Concomitant Loss of the Glyoxalase System and Glycolysis Makes the Uncultured Pathogen “Candidatus Liberibacter asiaticus” an Energy Scavenger

Abstract

Methylglyoxal (MG) is a cytotoxic, nonenzymatic by-product of glycolysis that readily glycates proteins and DNA, resulting in carbonyl stress. Glyoxalase I and II (GloA and GloB) sequentially convert MG into D-lactic acid using glutathione (GSH) as a cofactor. The glyoxalase system is essential for the mitigation of MG-induced carbonyl stress, preventing subsequent cell death, and recycling GSH for maintenance of cellular redox poise. All pathogenic liberibacters identified to date are uncultured, including "Candidatus Liberibacter asiaticus," a psyllid endosymbiont and causal agent of the severely damaging citrus disease "huanglongbing." In silico analysis revealed the absence of gloA in "Ca. Liberibacter asiaticus" and all other pathogenic liberibacters. Both gloA and gloB are present in Liberibacter crescens, the only liberibacter that has been cultured. L. crescens GloA was functional in a heterologous host. Marker interruption of gloA in L. crescens appeared to be lethal. Key glycolytic enzymes were either missing or significantly downregulated in "Ca. Liberibacter asiaticus" compared to (cultured) L. crescens. Marker interruption of sut, a sucrose transporter gene in L. crescens, decreased its ability to take up exogenously supplied sucrose in culture. "Ca. Liberibacter asiaticus" lacks a homologous sugar transporter but has a functional ATP/ADP translocase, enabling it to thrive both in psyllids and in the sugar-rich citrus phloem by (i) avoiding sucrose uptake, (ii) avoiding MG generation via glycolysis, and (iii) directly importing ATP from the host cell. MG detoxification enzymes appear to be predictive of "Candidatus" status for many uncultured pathogenic and environmental bacteria.IMPORTANCE Discovered more than 100 years ago, the glyoxalase system is thought to be present across all domains of life and fundamental to cellular growth and viability. The glyoxalase system protects against carbonyl stress caused by methylglyoxal (MG), a highly reactive, mutagenic and cytotoxic compound that is nonenzymatically formed as a by-product of glycolysis. The uncultured alphaproteobacterium "Ca. Liberibacter asiaticus" is a well-adapted endosymbiont of the Asian citrus psyllid, which transmits the severely damaging citrus disease "huanglongbing." "Ca. Liberibacter asiaticus" lacks a functional glyoxalase pathway. We report here that the bacterium is able to thrive both in psyllids and in the sugar-rich citrus phloem by (i) avoiding sucrose uptake, (ii) avoiding (significant) MG generation via glycolysis, and (iii) directly importing ATP from the host cell. We hypothesize that failure to culture "Ca. Liberibacter asiaticus" is at least partly due to its dependence on host cells for both ATP and MG detoxification.KEYWORDS ATP/ADP translocase, carbonyl stress, citrus greening, culturability, glycolysis, glyoxalase, huanglongbing, liberibacter, methylglyoxal, sugar transporter

Horizontal gene transfer to a defensive symbiont with a reduced genome amongst a multipartite beetle microbiome

“…The lack of Complex III would likely result in a decreased rate of ATP production in Lv-StB as observed in fungi with alternative oxidases that bypass Complexes III and IV (Duarte and Videira, 2009). Lv-StB may be similar to the psyllid endosymbiont “ Candidatus Liberibacter asiaticus”, which has lost both key glycolysis and glyoxalase genes and instead relies on the scavenging of ATP from the host (Jain et al, 2017).…”

Controlled replication of ‘Candidatus Liberibacter asiaticus‘ DNA in citrus leaf discs

“…Moreover, the absence in ‘ Ca. L. asiaticus ’ of genes shown to have significance for detoxification of methylglyoxal (MG; Jain et al , ) may predispose ‘ Ca. L. asiaticus ’ to MG sensitivity and thus make metabolism of glucose a suboptimal carbon source for this pathogen.…”

“…The ‘ Ca. L. asiaticus ’ genome encodes an apparently intact ATP/ADP transporter ( nttA ; Duan et al , ; Vahling et al , ; Jain et al , ), suggesting the pathogen acts like an ‘energy parasite’ by importing ATP directly from the host akin to the obligate intracellular bacteria Rickettsia prowazekii (Plano and Winkler, ; Driscoll et al , ) and Chlamydia trachomatis (Iliffe‐Lee and McClarty, ). It is possible that leaf tissue converts glucose to ATP and therefore that the increase in ‘ Ca.…”

Potential soil transmission of a novel Candidatus Liberibacter strain detected in citrus seedlings grown in soil from a huanglongbing infested citrus grove

“…The potential soil-borne nature of HLB was thought to be 365 non-credible when preliminary data were first presented at the Second International Research Conference 366 on Huanglongbing in 2011 (Nunes da Rocha et al 2011). One of the arguments for this skepticism was 367 that Las could not possibly survive in soil due to its very small genome, missing the genes for several 368 essential enzymes including those for glycolysis(Duan et al 2009; Hartung et al 2011;Jain et al 2017). 369In the meantime however, several other observations suggest the potential soil-borne nature of Las and 370 related Ca.…”

“…Liberibacter str. UFEPI, even if it could not multiply in 395 soil due to a lack of essential enzymes(Jain et al 2017). Alternatively, the pathogen could have been 396 vectored by contaminated soilborne insect vectors, nematodes or fungi, which could have transferred the 397 pathogen to young roots.…”