提出问题：Engineered low-lignin plants have reduced recalcitrance but often exhibit yield penalties, offsetting their gains in fermentable sugar yield.

结果1-CCR2(−/*)株系12：Here, CRISPR/Cas9-generated CCR2(−/*) line 12 poplars have one knockout CCR2 allele while the other contains a 3-bp deletion, resulting in a 114I115A-to-114T conversion in the corresponding protein.

结果2-表型：Despite having 10% less lignin, CCR2(−/*) line 12 grows normally. On a plant basis, the saccharification efficiency of CCR2(−/*) line 12 is increased by 25–41%, depending on the pretreatment.

结果3-遗传基础：Analysis of monoallelic CCR2 knockout lines shows that the reduced lignin amount in CCR2(−/*) line 12 is due to the combination of a null and the specific haploinsufficient CCR2 allele. Analysis of another CCR2(−/*) line shows that depending on the specific CCR2 amino-acid change, lignin amount and growth can be affected to different extents.

结论：Our findings open up new possibilities for stably fine-tuning residual gene function in planta.