Fig.2 | Symptoms of wild type and tin2 mutant infected maize plants. Leaves from plants infected with the solopathogenic strain SG200 expressing tin2 (left), two tin2 mutant strains in the SG200 background (middle) and a complemented strain (right) are shown. SG200 and the complemented strain induce large tumors and anthocyanin while the tin2 mutant infected leaves display smaller tumors and lack anthocyanin coloration..
As the individual contributions of most effector genes in cluster 19A to tumor formation were small, we studied the response of maize plants to the whole cluster mutant as well as to several individual mutants by array analysis. This revealed distinct plant responses, demonstrating that the respective effectors have discrete plant targets. We propose that the analysis of plant responses to effector mutant strains that lack a strong virulence phenotype may be a general way to visualize differences in effector function.
The Tin2 effector: The Tin2 effector in cluster 19A, which is responsible for anthocyanin biosynthesis (Fig.2), was studied in molecular detail. By expressing tin2 transiently without its signal peptide in maize epidermal cells, anthocyanin induction could be observed. This illustrates that Tin2 protein functions inside plant cells. Since anthocyanin induction was only detectable when the tissue was pre-infected with a tin2 mutant strain and was not seen in uninfected tissue, fungal proliferation most likely serves to elevate the synthesis of precursors for anthocyanins. By yeast-two-hybrid analysis we identified the maize protein kinase ZmTTK1 as only interaction partner of Tin2. By expressing ZmTTK1 in Nicotiana benthamiana and co-expressing Tin2 we were able to demonstrate that Tin2 stabilizes ZmTTK1. This occurs through masking a ubiquitin-proteasome degradation motif in the N-terminal domain of ZmTTK1. As result, an active kinase is detected (through its ability to phosphorylate myelin basic protein). Active ZmTTK1 controls activation of genes in the anthocyanin biosynthesis pathway, presumably through phosphorylation of the R-transcription factor, a known positive regulator of the anthocyanin biosynthetic genes. In line with this we observe that all anthocyanin biosynthetic genes are transcriptionally upregulated in tissue infected with U. maydis strains expressing tin2. After infection with a tin2 mutant, the anthocyanin biosynthetic genes are not induced. Instead, enhanced lignin biosynthesis is observed in mutant-infected tissue and several lignin biosynthetic genes are induced. Vascular bundles in particular show strong lignification. This is presumably limiting access of fungal hyphae to nutrients needed for massive proliferation late in infection and could explain the late proliferation defect of tin2 mutants in infected tissue. Consistent with this assertion, we observe that maize brown midrib mutants affected in lignin biosynthesis are hypersensitive to U. maydis infection. We speculate that Tin2 rewires metabolites into the anthocyanin pathway to lower their availability for other defense responses, in particular for lignin biosynthesis that uses the same precursor, p-coumarate, as the anthocyanin biosynthesis pathway. A model explaining these findings is shown in Fig. 3.