# Fluctuations in nitrate availability impact cytokinin biosynthesis through histone modifications of IPT3 in Arabidopsis roots for growth acclimation

## TL;DR

Plants adjust to changing nitrate levels by modifying the chromatin of the IPT3 gene, which controls cytokinin production and balances root and shoot growth.

## Contribution

The study reveals how chromatin modifications of IPT3 mediate cytokinin biosynthesis in response to fluctuating nitrate levels.

## Key findings

- IPT3 chromatin modifications are crucial for cytokinin output and growth acclimation to nitrate fluctuations.
- H3K4me3 and H3K27me3 histone modifications regulate IPT3 transcription in response to nitrate availability.
- Nitrate-driven chromatin dynamics fine-tune cytokinin biosynthesis to balance root and shoot growth.

## Abstract

Nitrate availability in soil is highly variable and often a limiting factor for crop growth. Plants must acclimate rapidly to these fluctuations. The phytohormone cytokinin (CK) plays a pivotal role in nitrate signaling as a secondary growth regulator. However, the mechanisms that regulate CK action in response to fluctuating nitrate levels remain poorly understood. Here, we show that chromatin modification of IPT3, a key gene in CK biosynthesis, is crucial for growth acclimation to variable nitrate supply. Transcriptional regulation of IPT3 drives CK output in response to nitrate availability, thereby balancing root and shoot growth. This rapid and dynamic regulation is mediated by two antagonistic histone H3 modifications: trimethylation of lysine 27 of histone H3 (H3K27me3) and H3K4me3. Using mutants defective in the deposition or removal of these modifications, we identify several chromatin effectors involved in these processes and confirm that nitrate-driven chromatin dynamics fine-tune CK biosynthesis. Our findings provide insights into the epigenetic regulatory mechanisms underlying CK biosynthesis and open new avenues faor enhancing plant acclimation to fluctuating nutrient environments.

This study unravels the temporal dynamics of plant responses to nitrate fluctuations, the relationship between growth acclimation and cytokinin biosynthesis, and the mechanisms underlying its transcriptional regulation. In particular, IPT3 is dynamically regulated by nitrate fluctuation, thereby controlling cytokinin output and balancing root and shoot growth. The transcriptional regulation of IPT3 is mediated by the interplay between H3K4me3 and H3K27me3 in response to nitrate availability, demonstrating that nitrate-driven chromatin dynamics fine-tune cytokinin biosynthesis to facilitate growth acclimation.