An early cellular transport step helps tune plant stress-hormone sensitivity

Abscisic acid, or ABA, helps plants coordinate germination, growth and responses to environmental stress. Its effects depend on receptors that recognize the hormone and trigger downstream signalling. Researchers already knew that moving these receptors through internal membrane compartments, including routes leading to their destruction, helps control hormone responsiveness. The new study examines an earlier part of that journey.

In a bioRxiv preprint posted on November 9, 2026, José Antonio Berlanga-Torres and colleagues report that the protein SEC16A, also called MAG5, helps maintain the distribution and abundance of two ABA receptors, PYR1 and PYL4. The work has not been certified by peer review.

Finding receptors near the cellular export machinery

The endoplasmic reticulum, or ER, is an interconnected membrane network. Specialized ER exit sites organize transport toward the Golgi apparatus, another compartment involved in processing and sorting cellular cargo. MAG5 acts as a scaffold at these exit sites, helping organize the COPII machinery responsible for ER export.

Using fluorescently tagged receptors and confocal microscopy, the researchers found PYR1 and PYL4 associated with the ER network and with some Golgi-associated structures. These were subpopulations: both receptors also showed their familiar distribution in the nucleus and surrounding cytoplasm, alongside fluorescence at the cell periphery.

Experiments included transient expression in Nicotiana benthamiana leaves and stable Arabidopsis lines. Importantly, one PYR1 reporter retained its native regulatory sequences, reducing concerns that the observed pattern arose solely from artificial overexpression. Biochemical separation of untagged proteins from wild-type Arabidopsis provided independent support: some PYR1 was recovered with membrane fractions enriched for ER markers.

PYR1 and PYL4 are soluble proteins, not proteins that span membranes. Their association with the ER and Golgi therefore raises questions about how they are recruited there. Fluorescence-complementation experiments placed receptor complexes with the membrane-associated protein CAR1 at these compartments. Disrupting CAR1’s membrane-binding capacity largely removed that pattern.

What changes when MAG5 is lost?

A subset of PYR1-containing structures was closely associated with MAG5-marked exit sites. Additional fluorescence-complementation experiments supported close association between MAG5 and both receptors. However, this method cannot distinguish direct binding from indirect proximity within a larger complex or membrane domain.

The team then examined Arabidopsis plants carrying the mag5-3 mutation. Receptor fluorescence showed a more prominent network-like pattern than in wild-type plants, suggesting altered intracellular organization.

Two drugs helped probe downstream trafficking. Brefeldin A reorganizes Golgi and early endosomal compartments into characteristic bodies. These bodies were fewer, smaller and abnormally shaped in mag5-3 cells, and receptor redistribution into them appeared less pronounced. Wortmannin enlarges late endosomal compartments; PYR1 accumulation in these drug-induced structures was markedly reduced in the mutant.

Protein measurements also showed increased PYR1 and PYL4 abundance in mag5-3 seedlings. Their corresponding messenger RNA levels did not differ significantly from wild type. This places the detected change downstream of transcript abundance, although it does not by itself identify the precise mechanism controlling protein accumulation.

Stronger responses without more hormone

The trafficking changes accompanied increased ABA sensitivity. Mutant seeds and seedlings showed stronger inhibition of germination and cotyledon greening under added ABA, and primary root growth was slightly more inhibited. Two other independent mag5 alleles also showed ABA hypersensitivity. Several ABA-responsive genes were more strongly induced in mag5-3.

Hormone measurements sharpened the interpretation. ABA concentrations were comparable between mutant and wild-type seedlings under control and salt-treatment conditions. With externally supplied ABA, the mutant actually accumulated less hormone. Increased hormone accumulation therefore did not explain its stronger responses.

Mutant leaves were warmer under both control and ABA-treated conditions, consistent with reduced evaporative cooling. Separate measurements found smaller stomatal apertures in seedling cotyledons. Under salt treatment, mutant germination and early establishment were more strongly inhibited. These observations do not establish improved drought survival or crop performance.

A checkpoint, with the mechanism still incomplete

The authors propose that impaired ER export limits receptor progression toward endosomal sorting and eventual vacuolar degradation, allowing receptors to accumulate and enhancing signalling. That is a mechanistic model, not a directly demonstrated sequence for individual receptor molecules. The study does not establish a direct late-endosomal role for MAG5 or determine how its loss changes receptor abundance at the plasma membrane.

This account is based on the supplied PDF text extraction and metadata. Figures, tables and supplementary material could not be independently assessed from that extraction. Within those limits, the results support an early trafficking contribution to ABA receptor regulation, while leaving the exact connections between export, receptor turnover and signalling unresolved.