Phytohormones are key signaling compounds, which are essential for the regulation of plant development, growth, and physiology. Furthermore, phytohormones regulate plant responses to biotic and abiotic factors in their environment. Several phytohormones such as salicylic acid (SA), jasmonic acid (JA) and ethylene (ET) have been established as central hormone signals in plant immunity. More recently, classical growth-regulating phytohormones such as abscisic acid, auxins, cytokinins or gibberellins have been shown to impact plant immunity, at least partially through interactions with SA-JA/ET-signaling. Besides these, other phytohormones such as strigolactones and brassinosteroids are known as regulators of the aforementioned processes.

Different plant pathogens can manipulate the host plant’s phytohormone balance in favor of their infection process. Moreover, the positive effects on plants by beneficial microorganisms, like growth promotion, and biotic and abiotic stress tolerance, are also mediated by phytohormones. Microbes can modulate the host plant’s phytohormone-based signaling in different ways by producing hormones themselves, by triggering the plant’s hormone production and by sensitizing plants for hormone responses. Besides that, microbial and plant-derived phytohormones can have a direct impact on microbes. Therefore, phytohormone roles in cross-kingdom communication, (inter- and intraspecific) microbial communication, and regulation of microbial physiology are of increasing interest.

A better understanding of phytohormone functions, particularly comparing beneficial and pathogenic microbes, will support the development of strategies in plant protection against pathogens and improve the use of beneficial microbes for specific purposes such as plant growth stimulation and increased tolerance to stressors. In this context, the identification of novel components interacting with phytohormone signaling and hormone cross-talk is of interest.

Therefore, this Research Topic will consider Research Articles as well as Reviews, Hypothesis and Theory, Perspective and Opinion articles, which address in a broad perspective the functions of phytohormones and phytohormone-related signaling in plant-microbe interactions. This includes the relevance of phytohormones for downstream effects induced in the interacting organisms as well as communication between organisms in specific interactions or at the community level. In particular, contributions addressing following aspects are welcome:
• Phytohormones in community communication, i.e. microbial and cross-kingdom communication (plant-microbe and microbe-plant)
• Phytohormone function in microbial physiology, particularly during their interaction with the host plant
• Novel functions of phytohormone-related components regulating important processes involved in (a) defense and/or infection establishment, and (b) beneficial effects in the host plant such as growth effects and/or stress tolerance
• Comparison of phytohormone functions in beneficial and pathogenic interactions with plants
• Practical applications of the knowledge related to phytohormone functions in plant-microbe interactions

Spontaneous submissions will be considered only if preceded by an abstract describing the focus of the planned contribution.

Studies focusing on the establishment of and processes in plant-microbe symbioses are not covered and will not be considered for publication in this Research Topic. Authors addressing these aspects should consider submission to the Research Topic The Role of Plant Hormones in Plant-Microbe Symbioses.

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Frontiers in Plant Science

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