The PB1 domain plays an essential role in auxin/indole-3-acetic acid (Aux/IAA) function in Physcomitrella, but oligomerization is not required for repression. (a) The developmental phenotypes of K and opca single and double mutants relative to the wild type and Δaux/iaa null mutant grown on medium with different auxin levels. (b) Quantitative PCR of auxin-responsive marker genes in the above lines treated with mock and 10 μM IAA for 1 h. White and black, IAA1a-luc; blue, IAA1a-lucK362A; purple, IAA1a-lucK,OPCA; gray, Δaux/iaa null mutant; darker colors represent 10 μM IAA-treated samples. Error bar represents ± SE. ‘a’ indicates that difference from IAA1a-luc is significant at P < 0.05 (Student’s t-test), n = 4. ‘b’ indicates that difference from Δaux/iaa is significant at P < 0.05 (Student’s t-test), n = 4. ‘s’ indicates that difference between conditions is significant at P < 0.05 (Student’s t-test), n = 4.