Alginate oligosaccharides enhance acid stress tolerance in maize roots through phenylpropanoid-mediated cell wall lignification

Soil acidification is a major constraint on crop production worldwide, and because the root is the first organ to encounter proton rhizotoxicity it is also the first site at which tolerance must be established, yet how exogenous biostimulants remodel root metabolism so as to improve acid tolerance remains poorly understood. We therefore tested whether alginate oligosaccharides (AO) enhance acid stress tolerance in maize (Zea mays L.) roots by shifting carbon allocation away from non-specific polysaccharide deposition and toward phenylpropanoid-mediated cell wall lignification. Combining growth phenotyping, untargeted metabolomics, bulk transcriptomics and single-cell RNA sequencing (scRNA-seq), we found that, relative to the pH 5.8 control, acid stress at pH 3.8 reduced plant height, shoot dry weight, total root length and root dry weight by 14.3%, 30.4%, 31.1% and 31.8%, respectively, whereas AO increased these traits under both pH levels, lowered relative electrolyte leakage by 24.2%, preserved root ultrastructure and elevated cis-zeatin, indole-3-acetic acid and indole-3-butyric acid. Metabolomic and transcriptomic profiling converged on starch–sucrose metabolism and phenylpropanoid biosynthesis: AO attenuated the acid-induced accumulation of trehalose, UDP-D-glucose and sucrose together with their biosynthetic transcripts, while activating ZmPAL1, ZmCYP73A14, ZmCCR1 and ZmPER1/12/54, so that callose and uronic acid declined whereas lignin accumulated. scRNA-seq of 14,622 root cells resolved 11 cell types and localized the AO-induced ZmCYP73A14 response to the epidermis, whereas ZmMATE1, ZmNRAMP4 and ZmSTOP1-A, which are associated with low-pH adaptation and metal transport, were activated in the cortex and endodermis. These results are consistent with phenylpropanoid-mediated cell wall lignification acting as a spatially organized mechanism of AO-conferred acid stress tolerance, and they support the deployment of AO as a residue-free biostimulant for maize on acidic soils.