Essential yet dispensable: The role of CINNAMATE 4-HYDROXYLASE in rice cell wall lignification.

This article investigates the role of CINNAMATE 4-HYDROXYLASE (C4H), a key enzyme in the phenylpropanoid pathway, in lignin biosynthesis and cell wall lignification in rice (Oryza sativa), a model grass species. Using CRISPR-Cas9 genome editing, rice mutants with knockouts in the three major C4H genes (OsC4H1, OsC4H2a, and OsC4H2b) were generated and analyzed for growth, lignin content, and cell wall composition. The study found that Class I (OsC4H1) and Class II (OsC4H2a and OsC4H2b) C4Hs cooperatively contribute to lignin biosynthesis, with OsC4H1 playing a predominant role in aerial tissues and all three contributing redundantly in roots. However, unlike in eudicots where C4H deficiency severely impairs lignification and development, rice mutants lacking C4H activity still produced substantial lignin and reached maturity, attributed to a grass-specific parallel pathway involving PHENYLALANINE/TYROSINE AMMONIA-LYASE (PTAL) that channels tyrosine into lignin biosynthesis independently of C4H. Isotopic feeding and chemical inhibition assays confirmed that the PTAL pathway compensates for the loss of the canonical PAL-C4H pathway, highlighting the essential yet partially dispensable role of C4H in grass lignification. These findings advance understanding of lignin biosynthesis regulation in grasses and have implications for bioengineering grass biomass for industrial applications.