The complete genome of 'Candidatus Phytoplasma phoenicium' highlights carboxylic acids as primary carbon and energy substrates in phytoplasmas.
TL;DR
The findings link genome reduction to host-dependent carboxylic acid metabolism in a quarantine-relevant phytoplasma, establishing a framework for comparative and functional studies of 16SrIX phytoplasmas and providing a basis for future investigations of AlmWB ecology and pathogenicity.
Abstract
Phytoplasmas are wall-less plant pathogens characterized by highly reduced genomes and limited metabolic capabilities, leading to an obligate host dependency. Since phytoplasma axenic cultivation has not yet been achieved, genomic approaches are essential to unravelling their biological and pathogenic traits. However, such insights are currently hindered by the scarcity of complete genomes and the lack of data for several clades, as is the case for the quarantine-relevant 'Candidatus Phytoplasma phoenicium' (group 16SrIX-B), the causal agent of the severe stone-fruit disease almond witches' broom (AlmWB). In this study, we present the complete genome sequence of the Lebanese strain F1A, which consists of a 552,248 bp chromosome with a 24.29% GC content and encodes 450 protein-coding genes. Comparative analyses with 16SrIX group draft genomes revealed high conservation among Lebanese AlmWB strains; however, comparisons with related 16SrIX-C phytoplasmas were limited by their low assembly completeness. Beyond providing a complete reference genome for this phytoplasma group, the F1A chromosome revealed distinct metabolic features. Strain F1A lacks the upper glycolytic pathway, indicating a metabolism that relies on glycerol-3-phosphate uptake and carboxylic acid fermentation. Notably, in addition to the conserved malate-to-acetate pathway, F1A encodes a complete citrate lyase complex, suggesting the potential for citrate utilization. Phylogenetic analysis of the associated 2-hydrocarboxylate symporter revealed the widespread occurrence of two phylogenetically distinct variants in phytoplasmas. Apart from the deduced metabolic capacities, the predicted effector repertoire shows similarities with those of other phytoplasmas, including proteins associated with branch proliferation and witches' broom symptoms. Overall, these findings link genome reduction to host-dependent carboxylic acid metabolism in a quarantine-relevant phytoplasma, establishing a framework for comparative and functional studies of 16SrIX phytoplasmas and providing a basis for future investigations of AlmWB ecology and pathogenicity.