A plethora of pathogenic microorganisms constantly attack plants. The Pseudomonas syringae species complex encompasses Gram-negative plant-pathogenic bacteria of special relevance for a wide number of hosts. P. syringae enters the plant from the leaf surface and multiplies rapidly within the apoplast, forming microcolonies that occupy the intercellular space. The constitutive expression of fluorescent proteins by the bacteria allows for visualization of the microcolonies and monitoring of the development of the infection at the microscopic level. Recent advances in single-cell analysis have revealed the large complexity reached by clonal isogenic bacterial populations. This complexity, referred to as phenotypic heterogeneity, is the consequence of cell-to-cell differences in gene expression (not linked to genetic differences) among the bacterial community. To analyze the expression of individual loci at the single-cell level, transcriptional fusions to fluorescent proteins have been widely used. Under stress

This work was supported by Project Grant RTI2018-095069- B-I00 funded by MCIN/AEI/10.13039/501100011033/ and by "ERDP A way of making Europe". J.S.R. was funded by Plan Andaluz de Investigación, Desarrollo e Innovación (PAIDI 2020). N.L.P. was funded by Project Grant P18-RT-2398 from Plan Andaluz de Investigación, Desarrollo e Innovación.

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