# Biostimulants in plant brassinosteroid hormone receptor BRI1 activation—a new system to evaluate activation capacity

## TL;DR

This paper introduces a new method to test how biostimulants activate a key plant growth receptor, BRI1, using a yeast-based system and a GFP reporter.

## Contribution

A novel yeast-based assay for evaluating biostimulant activation of the plant BRI1 receptor using a GFP reporter system.

## Key findings

- Biostimulants vary in their ability to activate the BRI1 receptor.
- Processing methods affect the BRI1 activation capacity of biostimulants from the same origin.
- The method enables high-throughput screening of biostimulants via BRI1 signaling.

## Abstract

The search for innovative and alternative chemical methods to manage plant growth is an ever‐increasing reality. Biostimulants, products of biological origin, have shown promise in improving various agronomic characteristics and boosting yield. However, the selection and characterization of biostimulant matrices is a complex process that requires rigorous evaluation adapted to the specific needs of each plant. Because mixtures of biologically active compounds are present in biostimulants, efficient methods are required to characterize their potential mode of action. In this study, a new approach was developed to assess the biological activity of biostimulants by activating specific plant receptors involved in key physiological processes. It is based on the heterologous expression in fission yeast of brassinosteroid receptor protein Brassinosteroid Insensitive 1 (BRI1), which is involved in plant growth and development, and its specific activation by brassinolide (BL). The method involves the identification of highly expressed genes in response to BL activation of the BRI1 receptor, to generate a GFP reporter gene system that is switched on when biostimulants activate the BRI1 receptor. The biostimulants selected for testing were hydrolysates of animal origin. The results not only revealed variations in BRI1 activation among biostimulants, but also highlighted that samples from the same origin exhibit different BRI1 activation capacities depending on their processing methods. This new method enables direct classification of the mode of action of biostimulants by assessing their ability to activate specific plant receptors, providing a valuable resource for biostimulant research and development.

Biostimulants improve plant growth and have shown promise in agronomic applications and boosting crop yield. Because biostimulants typically present as a mixture of active components, new methods are required to characterize their mode of action. Here, we describe a novel assay that evaluates plant biostimulants targeting the brassinosteroid receptor BRI1, a central component in plant growth and development. Expressing BRI1 in fission yeast enabled its functional activation by brassinolide (BL) and downstream transcriptomic profiling identified BL‐responsive genes via a GFP‐based reporter system. This platform offers a robust, specific, and high‐throughput method to screen new biostimulants that influence plant growth through BRI1 signalling.