Fig. 9.

WOX4 is required for ARF5 activity. a–f Toluidine blue-stained cross-sections at the stem base of wild-type and single and multiple mutant plants. Genotypes are indicated. Interfascicular regions are shown and ICD tissues are marked (red bar). Size bars represent 50 µm. g Quantification of ICD tissue extension at the stem base of plants shown in a–f. Statistical groups indicated by letters were determined by one-way ANOVA with post hoc Tamhane-T2 (CI 95%, Sample size n = 9–12). The T-bars that extend from the boxes (whiskers) are expected to include 95% of the data. h Scheme representing the proposed effect of ARF transcription factors on cambium activity. We suggest that ARF5 predominantly promotes xylem production by directly activating xylem-related genes and by repressing WOX4. Other ARFs, which still have to be identified, may activate WOX4 at the same time. ARF3 and ARF4 act positively on cambium activity from outside of the stem cell domain and independently from WOX4