Developmental plasticity is critical for plants to adapt to constantly changing environments. Plant trichomes and root hairs are specialized epidermal cells that paly crucial roles in defense against environmental stressors. Here, we report the isolation of an Arabidopsis mutant, a berrantly b ranched t richome 6-1 ( abt6-1), with both impaired trichomes and root hairs. Map-based cloning and allelic analyses confirmed that abt6-1 is a new mutant allele of SPIRRIG ( SPI), which encodes a beige and Chediak Higashi (BEACH) domain-containing protein. SPI has been reported to facilitate actin dependent root hair development by temporally and spatially regulating the expression of BRICK1 (BRK1), a subunit of the WAVE/SCAR actin nucleating promoting complex. Based on molecular and biochemical analyses, we found BRK1 is unstable and SPI mediates BRK1 stability. Functional loss of SPI results in the accumulation of steady-state of BRK1. Moreover, we found spi mutant root hairs are hypersensitive to salt stress and their initiation and elongation were entirely inhibited under NaCl treatment. Detailed examination of the actin cytoskeleton revealed that salt stress induces an altered actin organization in root hair and root epidermal cells that resemble those in the spi mutant, implying SPI may respond to salt stress by modulating actin cytoskeleton organization.