Probing actin-mediated root hair development

Chin et al. identify SPIRRIG and SCAR/WAVE as molecular determinants of actin-mediated root hair development in Arabidopsis.

 Background: Root hairs are tip growing cells that extend the radius of a root to optimize water and nutrient absorption. Root hair development is complex and involves the actin cytoskeleton and many proteins that specify the position of root hair emergence and tip growth. To understand the role of actin during root hair development, the dynamics of proteins that may affect actin organization must be visualized during this process by tagging the proteins of interest with a fluorescent protein. One such protein is a large 3571 amino acid BEACH domain-containing protein called SPIRRIG (SPI). In the absence of SPI, root hair tip growth is stunted and the position of root hair emergence is disrupted, suggesting that this protein is essential for root hair development.

Question: We wanted to generate a fully functional SPI fluorescent protein fusion, monitor its spatial and temporal localization during root hair development, and correlate such localization patterns to actin organization.

Findings: We successfully generated a fully functional fluorescent protein for SPI using a technique called recombineering in the Arabidopsis thaliana genome. The SPI protein fusion drove normal root hair development and localized to the tip of rapidly elongating root hairs. This localization pattern was correlated with filamentous-actin (F-actin) meshwork that also localized at the root hair tip. We also uncovered a potential link between SPI and selected components of the actin nucleating promoting complex WAVE/SCAR (W/SCR). A fully functional BRICK1 fluorescent protein fusion, a subunit of W/SCR, marked the position of root hair emergence but dissipated when SPI fluorescence intensified at the root hair tip during root hair growth.

Next steps: Our work links SPI to actin-mediated root hair development, but exactly how SPI modulates actin dynamics and W/SCR localization remain unknown. Additional biochemical and genetic studies could establish stronger mechanistic links between SPI, W/SCR, and actin-mediated root hair development. The simultaneous imaging of SPI, W/SCR, and actin during root hair development represents a significant step in addressing these questions.

Sabrina Chin , Taegun Kwon, Bibi Rafeiza Khan, J. Alan Sparks, Eileen L. Mallery, Daniel B. Szymanski, and Elison B. Blancaflor (2021). Spatial and temporal localization of SPIRRIG and WAVE/SCAR reveal roles for these proteins in actin-mediated root hair development. Plant Cell.