose and raffinose metabolism, and downregulated starch biosynthesis. We visualized gene networks between tissues inducing a source-sink shift. CaLas alters the hormone crosstalk, resulting in weak and ineffective tissue-specific plant immune responses necessary for bacterial clearance. Accordingly, expression of WRKYs (including WRKY70) was higher in fruits than in leaves. Systemic acquired responses were inadequately activated in young leaves, generally considered the sites where most new infections occur.