📄 plant biology

Chilling Nights Do Not Cause Starch Over-Accumulation and Trigger a Shift in Carbon Partitioning via SPS Toward Sucrose in Arabidopsis - Differing from Acclimation to Permanent Cold

Unlike permanent cold acclimation which induces starch over-accumulation, chilling nights in *Arabidopsis* trigger a distinct metabolic strategy characterized by increased sucrose synthesis via SPS and a shift in carbon partitioning toward soluble sugars without starch buildup.

Original authors: Bonn, C., Giesbrecht, O., Jordine, A., Becker, M., Jennings, S., van Aalst, M., van Dongen, J. T., Matuszynska, A. B., Fuertauer, L.

Published 2026-08-03

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: Bonn, C., Giesbrecht, O., Jordine, A., Becker, M., Jennings, S., van Aalst, M., van Dongen, J. T., Matuszynska, A. B., Fuertauer, L.

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ⚕️ This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer

Imagine plants as tiny, solar-powered factories that run on a strict day-and-night schedule. During the day, they use sunlight to cook up sugar, which they either burn immediately for energy or store in a pantry as starch for the night. But plants live in a world that isn't always predictable. Sometimes, the weather throws a curveball: a "chilling night" where the temperature drops near freezing, only to warm up again the next morning. Scientists have long known how plants handle a permanent cold snap (like a long, freezing winter), where they pack their pantries full of starch and sugar to survive the freeze. But what happens when the cold is just a temporary visitor? Does the plant panic and over-pack, or does it have a different trick up its sleeve? This question matters because our climate is changing, making these wild temperature swings more common, and understanding how plants react helps us predict how they will survive in a warmer, more chaotic world.

The researchers in this paper decided to play detective with a common weed called Arabidopsis thaliana to see how it handles these "chilling nights." They set up an experiment where the plants lived in a cozy 22°C (72°F) day but were subjected to a chilly 4°C (39°F) night. They tested two scenarios: one where the plant faced just a single cold night, and another where it endured seven nights in a row. They measured everything they could think of: how much sugar was floating around, how much starch was stored in the pantry, and how fast the plant's internal enzymes (the little workers that build and break down sugar) were running. To make sense of all these moving parts, they used a fancy computer model called an "augmented neural ordinary differential equation" (or ANODE for short). Think of this model as a super-smart traffic cop that looks at the snapshots of sugar levels and guesses how fast the sugar was actually moving between the different storage spots, even when they couldn't measure the speed directly.

Here is the twist: the plants didn't react the way scientists expected based on permanent cold. Usually, when plants get cold for a long time, they go into "survival mode" and over-accumulate starch, filling their pantries to the brim. But under these chilling nights, the plants did something completely different. Instead of hoarding starch, they shifted their strategy entirely toward making sucrose (table sugar). After seven nights of this cold-hot cycle, the plants had built up massive amounts of sucrose, glucose, and fructose, but their starch levels remained normal. They didn't over-pack the pantry; they just kept the cash register (sucrose) overflowing.

The paper suggests that this shift was driven by a specific worker in the plant's factory called Sucrose Phosphate Synthase (SPS). In the permanent cold, this worker doesn't change much, but under these chilling nights, the plant cranked up the SPS activity. The computer simulations showed that the rate at which SPS was making sucrose jumped up by about 2.6 times compared to normal conditions. At the same time, the plant seemed to slow down the workers that break sucrose back down, particularly one called fructokinase (FRCK), which dropped to about 40% of its normal speed. This created a bottleneck where sucrose and its components (glucose and fructose) piled up.

Interestingly, the plants didn't just over-accumulate sugar randomly; they seemed to be fine-tuning their response. After the very first cold night, the plants were a bit stressed and their sugar levels spiked, but by the seventh night, they had settled into a new rhythm. The study explicitly rules out the idea that these plants were just "stuck" in a panic mode; instead, they had adapted a distinct strategy. Unlike the permanent cold response, which relies on starch as a long-term energy reserve and freeze-protection, the chilling night strategy relies on soluble sugars like sucrose. The authors note that while they saw a huge drop in the predicted speed of the fructokinase worker, they couldn't fully explain why the glucose levels were so high just by looking at the enzyme speeds they measured, suggesting there might be other hidden factors at play.

In short, this paper reveals that plants are smarter than we thought. They don't just have one "cold response" switch. When faced with a night that's too cold but followed by a warm day, they don't try to fill their starch pantries. Instead, they switch their metabolic gears to flood the system with sucrose, likely to keep their cells flexible and ready for the next temperature swing. It's a different kind of survival dance, one that prioritizes quick energy and protection over long-term storage, proving that even a simple weed has a complex, adaptable plan for dealing with a changing world.

Technical Summary: Chilling Nights Do Not Cause Starch Over-Accumulation and Trigger a Shift in Carbon Partitioning via SPS Toward Sucrose in Arabidopsis

Problem Statement
Plants naturally encounter fluctuating environmental temperatures, particularly "chilling nights" (0–6 °C) followed by significantly warmer days (≥12 °C higher), which are common in temperate regions during spring and autumn. While the metabolic responses to permanent cold acclimation (continuous exposure to ~4 °C) are well-characterized—typically involving the accumulation of both starch and soluble sugars as energy reserves and cryoprotectants—the metabolic consequences of intermittent chilling nights remain poorly understood. Specifically, it is unclear whether the diurnal temperature shifts typical of natural environments trigger the same carbon partitioning strategies (e.g., starch over-accumulation) observed under constant cold stress.

Methodology
The study utilized Arabidopsis thaliana (Col-0) grown under controlled conditions (12h light/12h dark, 22 °C day/18 °C night) before being subjected to a chilling regime (4 °C night/22 °C day). The experimental design compared three conditions:

CTRL: Control plants (18 °C night/22 °C day).
CND1: Plants exposed to a single chilling night (immediate stress response).
CND7: Plants exposed to seven consecutive chilling nights (acclimated state).

Researchers performed a comprehensive time-series analysis, harvesting samples at multiple time points throughout the diurnal cycle (00 h, 06 h, 12 h, 15 h, 18 h, 21 h, 24 h). Key measurements included:

Metabolite Quantification: Concentrations of starch, sucrose, glucose, fructose, glucose-6-phosphate (G6P), and fructose-6-phosphate (F6P).
Enzyme Activity: Maximum activities ( $v_{max}$ ) of Sucrose Phosphate Synthase (SPS), acidic and neutral invertases (aINV, nINV), glucokinase (GLCK), and fructokinase (FRCK). Activities were corrected for temperature using the Arrhenius equation.
Physiological Parameters: Net photosynthesis and maximum quantum efficiency of Photosystem II ( $F_v/F_m$ ).

To infer continuous metabolic flux dynamics from discrete time-series data, the authors employed a Biologically Constrained Augmented Neural Ordinary Differential Equation (ANODE) model. This approach fixed the stoichiometry of the core carbon metabolic network while using neural networks to learn the reaction fluxes ( $v_{pred}$ ) constrained by experimentally measured $v_{max}$ values. An ensemble of 100 models was generated to estimate flux trajectories and quantify uncertainty.

Key Results

Distinct Acclimation Strategy: Principal Component Analysis (PCA) revealed that while the first chilling night induced a strong stress response separating night from day, the state after seven nights (CND7) formed a distinct cluster, indicating a unique acclimated metabolic state different from both the control and the immediate stress response.
Sucrose Accumulation and SPS Upregulation: Unlike permanent cold acclimation where sucrose accumulation is often linked to growth retardation, chilling nights induced a strong accumulation of sucrose, glucose, and fructose. By day 7, sucrose levels were approximately 3.1-fold higher than controls. Crucially, both the measured maximum activity ( $v_{max}$ ) and the ANODE-predicted rates ( $v_{pred}$ ) of Sucrose Phosphate Synthase (SPS) increased significantly (approx. 2.2-fold and 2.6-fold, respectively) during the day of the seventh chilling night.
Absence of Starch Over-Accumulation: Contrary to the well-documented starch over-accumulation seen in permanent cold, chilling nights did not lead to sustained starch over-accumulation. While starch levels were transiently higher at the end of the night, they realigned with control levels by midday. Consequently, the starch-to-sucrose ratio decreased significantly (approx. 0.4-fold relative to control), indicating a shift in carbon partitioning away from starch storage.
Hexose Dynamics and Enzyme Regulation: Glucose and fructose accumulated significantly (up to 11.5-fold and 14.0-fold, respectively, by day 7). The modeled rates for Fructokinase (FRCK) were consistently reduced (0.3–0.4-fold of control), suggesting a bottleneck in fructose phosphorylation that contributes to fructose accumulation. In contrast, Glucokinase (GLCK) rates showed a more complex pattern, decreasing during the cold night but recovering or increasing during the subsequent warm day.
Model Validation: The ANODE model successfully reconstructed diurnal flux trajectories, revealing that the increase in sucrose synthesis (SPS) and the reduction in starch accumulation are mechanistically linked, driven by elevated SPS capacity rather than a simple reduction in growth.

Significance and Claims
The paper claims to demonstrate that chilling nights invoke a fundamentally different acclimation strategy compared to permanent cold. While permanent cold triggers a metabolic reconfiguration characterized by the accumulation of both starch and soluble sugars to serve as energy reserves and cryoprotectants, intermittent chilling nights drive a specific shift in carbon partitioning toward sucrose biosynthesis without the accompanying starch over-accumulation.

The authors conclude that the upregulation of SPS capacity is a key regulatory mechanism in this response, potentially enhancing freezing tolerance through increased photosynthetic activity and sucrose availability rather than starch storage. This finding highlights that the metabolic response to temperature fluctuations (chilling nights) cannot be extrapolated from responses to constant cold stress, emphasizing the need to study natural, fluctuating temperature regimes to understand plant cold tolerance mechanisms accurately. The study further validates the utility of biologically constrained ANODE modeling in inferring dynamic metabolic fluxes from time-series metabolite and enzyme data where regulatory mechanisms are incompletely characterized.

More like this