# Arabidopsis photorespiration is not limited by mitochondrial serine hydroxymethyltransferase 1

## TL;DR

This study shows that increasing SHMT1 protein in Arabidopsis does not improve photorespiration or plant growth.

## Contribution

The study demonstrates that SHMT1 levels are not limiting for photorespiration in Arabidopsis.

## Key findings

- Overexpression of SHMT1 did not affect photosynthetic gas-exchange parameters or growth.
- Metabolomic analysis showed wild-type-like metabolite accumulation in transgenic plants.
- SHMT1 is already present at sufficient levels in wild-type Arabidopsis.

## Abstract

Genetic engineering of Arabidopsis thaliana reveals that mitochondrial serine hydroxymethyltransferase 1 (SHMT1) protein abundance does not limit the capacity of photorespiration, photosynthesis, or stomatal conductance.

Photorespiration safeguards photosynthesis from negative metabolic feedback inhibition caused by Rubisco’s major oxygenation byproduct, 2-phosphoglycolate, and its downstream metabolites. Previous studies have shown that photosynthetic limitations can be alleviated by enhancing endogenous photorespiratory flux through position-specific enzyme overexpression. Here, we investigated whether overexpression of mitochondrial serine hydroxymethyltransferase 1 (SHMT1) influences this regulatory circuit. However, the 1.3-threefold increase in SHMT1 protein abundance did not affect the visual phenotype, growth, or photosynthetic gas-exchange parameters, including CO2 assimilation rates and stomatal conductance, under either standard or photorespiration-promoting conditions. Metabolomic analyses further revealed wild-type-like steady-state accumulation patterns of photorespiratory and other primary metabolites in transgenic lines during both, illumination and darkness, respectively. These results indicate that SHMT1 is already expressed at sufficient levels in Arabidopsis wild type, as substantial increases in protein abundance did not enhance photorespiratory flux and growth under laboratory conditions.

The online version contains supplementary material available at 10.1007/s00425-026-04989-x.