‛Candidatus Liberibacter asiaticus’, the putative causal agent of citrus greening is not available in pure culture yet. In addition to trees of citrus and citrus relatives, ‛Ca. L. asiaticus’ can grow in Madagascar periwinkle (Catharanthus roseus). Using GC-MS, we compared the phloem sap composition in sweet orange ‛Valencia’ (Citrus sinensis) and periwinkle plants after the infection with ‛Ca. L. asiaticus’. Interestingly, in contrast to our previous studies of total leaf metabolites, we found that, compared to uninfected phloem sap, we found that the organic acids implicated in the TCA cycle including citrate, isocitrate, succinate, fumarate, and malate were reduced significantly in the infected phloem saps of both species. As a result of the reduction of organic acids content, the pH of infected phloem saps was increased. We hypothesize that the bacterial growth induces the mitochondrial TCA cycle in parenchyma cells to produce more of these compounds to be used as a bacterial carbon source. Once these compounds reach a low level in the phloem sap, the bacterium may send a signal, yet to be identified, to initiate a feedback loop to further induce the TCA cycle. Phloem blockage might be another reason behind the reduced the translocation of TCA cycle intermediates within the phloem. The net result, localized availability of organic acids likely benefits the bacterial growth and may explain the unequal distribution of ‘Ca L. asiaticus’ within infected trees. These findings may help in designing media for the pure culturing of ‛Ca. L. asiaticus’.
Changes in the expression of CrFTA, the Catharanthus roseus farnesyltransferase α-subunit gene, in response to a Candidatus Liberibacter asiaticus infection
