Content of cellulose, lignin, hemicellulose, and expression level of genes associated with secondary cell wall biosynthesis in roots of GL-3, MdMYB88, or MdMYB124 overexpression plants and MdMYB88/124 RNAi plants under drought conditions. A to C, Contents of cellulose (A), lignin (B), and hemicellulose (C). Plants were subjected to long-term drought stress for 2 months in a greenhouse. Data are means ± sd (n = 9). One-way ANOVA (Tukey test) was performed, and statistically significant differences are indicated by *P < 0.05 or **P < 0.01. D to F, Relative expression levels of MdCesA4 (D), MdCesA8 (E), and MdC4H (F). Plants were subjected to 20% PEG8000 for 0 or 6 h. Data are means ± sd (n = 3).