PT  - JOURNAL ARTICLE
AU  - Cecilia Borassi
AU  - Javier Gloazzo Dorosz
AU  - Martiniano M. Ricardi
AU  - Laercio Pol Fachin
AU  - Mariana Carignani Sardoy
AU  - Eliana Marzol
AU  - Silvina Mangano
AU  - Diana Rosa Rodríguez Garcia
AU  - Javier Martínez Pacheco
AU  - Yossmayer del Carmen Rondón Guerrero
AU  - Silvia M. Velasquez
AU  - Bianca Villavicencio
AU  - Marina Ciancia
AU  - Georg Seifert
AU  - Hugo Verli
AU  - José M. Estevez
TI  - A cell surface arabinogalactan-peptide influences root hair cell fate
AID  - 10.1101/787143
DP  - 2019 Jan 01
TA  - bioRxiv
PG  - 787143
4099  - http://biorxiv.org/content/early/2019/11/25/787143.short
4100  - http://biorxiv.org/content/early/2019/11/25/787143.full
AB  - Root hairs (RHs) develop from specialized epidermal cells called trichoblasts, whereas epidermal cells that lack RHs are known as atrichoblasts. The mechanism controlling root epidermal cell fate is only partially understood. Root epidermis cell fate is regulated by a transcription factor complex that promotes the expression of the homeodomain protein GLABRA 2 (GL2), which blocks RH development by inhibiting ROOT HAIR DEFECTIVE 6 (RHD6). Suppression of GL2 expression activates RHD6, a series of downstream TFs including ROOT HAIR DEFECTIVE 6 LIKE-4 (RSL4 [Yi et al. 2010]) and their target genes, and causes epidermal cells to develop into RHs. Brassinosteroids (BRs) influence root epidermis cell fate. In the absence of BRs, phosphorylated BIN2 (a Type-II GSK3-like kinase) inhibits a protein complex that directly downregulates GL2 [Chen et al. 2014]. Here, we show that the genetic and pharmacological perturbation of the arabinogalactan peptide (AG) AGP21 in Arabidopsis thaliana, triggers aberrant RH development, similar to that observed in plants with defective BR signaling. We reveal that an O-glycosylated AGP21 peptide, which is positively regulated by BZR1, a transcription factor activated by BR signaling, affects RH cell fate by altering GL2 expression in a BIN2-dependent manner. These results suggest that perturbation of a cell surface AGP disrupts BR responses and inhibits the downstream effect of BIN2 on the RH repressor GL2 in root epidermal cells. In addition, AGP21 also acts in a BR-independent, AGP-dependent mode that together with BIN2 signalling cascade controls RH cell fate.Significance In the plant Arabidopsis thaliana, the root epidermis forms in an alternating pattern atrichoblasts with trichoblast cells that end up developing root hairs (RHs). Atrichoblast cell fate is directly promoted by the transcription factor GLABRA2 (GL2) while the lack of GL2 allows RH formation. The loss of AGP21 peptide triggers an abnormal RH cell fate in two contiguous cells in a similar manner as brassinosteroid (BRs) mutants. In the absence of BR signaling, BIN2 (a GSK3 like-kinase) in a phosphorylated state, downregulate GL2 expression to trigger RH cell fate. The absence of AGP21 is able to repress GL2 expression and activates the expression of RSL4 and EXP7 root hair proteins.