STABILITY AND EXPRESSION OF AMPLIFIED EPSPS GENES IN GLYPHOSATE RESISTANT TOBACCO CELLS AND PLANTLETS

The stability and expression of amplified 5-enolpyruvylshikimate-3-pho sphate synthase (EPSPS) genes was examined in glyphosate resistant tob acco cells grown in glyphosate-free medium, and in plantlets regenerat ed from resistant cells. Amplified DNA was maintained in resistant cel ls grown in the absence of glyphosate for three years. Amplified EPSPS genes were retained in regenerated plantlets at levels comparable to those observed in the resistant cells, and EPSPS mRNA was overexpresse d (compared to unselected plantlets). However, glyphosate resistance i n cell lines grown in glyphosate-free medium declined 7-fold, and in r egenerated plantlets approximately 20-fold, compared to resistant cell s maintained under glyphosate selection. In plantlets, reduced resista nce correlated with lower levels of EPSPS mRNA. Plantlets regenerated from resistant cells exhibited morphological variation, and had an app roximate doubling of their nuclear genome size.