N-plumbaginifolia zeaxanthin epoxidase transgenic lines have unaltered baseline ABA accumulations in roots and xylem sap, but contrasting sensitivities of ABA accumulation to water deficit

A series of transgenic lines of Nicotiana plumbaginifolia with modified exp ression of zeaxanthin epoxidase gene (ZEP) provided contrasting ABA accumul ation in roots and xylem sap, For mild water stress, concentration of ABA i n the xylem sap ([ABA](xylem)) was clearly lower in plants underexpressing ZEP mRNA (complemented mutants and antisense transgenic lines) than in wild -type. In well-watered conditions, all lines presented similar [ABA](xylem) and similar ABA accumulation rates in detached roots. Plants could, theref ore, be grown under normal light intensities and evaporative demand. Both Z EP mRNA abundance and ABA accumulation rate in roots increased with water d eficit in all transgenic lines, except in complemented aba2-s1 mutants in w hich the ZEP gene was controlled by a constitutive promoter which does not respond to water deficit. These lines presented no change in root ABA conte nt either with time or dehydration. The increase in ZEP mRNA abundance in r oots with decreasing RWC was more pronounced in detached roots than in whol e plants, suggesting a difference in mechanism, In all transgenic lines, a linear relationship was observed between predawn leaf water potential and [ ABA](xylem), which could be reproduced in several experiments in the greenh ouse and in the growth chamber. It is therefore possible to represent the e ffect of the transformation by a single parameter, thereby allowing the use of a quantitative approach to assist understanding of the behaviour of tra nsgenic lines.