Changes in carbohydrate metabolism in Citrus sinensis infected with 'Candidatus Liberibacter asiaticus'

Changes in carbohydrate metabolism in sweet orange (Citrus sinensis) infected with 'Candidatus Liberibacter asiaticus', a purported cause of citrus Huanglongbing (HLB), were investigated. Starch levels in HLB-infected leaves with and without symptoms increased 3 center dot 1- and 7 center dot 9-fold, respectively, compared to healthy controls. In symptomless leaves, sucrose and fructose accumulated significantly (P < 0 center dot 05) in both midribs and lobes, and glucose only in the midribs (greater than fivefold); whereas maltose levels were reduced to 64 center dot 6% and 86 center dot 8% in the midribs and foliar lobes, respectively, of the values in healthy leaves. In leaves with symptoms, sucrose and glucose remained at high levels compared to healthy leaves, whilst no accumulation of fructose was observed; by contrast, the maltose content decreased to as low as 49 center dot 6% of that in healthy leaves. Fourfold induction of cell-wall-bound invertase activity was detected in both types of leaves on diseased plants. Additionally, the expression profiles of starch breakdown genes suggested that the transcription of DPE2 and MEX1 was down-regulated. Together with the reduction of maltose, it is suggested that the impairment of starch breakdown may contribute to the starch accumulation in infected leaves. The imbalance of carbohydrate partitioning and its relation to disease physiology are discussed.