Bacteria hijacks plant defense system to enter leaves

Jim Crocker
19th November, 2025

Bacteria hijacks plant defense system to enter leaves

The bacterium Salmonella enterica serovar Typhimurium 14028s reopens plant stomata—pores on leaf surfaces—by producing auxin, a plant hormone, enabling bacterial entry and potentially increasing the risk of foodborne illness.

Image adapted from: Fochs et al. / CC BY (Source)

Key Findings

  • Researchers found Salmonella can temporarily close plant stomata, but then reopen them, unlike some other bacteria
  • Both Salmonella and the plant produce a hormone called auxin, which is essential for Salmonella to reopen the stomata and invade the plant
  • Blocking auxin signaling in the plant makes it resistant to Salmonella reopening stomata, highlighting a new vulnerability in plant defenses
Plant leaves are not sterile surfaces; they host a complex community of microorganisms, a field of study historically less explored than the microbial life in the soil surrounding plant roots[2]. However, understanding this ‘phyllosphere’ – the aerial environment of plants – is increasingly important, particularly concerning plant health and food safety. A significant concern is how harmful bacteria, like Salmonella, can colonize fresh produce, leading to foodborne illness. Recent research has shown that Salmonella can enter plants through small pores on the leaf surface called stomata[3]. Stomata are crucial for plant gas exchange, but also represent potential entry points for pathogens. Plants have defense mechanisms, collectively known as stomatal immunity, to close these pores when they detect a threat. However, some bacteria can overcome this defense. Researchers from the University of California, Davis and The Ohio State University have investigated how Salmonella enterica serovar Typhimurium strain 14028s (STm 14028s) manages to reopen stomata after the plant initially attempts to close them[1]. The study revealed a surprising mechanism: the bacterium and the plant both produce a plant hormone called auxin, and this auxin production is essential for STm 14028s to reopen the stomata. Auxin is known to regulate plant growth and development, but its role in plant-pathogen interactions was previously unclear. The researchers found that STm 14028s produces auxin directly on the leaf surface using an enzyme called indole pyruvate decarboxylase (ipdC). Crucially, the bacterium also triggers the plant itself to produce more auxin, specifically through a plant enzyme called YUC5. To demonstrate this, the researchers performed several experiments. They showed that when STm 14028s was unable to produce auxin due to a mutation in the ipdC gene, it couldn’t effectively reopen stomata. They also demonstrated that when plants lacked functional auxin signaling pathways – meaning they couldn’t respond to auxin – they were resistant to STm 14028s reopening the stomata. This was tested using mutant plants with defects in auxin signaling components (tir1-10, axr1-3, and aux1-7) and by using chemicals that block auxin signaling. Interestingly, this auxin-mediated stomatal reopening is specific to STm 14028s. Another bacterium, Pseudomonas syringae, which also infects plants, does not rely on auxin to reopen stomata. This suggests a unique strategy employed by Salmonella to overcome plant defenses. The researchers even showed that transferring the ipdC gene from Salmonella into E. coli – a bacterium that doesn’t normally induce plant auxin production – partially restored the ability to reopen stomata, further confirming the role of auxin production. This discovery builds upon earlier work showing that bacterial chemotaxis – their ability to move towards chemical signals – is vital for plant pathogen infection[4]. Salmonella’s ability to sense and respond to chemical cues, including those related to plant metabolism, likely guides it towards stomata and facilitates this auxin-mediated reopening. The study also aligns with the understanding that stomata are key interaction sites between plants and pathogens, where a constant ‘battle’ for control takes place[5]. The dynamic opening and closing of stomata during infection, as previously observed, may be directly linked to this auxin signaling pathway. The findings highlight a previously unknown vulnerability in plant immunity and offer potential targets for developing strategies to prevent Salmonella contamination of crops. By understanding how Salmonella manipulates plant hormone signaling, researchers may be able to develop methods to disrupt this process and enhance plant resistance to infection.

AgricultureBiochemPlant Science

References

Main Study

1) Salmonella enterica exploits the auxin signaling pathway to overcome stomatal immunity

Published 17th November, 2025

https://doi.org/10.1371/journal.ppat.1013662

Related Studies

2) Microbial life in the phyllosphere.

https://doi.org/10.1038/nrmicro2910

3) Internalization of Salmonella enterica in leaves is induced by light and involves chemotaxis and penetration through open stomata.

https://doi.org/10.1128/AEM.01084-09

4) The effect of bacterial chemotaxis on host infection and pathogenicity.

https://doi.org/10.1093/femsre/fux052

5) Fighting for Survival at the Stomatal Gate.

https://doi.org/10.1146/annurev-arplant-070623-091552


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