A Gly65Val substitution in an actin, GhACT_LI1, disrupts cell polarity and F-actin organization resulting in dwarf, lintless cotton plants

Actin polymerizes to form part of the cytoskeleton and organize polar growth in all eukaryotic cells. Species with numerous actin genes are especially useful for the dissection of actin molecular function due to redundancy and neofunctionalization. Here, we investigated the role of a cotton (Gossypium hirsutum) actin gene in the organization of actin filaments in lobed cotyledon pavement cells and the highly elongated single-celled trichomes that comprise cotton lint fibers. Using mapping-by-sequencing, virus-induced gene silencing, and molecular modeling, we identified the causative mutation of the dominant dwarf Ligon lintless Li-1 short fiber mutant as a single Gly65Val amino acid substitution in a polymerization domain of an actin gene, GhACT_LI1 (Gh_D04G0865). We observed altered cell morphology and disrupted organization of F-actin in Li-1 plant cells by confocal microscopy. Mutant leaf cells lacked interdigitation of lobes and F-actin did not uniformly decorate the nuclear envelope. While wild-type lint fiber trichome cells contained long longitudinal actin cables, the short Li-1 fiber cells accumulated disoriented transverse cables. The polymerization-defective Gly65Val allele in Li-1 plants likely disrupts processive elongation of F-actin, resulting in a disorganized cytoskeleton and reduced cell polarity, which likely accounts for the dominant gene action and diverse pleiotropic effects associated with the Li-1 mutation. Lastly, we propose a model to account for these effects, and underscore the roles of actin organization in determining plant cell polarity, shape and plant growth. Significance Statement Here we used mapping-by-sequencing, virus-induced gene silencing, and molecular modeling to determine the causative mutation of the dominant dwarf Ligon-lintless Li-1 short fiber mutant in cotton. This actin isoform point mutation likely disrupts processive elongation of F-actin, resulting in a disorganized cytoskeleton and reduced cell polarity. We suggest that stem-specific expression of actin isoforms with mutations similar or orthologous to the Gly65Val mutation of Li-1 might provide another avenue to introduce beneficial dwarfism to crops.