A Transgenic Mutant Defective in Cell Elongation and Cellular Organization during Both Root and Shoot Development in Lettuce, Lactuca sativa

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A recessive mutation affecting both root and shoot development was isolated from transformants of lettuce cv. Diana co-transformed with maize Ac transposase and Ds. Mutant phenotype in the progeny was co-segregated with the T-DNA containing Ac transposase indicating that the mutation was caused by insertional mutagenesis. Mutation in this novel genetic locus, designated as ABNORMAL ROOT AND SHOOT(ARS), showed a dwarf phenotype with short thick roots, short hypocotyl and abnormal filamentous leaves without any further reprodective development. The anatomical analysis revealed that the arsmutant root phenotype is primarily due to the lack of cell elongation and to the abnormal increase in cell number in the cortex region in the roots. ars mutants are able to initiate normal leaf primodia, but, the cell elongation and cellular organization of the developing leaf primodia is impaired and results in sequentially abnormal development of the leaf. ars mutants also display photomorphogenic development in darkness by producing open cotyledons, developing vegetative leaves, and short hypocotyls. This suggests that the ARS gene may also be involved in the regulation of cell elongation in the hypocotyl in the absence of light. Abnormal development in ars mutations can not be normalized by exogenous application of phytohormones such as gibberellin and brassinosteroids, indicating that the mutant is not impaired in the biosynthesis of these two hormones.