A transgenic mutant defective in cell elongation and cellular organizationduring both root and shoot development in lettuce, Lactuca sativa

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