Plants keep up a precise rhythm to partner with bacteria

The marvel of symbiosis highlights the intricate interplay between different organisms, each playing a crucial part in their mutual survival and progress.

This biological phenomenon demonstrates how life forms, despite their differences, can forge beneficial relationships that enhance their existence.

Plant-bacteria symbiosis in action

A particular and striking example of such interdependence is found in the relationship between leguminous plants and soil bacteria, also known as Rhizobia. This fascinating partnership allows legumes to thrive in low-nitrogen environments, a feat that is challenging for many plants.

Rhizobia have a unique ability to convert atmospheric nitrogen into a form the plants can utilize – ammonium. This nitrogen fixation process is vital for the legumes’ growth and development, enabling them to flourish even in nutrient-poor soils.

But where do these beneficial bacteria reside? Quite intriguingly, they find a home within specialized structures called root nodules that form on the roots of leguminous plants.

These nodules provide a protective environment where the Rhizobia can live and function, facilitating a continuous exchange of nutrients that supports the health and productivity of both partners in this remarkable symbiotic relationship.

This mutualistic interaction not only underscores the complexity and elegance of natural processes but also has significant implications for sustainable agriculture and ecosystem management.

Unraveling plant-bacteria interaction

The interaction, however, is far from simplistic. An uncontrolled formation of numerous root nodules can significantly hinder root function, impeding the plant’s ability to absorb water and nutrients effectively.

A precise regulation over both the distribution and the amount of root nodules is required for the plants to survive. The intricate mechanisms underpinning this regulation remained largely undiscovered.

However, recent studies on Lotus japonicus, a model legume plant, have begun to unveil these complexities.

The studies have shed light on the sophisticated biological controls that manage nodule formation, ensuring that the symbiotic relationship between legumes and Rhizobia is both efficient and beneficial.

This groundbreaking research is a collaborative effort conducted by the National Institute for Basic Biology, Nara Institute of Science and Technology, Hokkaido University, Kwansei Gakuin University, RIKEN, and Aichi University of Education.

Dancing with the rhythm

The interaction between legume roots and Rhizobia is dictated by a rhythmic gene expression occurring every six hours. This rhythmic pattern influences the root regions susceptible to Rhizobial infection and the distribution of nodules.

“When Rhizobia infect legume roots, root epidermal cells form infection threads, membranous tube-like structures guiding the bacteria to the inner root tissue where they can fix nitrogen,” explained study co-author Dr. Takashi Soyano.

“We noticed that these oscillation regions coincide with areas where infection threads are densely formed, leading us to think that this rhythmic gene expression might be related to the determination of nodule formation sites.”

Cytokinin: The symbiotic maestro

Moreover, another key discovery centered on the crucial role of the plant hormone cytokinin in this intricate process. This hormone is pivotal for maintaining the rhythmic gene expression that governs the interaction between legume roots and Rhizobia.

The research team observed distinct oscillatory patterns of genes involved in cytokinin biosynthesis, metabolism, and signaling following rhizobial inoculation.

The findings suggest that cytokinin is not only essential for initiating nodule formation but also for regulating the periodic responses that ensure an optimal balance of root nodules.

Linking symbiosis and evolution

Root nodule symbiosis is not an isolated phenomenon. It occurs in the monophyletic nitrogen-fixing clade, inclusive of four orders – Fabales, Rosales, Cucurbitales, and Fagales – indicating a shared evolutionary acquisition to interact with nitrogen-fixing bacteria.

The legume family in the order Fabales, where most species engage in root nodule symbiosis, uniquely incorporated the cytokinin pathway as an important regulatory module for the symbiosis.

“The discovery of periodic cytokinin responses was unexpected, raising several questions, including the molecular mechanisms that establish this periodicity and how these periodic responses shape the infection regions,” said Dr. Soyano.

Addressing these questions can further our understanding of the regulatory mechanisms of root nodule symbiosis and propel research on the spatial control of organ development through periodic responses mediated by plant hormones.

The study is published in the journal Science.

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