Physiological Behavior of Two Tobacco Lines Expressing EPSP Synthase Resistant to Glyphosate

Among numerous tobacco transgenic lines expressing an altered gene encoding glyphosate-resistant EPSP synthase, two were chosen for detailed physiological studies, as they were both highly resistant to glyphosate in the growth chamber (HR and WR lines) while one of them (WR) was susceptible to the herbicide in the field. The two transformed lines and the susceptible line (S) grew similarly. Foliar applied glyphosate penetrated into the plants to the same extent in the three lines and phloem transfer from the source leaves to the sinks was also similar. In the transgenic lines, EPSP synthase was overexpressed in all leaves, with an average overexpression factor reaching 3 to 15, with the highest value in HR. The averageI50for glyphosate of the leaf enzyme was approximatly 700-fold that from S for WR and 6 to 22 times lower for HR. The most physiologically significant difference between the two transgenic lines was that WR had a very low EPSP synthase content in its roots. Furthermore, the enzyme present in the roots was very sensitive to glyphosate. In contrast, the roots of the HR line contained a well-measurable pool of resistant enzyme. This difference probably explains why HR was resistant to glyphosate in the field, allowing a good rate of lignification to occur and ensuring high water movements inside the plant in order to compensate for the transpiration loss.