Submitted to: Journal of Experimental Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/13/2002
Publication Date: N/A
Interpretive Summary: We have identified a lettuce (Lactuca sativa L.) mutant that exhibits a horizontal, or creeping, growth habit after initiation of flowering (bolting). Genetic studies were performed by crossing the mutant lettuce with normal lettuce varieties that grow straight upwards after bolting. Results showed that one recessive gene controls the horizontal growth, and we have named this gene weary, abbreviated wry. Roots and shoots of normal plants that have been re-oriented with respect to gravity (i.e. turned on their sides) show a rapid gravitropic response, with shoots bending to grow upwards and roots bending to grow downwards. Amyloplasts are starch-containing organelles that are found in the endodermis of higher plants, and are thought to be necessary for perceiving gravity. Studies conducted by re-orienting mutant and normal lettuce plants in darkness (to remove the effects of phototropism) revealed that above-ground parts of weary plants (stems and seedling hypocotyls) do not respond normally to gravity. Roots of weary plants respond normally to gravity. Microscopic analysis of weary seedling hypocotyls showed that these tissues have abnormally distributed amyloplasts, and that stems of weary plants lack amyloplasts altogether. The phenotype of the weary mutant is similar to that of other plant mutants that are defective in radial patterning process that occurs during plant development and gravitropism.
Technical Abstract: A lettuce (Lactuca sativa L.) mutant that exhibits a procumbent growth habit was identified and characterized. In two wild type (WT) genetic backgrounds, segregation patterns revealed that the mutant phenotype was controlled by a recessive allele at a single locus, which was designated weary. Hypocotyls and inflorescence stems of plants homozygous for the weary allele exhibited reduced gravitropic responses compared with WT plants, but roots exhibited normal gravitropism. Microscopic analysis revealed differences in the radial distribution of amyloplasts in hypocotyls and inflorescence stem cells of weary and WT plants. Amyloplasts occurred in a single layer of endodermal cells in WT hypocotyls and inflorescence stems. In contrast, amyloplasts were observed in several layers of cortical cells in weary hypocotyls, and weary inflorescence stem cells lacked amyloplasts entirely. These results are consistent with the proposed role of sedimenting amyloplasts in shoot gravitropism of higher plants. The phenotype associated with the weary mutant is similar to that described for the Arabidopsis mutant sgr1/scr, which is defective in radial patterning and gravitropism.