‘Candidatus Liberibacter asiaticus’ Expands and Scavenges the Nutritional Choline Pool in its Host Grapefruit (Citrus × paradisi) Leaves

Phosphatidylcholine (PtdCho) is an unusual membrane phospholipid present in some endosymbiotic and intracellular pathogenic prokaryotes. ‘Candidatus Liberibacter asiaticus’ (CLas) is a phloem-limited, uncultured, fastidious α-Proteobacterium associated with the devastating citrus “greening” disease (Huanglongbing). Phylogenetically related but nonpathogenic L. crescens (Lcr) was used as a culturable surrogate to examine PtdCho biosynthesis in pathogenic CLas. Genes encoding key enzymes for two alternative PtdCho biosynthetic routes are present in the Lcr genome, viz. the one-step CDP-choline (pcs-encoding phosphatidylcholine synthase) and the three-step methyl-transferase pathway (pmt-encoding phospholipid N-methyltransferase). However, only the CDP-choline pathway genes for incorporating exogenous Cho were identified in the CLas genome. Exogenous Cho enhanced growth and alleviated osmotic stress in wild-type Lcr and in the pmt insertion mutant strains when cultured in sugar-rich medium. Quantitative RT-PCR analyses confirmed active uptake and condensation of nutritional Cho into PtdCho by CLas in both its plant host and psyllid vector. CLas-infected grapefruit leaves showed transcriptional activation of Cho biosynthesis genes and 2.8-fold higher levels of Cho. In plant cells, the compatible osmolyte glycine-betaine (GlyBet) is also derived from Cho. Expression of GlyBet biosynthesis genes and the GlyBet content was similar in both CLas-infected and healthy leaf tissue. The data presented here suggest that CLas likely exploits the Cho biosynthetic pathway in citrus hosts to expand the nutritional Cho pool.