The MYB36 transcription factor orchestrates Casparian strip formation

Abstract

The endodermis in roots acts as a selectivity filter for nutrient and water transport essential for growth and development. This selectivity is enabled by the formation of lignin-based Casparian strips. Casparian strip formation is initiated by the localization of the Casparian strip domain proteins (CASPs) in the plasma membrane, at the site where the Casparian strip will form. Localized CASPs recruit Peroxidase 64 (PER64), a Respiratory Burst Oxidase Homolog F, and Enhanced Suberin 1 (ESB1), a dirigent-like protein, to assemble the lignin polymerization machinery. However, the factors that control both expression of the genes encoding this biosynthetic machinery and its localization to the Casparian strip formation site remain unknown. Here, we identify the transcription factor, MYB36, essential for Casparian strip formation. MYB36 directly and positively regulates the expression of the Casparian strip genes CASP1, PER64, and ESB1. Casparian strips are absent in plants lacking a functional MYB36 and are replaced by ectopic lignin-like material in the corners of endodermal cells. The barrier function of Casparian strips in these plants is also disrupted. Significantly, ectopic expression of MYB36 in the cortex is sufficient to reprogram these cells to start expressing CASP1–GFP, correctly localize the CASP1–GFP protein to form a Casparian strip domain, and deposit a Casparian strip-like structure in the cell wall at this location. These results demonstrate that MYB36 is controlling expression of the machinery required to locally polymerize lignin in a fine band in the cell wall for the formation of the Casparian strip.

Caspary’s conductor

“…It revealed that leaf ionomics represents a powerful tool to uncover root barrier mutants. Kamiya et al (6) demonstrate that 11250, a previously isolated Arabidopsis mutant (12), has elevated levels of sodium, magnesium, and zinc, and reduced calcium, manganese, and iron in leaves. This ion profile is very similar to that of esb1 and casp1 casp3.…”

“…Unfortunately, chromosomal rearrangements in 11250 delayed the identification of MYB36 as the causal defective gene. To see whether the CS is affected in the myb36 mutant, Kamiya et al (6) inspected the distribution of lignin-like materials in the endodermis. No CS lignin was present in the middle of the endodermal cell wall of myb36.…”

“…Kamiya et al (6) then tested the CS functionality using the apoplastic tracer propidium iodide (PI). They found that in regions of myb36 roots where a CS is expected and lignin-like material is present in the corners, PI easily penetrated the endodermis.…”

“…To test the hypothesis that MYB36 controls the expression of instruments involved in CS formation, Kamiya et al (6) conducted chromatin immunoprecipitation quantitative PCR. This experiment showed that MYB36 regulates CASP1, PER64, and ESB1, presumably by directly connecting with their promoters.…”

“…Therefore, Kamiya et al (6) conducted genome-wide transcriptomics in roots of myb36 mutants. From the initial set of 77 differentially expressed genes, they selected endodermis-expressed genes and deselected esb1-effected genes, because they potentially respond to ectopic suberin and lignin; this manifested a shortlist of genes directly regulated by MYB36.…”