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Journal of Experimental Botany, Vol. 54, No. 385, pp. 1259-1268, April 1, 2003
© 2003 Oxford University Press

Characterization and genetic analysis of a lettuce (Lactuca sativa L.) mutant, weary, that exhibits reduced gravitropic response in hypocotyls and inflorescence stems

Received 9 September 2002; Accepted 13 January 2003

Rebecca C. Grube1,, Eric B. Brennan and Edward J. Ryder

US Department of Agriculture, Agricultural Research Service, 1636 E. Alisal St., Salinas, CA 93905, USA

1 To whom correspondence should be addressed. Fax: +1 831 755 2814. e-mail: rgrube{at}pw.ars.usda.gov

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 hypocotyl and inflorescence stem cells of weary and WT plants. Amyloplasts occurred in a single layer of endodermal cells in WT hypocotyls and inflorescence stems. By 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.

Key words: Amyloplast, Asteraceae, endodermis, gravitropic mutant, gravitropism, Lactuca sativa, lettuce.


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