1. Figure S1. Figure S1. Relative expression levels of YUC family member transcripts in roots under high and low N conditions. Seedlings 4 days after germination were transferred to agar medium containing 3 mM NH 4 NO 3 (HN) or 0.1 mM NH 4 NO 3 (LN). After being grown vertically for 7 days, the roots were sampled for quantitative real-time RT-PCR analysis of YUC family member genes. cDNA was standardized by reference to an actin standard. Data are presented as means ± SE. Relative expression

2. Figure S2. Figure S2. Primary root length in Arabidopsis wild type (Col-0), tar2-c, complementation lines of tar2-c, and TAR2 over expression lines under high and low N conditions. Seedlings 4 days after germination were transferred to agar medium containing 3 mM NH 4 NO 3 (HN) or 0.1 mM NH 4 NO 3 (LN). After being grown vertically for 7 days, the primary root length was measured. Comp-2 and -5 indicate complementation lines of tar2-c. OE-22 and -39 indicate TAR2 overexpression lines. Values are given as mean ± SD of at least 15 plants of each line from three independent experiments. Different letters above the column indicates statistically significant differences at the P < 0.05 level. Primary root length (cm) Col-0tar2-c comp-2 comp-5 OE-22OE-39 a b a b a b a b a b a b

3. Figure S3. tar2-ctar2-2 ATG TAA AT4G24670.1 500 bp Figure S3. Predicted intron/exon structure of TAR2 (At4g24670) and T-DNA insertion sites of the tar2-c and tar2-2 mutants.

4. Figure S4. Figure S4. Relative expression levels of TAR2 in Col-0, tar2-c, complementation construct lines, and TAR2 over expression lines. Seedlings 7 days after germination grown on ½ strength MS medium were collected for quantitative real-time RT-PCR analysis of TAR2 in Arabidopsis wild type (Col-0), tar2- c, complementation lines (comp-2 and -5), and TAR2 over expression lines (OE-22 and -39). The relative expression level was calculated as the ratio of TAR2/actin. Values are given as means ± SE of three replicates. Relative expression Col-0tar2-c comp-2 comp-5 OE-22 OE-39

5. Figure S5. tar2-2Col-0 Figure S5. Lateral root number in Arabidopsis wild type (Col-0) and tar2-2 Seedlings 4 days after germination were transferred to agar medium containing 3 mM NH 4 NO 3 (HN) or 0.1 mM NH 4 NO 3 (LN). After being grown vertically for 7 days, the lateral root number was counted. Values are given as mean ± SD of at least 15 plants of each line, from three independent experiments. Different letters above the column indicates statistically significant differences at the P < 0.05 level. LR number a b b b

6. HN LN TAR2 pro ::GUS Figure S6. TAR2 pro ::GUS staining in primary roots under high and low N conditions. Seedlings of Arabidopsis transgenic line harboring TAR2 pro ::GUS 4 days after germination were transferred to agar medium containing 3 mM NH 4 NO 3 (HN) or 0.1 mM NH 4 NO 3 (LN), and were grown for 2 days. Bar =1 mm. Figure S6

7. Relative expression level Figure S7 Figure S7. Time course of TAR2 expression levels in response to N supply. Seedlings of the Arabidopsis transgenic line harboring TAR2 pro ::GUS 4 days after germination were transferred to agar medium containing 3 mM NH 4 NO 3 (HN) or 0.1 mM NH 4 NO 3 (LN), and were grown for the durations indicated. Relative expression level was expressed as the ratio of TAR2/actin. Data are presented as means ± SE of three replicates. Different letters above the column indicates statistically significant differences at the P < 0.05 level. a b c c c c

8. HN LN TAR2 pro ::GUS Stage A Stage B Emerging LR Emerged LR Figure S8. Figure S8. TAR2 pro ::GUS staining in different developmental stages of lateral roots under high and low N conditions. Seedlings of Arabidopsis transgenic line harboring TAR2 pro ::GUS 4 days after germination were transferred to agar medium containing 3 mM NH 4 NO 3 (HN) or 0.1 mM NH 4 NO 3 (LN), and were grown for 4 days. Bar =100 μm.

9. Figure S9. Figure S9. DR5::GUS staining in the shoots of Arabidopsis wild type (Col-0), tar2-c mutant, and TAR2 overexpressor plants under high and low N conditions. Arabidopsis plants expressing DR5::GUS in wild type (Col-0), tar2-c, and TAR2 overexpressor were treated for 4 days with 3 mM NH 4 NO 3 (HN) or 0.1 mM NH 4 NO 3 (LN). Bar =1 mm. tar2-cCol-0 HN LN TAR2-OE39

10. tar2-cCol-0 HN LN TAR2-OE Figure S10. Figure S10. DR5::GUS staining in the roots of Arabidopsis wild type (Col-0) and tar2-c mutant under high and low N conditions. Arabidopsis plants expressing DR5::GUS in tar2-c, wild type (Col-0), and TAR2 overexpression lines were treated for 4 days with 3 mM NH 4 NO 3 (HN) or 0.1 mM NH 4 NO 3 (LN). Bar =100 μm.