Abstract
Citrus canker is an economically important disease that affects orange production in some of the most important producing areas around the world. It represents a great threat to the Brazilian and North American citriculture, particularly to the states of São Paulo and Florida, which together correspond to the biggest orange juice producers in the world. The etiological agent of this disease is the Gram‐negative bacterium Xanthomonas citri subsp. citri (Xcc), which grows optimally in laboratory cultures at ~30°C. To investigate how temperatures differing from 30°C influence the development of Xcc, we subjected the bacterium to thermal stresses, and afterward scored its recovery capability. In addition, we analyzed cell morphology and some markers of essential cellular processes that could indicate the extent of the heat‐induced damage. We found that the exposure of Xcc to 37°C for a period of 6 h led to a cell cycle arrest at the division stage. Thermal stress might have also interfered with the DNA replication and/or the chromosome segregation apparatuses, since cells displayed an increased number of sister origins side‐by‐side within rods. Additionally, Xcc treated at 37°C was still able to induce citrus canker symptoms, showing that thermal stress did not affect the ability of Xcc to colonize the host citrus. At 40–42°C, Xcc lost viability and became unable to induce disease symptoms in citrus. Our results provide evidence about essential cellular mechanisms perturbed by temperature, and can be potentially explored as a new method for Xanthomonas citri synchronization in cell cycle studies, as well as for the sanitation of plant material.
Highlights
Bacterial cell cycle synchronization is useful in the isolation of subgroups of cells within a culture exhibiting different morphologies and/or physiological behaviors
Considering that Xanthomonas citri subsp. citri (Xcc) spends part of its life cycle as an epiphyte, bacterium has to deal with environmental temperature oscillations that are quite different from the optimum cultivation temperature used in the laboratory (28–30°C)
By investigating the effects of thermal stress for periods of 6 h on the growth of Xcc, we observed: first, the cell cycle is arrested at division when the bacterium is exposed to 37°C, and in addition, the cell division arrest detected does not seem to interfere with the ability of Xcc to colonize the host citrus
Summary
Bacterial cell cycle synchronization is useful in the isolation of subgroups of cells within a culture exhibiting different morphologies and/or physiological behaviors. Among the techniques employed to obtain synchronization, centrifugation gradients have been widely explored in Caulobacter crescentus (Schrader and Shapiro 2015), a bacterial model for cell cycle and differentiation studies This is probably the less disturbing method used since it does not require any specific cell lineage or mutant to be applied, and it allows the isolation of large amounts of cells (Lin et al 2012; Schrader and Shapiro 2015). Inhibition at the initiation of DNA replication was reported in E. coli mutants carrying thermo-sensitive dnaC alleles (Withers and Bernander 1998) This method of cell cycle arrest induced by thermal stress seems an attractive and easy procedure for cell synchronization, it has some disadvantages, such as the requirement for specific mutations, the occurrence of abnormal restarts of DNA replication following the stress, and the fact that the synchronization status cannot be kept for long periods
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