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Fig. S1 Phenotypes of Arabidopsis wild-type (WT), mpk3 and mpk6 seedlings.

Fig. S2 Effect of MKK9 activity on primary root length and lateral root density of Arabidopsis.

Fig. S3 Anthocyanin and Pi (phosphate; H2PO4 and HPO42−) accumulation in Arabidopsis Col wild-type (WT) seedlings under Murashige and Skoog (MS) and low Pi (LP) conditions with or without dexamethasone (DEX).

Fig. S4 Anthocyanin and Pi (phosphate; H2PO4 and HPO42−) contents in Arabidopsis wild-type (WT) and mkk9 seedlings after transfer to Murashige and Skoog (MS) or low Pi (LP) medium.

Fig. S5 Immunoblot analyses of MKK9 mutant transgene expression, and MPK3 and MPK6 kinase activity assays of MKK9 mutant transgenic and crossed Arabidopsis seedlings.

Fig. S6 T-DNA insertion of Arabidopsis WRKY33 and WRKY75 mutants.

Fig. S7 Activation of MKK9-MPK3/MPK6 in Arabidopsis seedlings enhances Pi (phosphate; H2PO4 and HPO42−) accumulation and suppresses anthocyanin production through a camalexin-independent pathway.

Fig. S8 Detection of the interaction between Arabidopsis PHR1 and MPK3 or MPK6, phosphorylation by MPK3 and MPK6 on PHR1, and regulation of PHR1 on MKK9-MPK3/MPK6-enhanced Pi (phosphate; H2PO4 and HPO42−) accumulation.

Table S1 Oligonucleotides used in this study