Transcript accumulation for MYB genes and secondary cell-wall-related genes in differentiating compression wood and opposite wood. a) Compression wood and opposite wood formed in a leaning spruce seedling after 21 days of treatment, compared to the control from vertical seedling. Exposed wood (compression wood is light brown) and wood cross-sections (10 μm thick) were stained by the safranin-orange procedure  (magnification, ×40). Steady-state mRNA levels were determined as in Figures 4 and 5 for cell-wall-related genes (b) and for several PgMYB genes (c) in the compression wood (left panels) and opposite side wood (right panels) of spruce seedlings leaning at a 45° angle from vertical. Continuous lines indicate genes with significant variation, and standard error bars are shown three trees (biological replicates) with two independent technical repetitions). Discontinuous lines indicate examples of gene transcripts that do not fluctuate in abundance. The zero time point represents vertical control trees only. PgMYB4 (1/15) means that mRNA level is divided by 15.