Orange fruit from trees infected by ‘ Candidatus Liberibacter asiaticus’ ( CaLas) often do not look fully mature and exhibit off-flavours described as bitter, harsh, and metallic rather than juicy and fruity. Although previous studies have been carried out to understand the effect of CaLas on the flavour of orange juice using metabolomic methods, the mechanisms leading to the off-flavour that occurs in Huanglongbing (HLB)-symptomatic fruit are not well understood. In this study, fruits were collected from symptomatic and healthy Valencia sweet orange ( Citrus sinensis) trees grafted on Swingle ( C. paradisi X Poncirus trifoliata) rootstock. Isobaric tags for relative and absolute quantification (iTRAQ) and gas chromatography-mass spectrometry (GC-MS) were used to measure the proteins, sugars, organic acids, amino acids, and volatile terpenoids. The results showed that most of the differentially expressed proteins involved in glycolysis, the tricarboxylic acid (TCA) cycle and amino-acid biosynthesis were degraded, and terpenoid metabolism was significantly downregulated in the symptomatic fruit. Valencene, limonene, 3-carene, linalool, myrcene, and α-terpineol levels were significantly lower in fruit from CaLas-infected trees than from healthy trees. Similar phenomena were observed for sucrose and glucose. Our study indicated that off-flavour of symptomatic fruit was associated with a reduction in the levels of terpenoid products and the downregulation of proteins in glycolysis, the TCA cycle, and the terpenoid biosynthesis pathway.
The bacteria Candidatus Liberibacter asiaticus ( CaLas) causes orange trees to produce poor-tasting fruit thanks to the decreased production of flavour-enhancing proteins, sugars, and metabolites. The University of Florida’s Frederick Gmitter and his team of US and Chinese scientists profiled the proteins and metabolites of healthy Valencia sweet orange trees infected with CaLas, a bacterial pathogen that causes the citrus disease Huanglongbing and reduces the quantity and quality of fruit and juice. The researchers found 123 differentially-expressed proteins and decreased numbers of taste-enhancing constituents, including those mediated by key energy-producing processes. This degradation involved a class of chemicals called terpenoids, which the authors link to poor quality fruit. These results provide insights into the pathogenesis of CaLas infection and could empower future studies to prevent the impact of the bacteria and Huanglongbing infection.