# Tonoplast sucrose transporter SUT4‐dependent sugar partitioning modulates phenological transitions and reproductive success in poplar

## TL;DR

A study shows that a specific sugar transporter in poplar trees, SUT4, is crucial for seasonal growth and reproduction, and its absence disrupts timing and fertility.

## Contribution

The study reveals SUT4's role in sugar partitioning and seasonal development under field conditions, not just greenhouse settings.

## Key findings

- Loss of SUT4 causes earlier leaf senescence, delayed spring growth, and reduced stem growth in poplar.
- SUT4 mutants produce sterile ovules despite normal-looking flowers and show disrupted sugar and raffinose dynamics.
- Mutants exhibit elevated stress and downregulated circadian clock and proanthocyanidin biosynthesis genes.

## Abstract

Climate uncertainty is intensifying the need for greater plasticity in carbohydrate reserve utilization to support winter survival and spring growth in woody perennials. In poplar, the single‐copy SUT4, which encodes a tonoplast‐localized sucrose transporter, and the SUT5/SUT6 genome duplicates, which encode plasma membrane‐localized transporters, are expressed year‐round, with SUT4 showing the highest expression during cool seasons. Given its role in vacuolar sucrose efflux and winter‐predominant expression, SUT4 may play a key role in modulating seasonal carbohydrate dynamics. While SUT4‐knockdown and knockout effects have been studied under greenhouse conditions, their impact under field conditions remains unexplored. Here, we report a field‐based study comparing CRISPR knockout mutants of winter‐expressed SUT4 and SUT5/SUT6 in Populus tremula × alba. We show that sut4, but not sut5/6, mutants exhibited earlier autumn leaf senescence, delayed spring bud flush, reduced stem growth, and altered sugar partitioning in winter xylem and bark relative to controls. After 2 years in the field, all genotypes flowered before leaf flush in early spring; however, sut4 mutants produced sterile ovules despite developing normal‐looking catkins. Metabolic profiling revealed disrupted sucrose and raffinose dynamics in elongating sut4 catkins. This was accompanied by transcriptomic signatures of elevated stress and downregulation of proanthocyanidin biosynthesis and circadian clock genes. These findings highlight the critical role of SUT4 in coordinating sugar allocation, stress responses, and seasonal development in poplar.

This field study demonstrates that the loss of SUT4, the most highly expressed sucrose transporter during cool seasons, disrupts phenology, growth, and fertility in poplar. Altered sugar and raffinose dynamics in the mutants, along with dysregulated stress and circadian clock genes, underscore SUT4's role in coordinating sugar allocation and seasonal developmental transitions under natural environmental conditions.