How do plants grow more branches and become bushy?

For many plants, increased branching translates to higher fruit yields. But what drives a plant to develop more branches? 

New research from the University of California, Davis, has revealed how plants degrade the hormone strigolactone, which suppresses branching, thereby becoming more “bushy.” 

Understanding how strigolactone is regulated could have significant implications for numerous crop plants.

The role of strigolactone 

The study, published in the journal Nature Communications, delves into the mechanisms behind strigolactone regulation. 

“Being able to manipulate strigolactone could also have implications beyond plant architecture, including on a plant’s resilience to drought and pathogens,” said Nitzan Shabek, the senior author of the study and an associate professor in the UC Davis Department of Plant Biology who specializes in biochemistry and structural biology.

Discovered only in 2008, strigolactone has emerged as a crucial hormone in plant research. Besides controlling branching, it fosters beneficial interactions between mycorrhizal fungi and plant roots and helps plants cope with stresses like drought and high salinity.

Mystery of strigolactone breakdown

While much is known about how plants synthesize hormones like strigolactone, the mechanisms of their breakdown remain largely unexplored. 

Recent research pointed to enzymes called carboxylesterases, which are found across all life forms, including humans, as potential players in strigolactone degradation. 

Plants produce over 20 types of carboxylesterases, but only CXE15 and CXE20 have been speculated to link to strigolactone degradation. Shabek’s team aimed to uncover the specifics of this degradation process.

“Our lab is interested in mechanisms, meaning we don’t want to just know that a car can drive, we want to know how it’s driving; what’s going on inside the engine,” Shabek explained.

Focus of the research 

To confirm whether CXE15 and CXE20 are involved in strigolactone regulation, the researchers began by constructing 3D models of the enzymes’ molecular structure. 

This project was initially led by undergraduate researcher Linyi Yan, who grew and purified the carboxylesterase proteins in the lab. According to Shabek, this student-led initiative quickly expanded into a more substantial research endeavor.

Postdoctoral fellow Malathy Palayam utilized x-ray crystallography and computer simulations to determine the three-dimensional atomic structure of the enzymes and conducted biochemical experiments to compare how these enzymes degrade the hormone.

The enzyme responsible for bushy plants

The experiments revealed that CXE15 was significantly more efficient at breaking down strigolactone compared to CXE20, which binds to the hormone but fails to degrade it effectively. Their 3D models unveiled a new finding: a specific region of CXE15 allows the enzyme to alter its shape dynamically.

“CXE15 is a very effective enzyme – it can completely destroy the strigolactone molecule in milliseconds,” Shabek said. “When we zoomed in, we realized that there is a dynamic area in the enzyme’s structure which is required for it to function in this way.”

By examining CXE15’s structure, Shabek and his team identified specific amino acids crucial for the enzyme’s ability to dynamically bind to strigolactone. To validate these findings, they genetically engineered a mutant version of CXE15 with an altered dynamic region. 

This mutant enzyme exhibited a reduced capacity to degrade strigolactone both in vitro and when tested in Nicotiana benthamiana plants.

Future research directions

Shabek emphasized the future directions of this research: “In this study we were really interested in elucidating these enzymes’ mechanism and structure, but future studies can begin investigating how they affect plant growth and development.” He expressed interest in exploring how carboxylesterase enzymes are produced in different plant tissues, such as roots and stems.

Understanding the intricacies of how strigolactone is broken down opens the door to new possibilities in plant biology and agriculture. By potentially manipulating this hormone, scientists could influence plant architecture, making crops more productive and resilient. 

This could lead to advances in how crops are grown, improving yields and resistance to environmental stresses, which are crucial for sustainable agriculture and food security.

“Taking a broader perspective, the carboxylesterase superfamily plays fundamental roles in numerous essential metabolic pathways across all kingdoms of life. We unveil a molecular architecture and intrinsic dynamics for carboxylesterases that have not been observed before, thereby adding an important layer of complexity to the mode of action of these crucial enzymes,” the authors concluded.

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