Overcoming the yield penalty of lignin-modified plants

Engineering plants for reduced lignin amount or altered structure is often associated with a yield penalty. Several hypotheses have been put forward to explain the molecular basis of the yield penalty. It has been shown that lignin modification often results in a collapse of vessels, which negatively affects water and nutrient transport in the plant. We have shown that the lignin reduction in cse and ccr mutants results in smaller plants, and that restoring lignin in the xylem vessels restores growth (Vargas et al., 2016; De Meester et al., 2018; 2021). Another strategy to avoid growth phenotypes is to engineer weak alleles that subtly reduce enzyme activity, as we have demonstrated by engineering the combination of a null and a haploinsufficient CCR2 allele by CRISPR/Cas9 in poplar (De Meester et al., 2020)

Lignin amount can be reduced without yield penalty by engineering weak alleles. Wild type poplar (left), a *ccr2* mutant with two null *ccr2* alleles (middle), a *ccr2* mutant with a null and a haploinsufficient ccr2 allele (right) (De Meester et al. 2020).

The biomass penalty of low-lignin mutants can be overcome by restoring lignin biosynthesis in the vasculature. Wild type Arabidopsis (left), the low lignin *ccr1* mutant (middle), and the vessel-complemented *ccr1* mutant (right) (De Meester et al., 2018).