Addressing the challenge of early osmotic stress to pea seedling growth: root arabinogalactan-proteins at the frontline

Legumes are a major source of plant proteins, with demand expected to rise sharply, while climate change threatens crop productivity. Pea ( Pisum sativum L.) is particularly sensitive to water deficit during flowering and pod filling, but early seedling growth is also a critical stage, as root elongation depends on sustained water supply. Because rainfall at this stage is unpredictable, it is essential to understand root-associated molecules that may influence plant-available moisture. Here, we investigated the composition of root cell wall glycomolecules and their exudation, including the root extracellular trap (RET), under polyethylene glycol (PEG)-induced osmotic stress. The RET, composed of border cells and mucilage, forms a polysaccharide-rich environment at the root–soil interface implicated in stress alleviation. Our results show that PEG stress (i) increased root hair density, (ii) modified arabinogalactan-protein (AGP) epitope distribution across root regions, (iii) altered AGP composition in both root tissue and RET, affecting their secretion at the surface, and (iv) enhanced proline biosynthesis via induction of P5CS2 (Δ¹-pyrroline-5-carboxylate synthase). We also developed expression markers to assess pea root responses to osmotic stress. These findings highlight the involvement of AGPs and the RET in the early stress response of pea seedlings.