How two protein modifications help coordinate plant metabolism and growth

Plants must match growth to both their hormonal signals and the carbon they can use. A study in Arabidopsis suggests that two competing chemical tags on one enzyme help make that connection: SPINDLY adds a sugar-derived fucose group, while the kinase BIN2 adds phosphate. The authors report that the tags oppose one another on PCK1, an enzyme involved in making sugars from other carbon compounds.

Plants cannot simply grow whenever growth-promoting signals arrive: they also need carbon and energy to build new tissue. How plants coordinate these inputs is not fully understood. A preprint by Hongliang Zhang and colleagues examines one possible molecular link in Arabidopsis thaliana: whether a sugar-dependent protein modification can counter a hormone-signaling enzyme’s effects on a key metabolic protein.

The study focuses on PCK1, an enzyme that helps convert carbon from compounds such as stored lipids and amino acids into material used to make sugars. This process is particularly relevant to young seedlings before photosynthesis is fully established. The authors’ earlier work had identified BIN2, a kinase associated with brassinosteroid signaling, as a protein that phosphorylates and inhibits PCK1. Here they ask whether SPINDLY (SPY), which attaches the sugar-derived group O-fucose to proteins, modifies PCK1 and affects that phosphorylation.

To investigate, the researchers combined experiments in Arabidopsis seedlings with protein studies in tobacco leaves and tests using purified proteins. They used genetic variants with reduced SPY function, chemical treatments that alter brassinosteroid or BIN2 activity, protein pull-down and interaction assays, mass spectrometry to identify modification sites, and measurements of PCK enzyme activity and seedling hypocotyl length. The mix of approaches matters: some tests examine the plant-level consequences, while others probe whether the proteins can interact and modify one another directly.

The evidence supports a model in which SPY binds PCK1 and adds O-fucose to a region containing Ser-62 and Thr-66, positions also identified as BIN2 phosphorylation sites. The researchers found that SPY-associated O-fucosylation and BIN2-associated phosphorylation were inversely related: when SPY activity was reduced or BIN2 activity promoted, PCK1 phosphorylation tended to rise and its activity fell. In assays, PCK1 isolated from tissue expressing SPY was less readily phosphorylated by BIN2. The results suggest the modifications compete at, or close to, the same regulatory region; they do not establish that every observed effect comes from direct competition at an identical amino-acid position.

Seedlings with spy mutations had more phosphorylated PCK1, lower PCK activity and shorter hypocotyls—the stem-like region below the seed leaves. Treatments with brassinolide, a brassinosteroid, or bikinin, a BIN2/GSK3-family kinase inhibitor, reduced phosphorylation and improved PCK activity; the authors also report suppression of the short-hypocotyl phenotype under the tested conditions. This pattern is consistent with excess BIN2 activity contributing to the effects of losing SPY. It does not mean PCK1 alone explains the growth defect: adding sucrose rescued the short hypocotyl of pck1 plants, but not spy plants. SPY modifies many proteins, so other targets likely contribute.

The work offers a possible molecular explanation for how sugar-related information and hormone signaling can converge on metabolism. Its strongest specific case is PCK1 in the experimental Arabidopsis settings studied. The authors suggest that similar antagonism could affect other proteins targeted by both SPY and BIN2, but that broader role remains a proposal; identifying and testing additional targets will be necessary.

There are also limits to what can be concluded from the available evidence. Some interaction and modification experiments used transiently expressed proteins in Nicotiana benthamiana, rather than only native proteins in Arabidopsis. Several activity experiments report three biological replicates, and the work uses particular mutant lines, growth conditions and chemical treatments. These experiments support the proposed mechanism, but do not by themselves show how important it is across plant tissues, developmental stages or natural environments.

This article is based on a bioRxiv preprint, posted November 12, 2026, and marked as not certified by peer review. Its findings should therefore be treated as provisional pending peer review and independent replication. The source reports that data and materials are available from the corresponding author on request; readers should consult the full paper for experimental details and figure-level results.