Abstract
BackgroundLiberibacter crescens is the closest cultured relative of four important uncultured crop pathogens. Candidatus. L. asiaticus, L. americanus, L. africanus cause citrus greening disease, while Ca. L. solanacearum causes potato Zebra chip disease. None of the pathogens grows in axenic culture. L. crescens grows in three media: a BM-7, a serum-free Hi® Grace’s Insect Medium (Hi-GI), and a chemically-defined medium called M15. To date, no optimal growth parameters of the model species L. crescens have been reported. Studying the main growth parameters of L. crescens in axenic culture will give us insights into the lifestyle of the Ca. Liberibacter pathogens.ResultsThe evaluation of the growth parameters—pH, aeration, temperature, and buffering capacity—reflects the optimal living conditions of L. crescens. These variables revealed that L. crescens is an aerobic, neutrophilic bacterium, that grows optimally in broth in a pH range of 5.8 to 6.8, in a fully oxygenated environment (250 rpm), at 28 °C, and with monosodium phosphate (10 mM or 11.69 mM) as the preferred buffer for growth. The increase of pH in the external media likely results from the deamination activity within the cell, with the concomitant over-production of ammonium in the external medium.ConclusionL. crescens and the Ca. Liberibacter pathogens are metabolically similar and grow in similar environments—the phloem and the gut of their insect vectors. The evaluation of the growth parameters of L. crescens reveals the lifestyle of Liberibacter, elucidating ammonium and phosphate as essential molecules for colonization within the hosts. Ammonium is the main driver of pH modulation by active deamination of amino acids in the L. crescens amino acid rich media. In plants, excess ammonium induces ionic imbalances, oxidative stress, and pH disturbances across cell membranes, causing stunted root and shoot growth and chlorosis—the common symptoms of HLB-disease. Phosphate, which is also present in Ca. L. asiaticus hosts, is the preferred buffer for the growth of L. crescens. The interplay between ammonium, sucrose, potassium (K+), phosphate, nitrate (NO3−), light and other photosynthates might lead to develop better strategies for disease management.
Highlights
Liberibacter crescens is the closest cultured relative of four important uncultured crop pathogens
Since phosphate was the preferred buffer for L. crescens growth, phosphate concentrations based on the levels found in the citrus phloem were studied for L. crescens growth [11]
The evaluation strategy for the growth of L. crescens opens new avenues to understand the lifestyle of Liberibacter species and the potential interactions within the hosts
Summary
Liberibacter crescens is the closest cultured relative of four important uncultured crop pathogens. L. asiaticus, L. americanus, L. africanus cause citrus greening disease, while Ca. L. solanacearum causes potato Zebra chip disease. Studying the main growth parameters of L. crescens in axenic culture will give us insights into the lifestyle of the Ca. Liberibacter pathogens. The genus Liberibacter comprises reduced-genome, fastidious α-proteobacteria including the not yet cultured Ca. Liberibacter species, which are responsible for the devastating citrus greening, or Huanglongbing (HLB), and potato zebra chip (ZC) diseases worldwide [1]. The genus Liberibacter includes the endophytic bacterium Ca. L. europaeus found in carrots and celery as well as the only culturable species, L. crescens, which was isolated from mountain papaya in 1995 [4, 5]. Since L. crescens is the sole cultured member of the genus Liberibacter, it is the best and closest model bacteria to study the lifestyle and metabolism of Ca. Liberibacter pathogens
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