The study of the lignin biosynthesis and ways of its modification by genetic engineering methods are important in both scientific and practical terms. The mechanisms of its regulation have not yet been well studied, and no efficient methods of lignification decrease have been developed, which would not cause any undesirable attributes in the phenotype. We have used the method of synthesis of small interfering RNAs to the fourth exon of the 4-Coumarate: Coenzyme A Ligase gene in transgenic aspen plants, which resulted in the decreasing expression of the 4CL1 gene in the plants of the PtXIII4CL2c, PtXIII4CL4a, and PtXVI4CL9b lines by 72%, 45%, and 75%, accordingly. The 4CL1 gene expression inhibition was accompanied by the growth (height) speed change in greenhouse conditions. The height of cloned plants PtXIII4CL2c and PtXVI4CL9b was validly lower than the control values by 20% and 12%, accordingly. At the same time, the PtXIII4CL4a clone was higher than the control plants by 23%. The total lignin content in the wood of 6-month old cloned plants PtXIII4CL2c, PtXIII4CL4a, and PtXVI4CL9b decreased by 23%, 13%, and 16%, accordingly. We detected inverse correlation between the decrease in the lignin content and the development of the redbrown color of the wood. The correlation ratio was-0.87. Thus, the growth reduction was observed when the 4CL1 gene expression was inhibited by more than 70%, while the lignin content decrease and color change took place in all lines under study.