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The MYB36 transcription factor orchestrates Casparian strip formation
Wednesday, 2015/08/19 | 08:20:34

Takehiro Kamiya, Monica Borghi, Peng Wang, John M. C. Danku, Lothar Kalmbach, Prashant S. Hosmani, Sadaf Naseer, Toru Fujiwara, Niko Geldner, and David E. Salt

 

Significance

Casparian strips play a critical role in sealing endodermal cells in the root to block uncontrolled extracellular uptake of nutrients and water. Building Casparian strips requires the construction of extracellular lignin structures that encircle cells within the cell wall and that are anchored to the plasma membranes of adjacent cells to form tight seals between them. The transcription factor we have discovered, and the set of genes it regulates, now provides us with the detailed “parts list” necessary to build Casparian strips. This finding has clear implications for better understanding the nature of tight cellular junctions in biology and also has practical implications of agricultural, offering the potential for improved water and nutrient use efficiencies and enhanced resistance to abiotic stresses.

 

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 CASP1GFP, 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.

 

See: http://www.pnas.org/content/112/33/10533.abstract.html?etoc

PNAS August 18, 2015 vol. 112 no. 33 10533-10538

 

Fig. 1.

Disruption of MYB36 alters the leaf ionome and CASP1 expression. (A) Principal component analysis based on the concentration of 20 elements in shoots (n = 15). (B) Mutants identified by measuring level of accumulation of CASP1–GFP expressed from the CASP1 native promoter. (C) Mutation sites in the myb36 mutants. The myb36-1 mutant may have a large change in the promoter region because we were unable to amplify this region; myb36-2 contains a T-DNA insertion in the second exon; myb36-3 contains an Arg-to-Trp substitution in the second MYB domain repeat, and myb36-4 contains a splice-site mutation after the second intron. (D–G) Endodermal localization of MYB36 using a MYB36–GFP fusion protein expressed from the MYB36 native promoter. Magenta [propidium iodide (PI)], cell wall. E and G were taken with the same microscope settings. The dotted box in E is shown in F with the brightness of the GFP signal artificially enhanced. Arrowheads (E and F) point to examples of MYB36–GFP in the endodermal cell layer. [Scale bars: 100 µm (B and D); 50 µm (E and G).]

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