Fig. 5: Auxin biosynthesis acts epistatic to and downstream of BR signaling to regulate LR response to low N.

a–c Appearance of plants (a), primary root length (b) and average lateral root length (c) of wild-type (Col-0), bsk3, yuc8 and bsk3 yuc8 plants grown under high N (HN, 11.4 mM N) or low N (LN, 0.55 mM N). Horizontal lines show medians; box limits indicate the 25th and 75th percentiles; whiskers extend to 1.5 times the interquartile range from the 25th and 75th percentiles. Numbers below each box indicates the number of plants assessed for each genotype under the respective N condition. d Appearance of bsk3,4,7,8 mutant plants grown at HN or LN in the presence or absence of 50 nM IAA. e The LR response of bsk3 and bsk3,4,7,8 plants to low N is rescued in presence of exogenous IAA. Dots represent means ± SEM. Number of individual roots analyzed in HN/LN: n = 19/22 (mock) and 17/17 (50 nM IAA) for Col-0; 15/15 (mock) and 17/17 (50 nM IAA) for bsk3; 17/16 (mock) and 18/18 (50 nM IAA) for bsk3,4,7,8. Average LR length was assessed 9 days after transfer. f–g Transcript levels of YUC8 in bsk3,4,7,8 (f) and BZR1 loss- (bzr1) or gain-of-function (bzr1-1D) mutants (g). Expression levels were assessed in roots by qPCR and normalized to ACT2 and UBQ10. Bars represent means ± SEM (n = 4 for Col-0, bzr1, bzr1-1D, and three independent biological replicates for bsk3,4,7,8 at both N conditions). h–i Representative images (h) and ratio of mDII-ntdTomato and DII-n3xVenus fluorescence signals (i) in mature LR tips of wild-type plants grown for 7 days on HN or LN in the presence or absence of 1 µM brassinazole, a BR biosynthesis inhibitor. j–k Representative images (j) and ratio of mDII-ntdTomato and DII-n3xVenus fluorescence signals (k) in mature LR tips of Col-0/R2D2 and bzr1-1D/R2D2. In (h–j), Scale bars, 100 µm. In (h–k), DII-n3xVenus and mDII-ntdTomato fluorescence was quantified in epidermal cells of mature LRs. Dots represent means ± SEM (n = 20 roots). Different letters in (b, c, e–g) indicate significant differences at P < 0.05 according to one-way ANOVA and post hoc Tukey test.