Genetic analysis of the tomato inquieta mutant links the ARP2/3 complex to trichome development

Trichomes are hair-like structures on the aerial surface of many plant species. Trichomes are well characterized for their role as physical barriers and chemical defense against herbivore attack. Here, we describe the characterization of a monogenic recessive mutant of tomato (Solanum lycopersicum) called inquieta (ini). All trichome types on ini plants showed distinct morphological defects (e.g., swelling) that are known to be associated with defects in the actin cytoskeleton. Genetic mapping experiments positioned the Ini locus within a 1.5 cM interval on chromosome 11 that contains the tomato homolog of the Arabidopsis ARPC2A gene, which encodes a protein involved in nucleating the polymerization of actin filaments. Use of ARPC2A as a molecular marker showed that this gene strictly co-segregates with the target locus in a mapping population of 135 F-2 plants. Reverse transcriptase (RT)-PCR and genomic PCR experiments showed that full-length ARPC2A is amplified in wild-type but not in the ini mutant. Flanking PCR and Southern blot analysis showed that the ini mutation corresponds to a complex similar to 6-kb insertion in the 5(th) intron of ARPC2A. These results provide molecular evidence that altered trichome development in the ini mutant is caused by a defect in actin cytoskeleton formation.