Lignin is an essential polymer in vascular plants that plays key structural roles in vessels and fibers. Lignification is induced by external inputs such as wounding, but the molecular mechanisms that link this stress to lignification remain largely unknown. In this work, we provide evidence that three maize (Zea mays) lignin repressors, MYB11, MYB31, and MYB42, participate in wound-induced lignification by interacting with ZML2, a protein belonging to the TIFY family. We determined that the three R2R3-MYB factors and ZML2 bind in vivo to AC-rich and GAT(A/C) cis-elements, respectively, present in a set of lignin genes. In particular, we show that MYB11 and ZML2 bind simultaneously to the AC-rich and GAT(A/C) cis-elements present in the promoter of the caffeic acid O-methyl transferase (comt) gene. We show that, like the R2R3-MYB factors, ZML2 also acts as a transcriptional repressor. We found that upon wounding and methyl jasmonate treatments, MYB11 and ZML2 proteins are degraded and comt transcription is induced. Based on these results, we propose a molecular regulatory mechanism involving a MYB/ZML complex in which wound-induced lignification can be achieved by the derepression of a set of lignin genes.

Caffeoyl coenzyme A 3-O-methyltransferase (CCoAOMT) and caffeic acid-O-methyltransferase (COMT) are key enzymes in the biosynthesis of coniferyl and sinapyl alcohols, the precursors of guaiacyl (G) and syringyl (S) lignin subunits. The function of these enzymes was characterized in single and double mutant maize plants. In this work, we determined that the comt (brown-midrib 3) mutant plants display a reduction of the flavonolignin unit derived from tricin (a dimethylated flavone), demonstrating that COMT is a key enzyme involved in the synthesis of this compound. In contrast, the ccoaomt1 mutants display a wild-type amount of tricin, suggesting that CCoAOMT1 is not essential for the synthesis of this compound. Based on our data, we suggest that CCoAOMT1 is involved in lignin biosynthesis at least in midribs. The phenotype of ccoaomt1 mutant plants displays no alterations, and their lignin content and composition remain unchanged. On the other hand, the ccoaomt1 comt mutant displays phenotypic and lignin alterations similar to those already described for the comt mutant. Although stems from the three mutants display a similar increase of hemicelluloses, the effect on cell wall degradability varies, the cell walls of ccoaomt1 being the most degradable. This suggests that the positive effect of lignin reduction on cell wall degradability of comt and ccoaomt1 comt mutants is counteracted by changes occurring in lignin composition, such as the decreased S/G ratio. In addition, the role of the flavonolignin unit derived from tricin in cell wall degradability is also discussed.

The plant cell wall is a plastic structure of variable composition that constitutes the first line of defence against environmental challenges. Lodging and drought are two stressful conditions that severely impact maize yield. In a previous work, we characterised the cell walls of two maize inbreds, EA2024 (susceptible) and B73 (resistant) to stalk lodging. Here, we show that drought induces distinct phenotypical, physiological, cell wall, and transcriptional changes in the two inbreds, with B73 exhibiting lower tolerance to this stress than EA2024. In control conditions, EA2024 stalks had higher levels of cellulose, uronic acids and p‐coumarate than B73. However, upon drought EA2024 displayed increased levels of arabinose‐enriched polymers, such as pectin‐arabinans and arabinogalactan proteins, and a decreased lignin content. By contrast, B73 displayed a deeper rearrangement of cell walls upon drought, including modifications in lignin composition (increased S subunits and S/G ratio; decreased H subunits) and an increase of uronic acids. Drought induced more substantial changes in gene expression in B73 compared to EA2024, particularly in cell wall‐related genes, that were modulated in an inbred‐specific manner. Transcription factor enrichment assays unveiled inbred‐specific regulatory networks coordinating cell wall genes expression. Altogether, these findings reveal that B73 and EA2024 inbreds, with opposite stalk‐lodging phenotypes, undertake different cell wall modification strategies in response to drought. We propose that the specific cell wall composition conferring lodging resistance to B73, compromises its cell wall plasticity, and renders this inbred more susceptible to drought.

Stalk-lodging and drought are major constraints limiting maize productivity. We previously characterized B73 as stalk-lodging resistant but drought-sensitive maize inbred, while EA2024 is stalk-lodging susceptible and drought-tolerant. Using these inbreds, we performed a comparative proteomic analysis under well-watered and drought conditions. Our results revealed that drought induces extensive proteomic reprogramming in B73, while the EA2024 proteome remains unaltered. Comparative analyses identified 481 differentially accumulated proteins between inbreds across conditions. Among them, 100 proteins were differentially accumulated in well-watered conditions but showed no significant (or attenuated) differences upon drought. We defined these B73 proteins as having a convergent response to drought. Their functional characterization revealed that one third relates to upstream elements of the photosynthetic machinery, suggesting that B73 reprograms its energetic machinery to achieve an organization similar to EA2024. We also identified 381 proteins differentially accumulated between inbreds upon drought, defined as having a divergent response to drought. In this case, B73 over-accumulates proteins mainly involved in cofactors, vitamins metabolism and signaling and under-accumulates proteins linked to downstream photosynthetic components and biosynthetic processes. Thus, the convergent response suggests a partial shift of drought sensitive B73 toward the baseline of drought resistant EA2024. However, the broader divergent response of B73 suggests an inbred-specific metabolic reprogramming associated with reduced energy production, metabolic impairment and altered signaling cascades. Our findings open the possibility that stalk−lodging resistance in B73 was acquired at the expense of drought tolerance, prompting further investigation into the correlation between these two traits across a broader range of maize inbreds.