How bacteria coordinate attacks on plants

Bacteria Pseudomonas syringae They do not operate as homogeneous armies. Instead, they organize themselves into specialized squads that alternate between producing toxins to sabotage the plant’s immune system and using flagella to migrate. This division of labor, revealed by European researchers, redefines the understanding of bacterial virulence in agricultural crops.

Using confocal microscopy and flow cytometry, scientists identified remarkable phenotypic heterogeneity. In bean leaves (Phaseolus), each bacterial cell adopts a distinct behavior: either it activates genes of the type III secretion system (T3SS) to inject toxic proteins into the plant cells, or it produces flagella to move around. They rarely do both at the same time.

This emergent behavior does not depend on genetic differences. Even clonal populations revealed distinct patterns of gene expression, resulting from stochastic and environmental factors. Bacteria close to host cells preferentially activate the T3SS. As the infection progresses, others, further away, start producing flagella. This spatial distribution suggests a functional architecture within the apoplastic microcolonies.

The toxins secreted by T3SS function as “common goods,” suppressing plant immunity for the collective benefit. This creates an environment conducive to the escape of motile bacteria with activated flagella from the plant tissue before necrosis occurs. Early escape increases the chances of survival and dissemination, especially under the humid conditions simulated in the sprayed leaf experiment.

The metabolic costs of these behaviors are real. Expressing T3SS reduces bacterial growth, as demonstrated by experiments with mutants that grow faster when deprived of this system. The production of flagella, on the other hand, imposes a smaller but still measurable cost. When both functions are activated simultaneously, the damage is exacerbated.

These findings point to a form of bacterial cooperation rarely documented in plant pathogens. Phenotypic heterogeneity confers adaptive advantages to the group as a whole. Cell-specific specialization creates a coordinated network of actions that promotes efficient leaf colonization and orderly host exit.

More than a biological phenomenon, it is a sophisticated evolutionary strategy. By preventing all individuals from simultaneously activating costly and immunogenic systems, bacteria balance efficiency and discretion. This logic is reminiscent of the organization of multicellular systems, where the distribution of tasks maximizes the survival of the group.

This model, based on "division of labor", contrasts with the "bet-hedging" hypothesis, in which different behaviors emerge as random preparation for future events. In the case of P. syringae, cooperative functionality and spatial distribution of phenotypes indicate more refined biological planning.

More information at doi.org/10.1038/s41564-025-01966-0