Comparison of hormonal responses to heat, drought and combined stress in tobacco plants with elevated proline content
Abstract
Heat stress (40oC) caused a transient increase in the level of bioactive cytokinins (CK), accompanied by down-regulation of the activity of cytokinin oxidase/dehydrogenase (CKX, EC 1.4.3.18/1.5.99.12) in tobacco plants constitutively over-expressing a gene for the proline (Pro) biosynthetic enzyme INCREMENT 2-pyrroline-5-carboxylate synthetase (P5CSF129A; EC 2.7.2.11/1.2.1.41) as well as the wild-type. Concomitantly, a transient decrease of abscisic acid was observed.
Following drought stress, bioactive CK levels decreased (to lower extent in upper leaves and in roots), more in the wild-type. Exposure to heat stress (40oC for 2 h) at the end of 10-d drought period was associated with stimulation of the activity of CKX in upper leaves and roots, coinciding with down-regulation of bioactive CKs in whole plants.
All stresses were associated with elevation of free auxin, indolylacetic acid, in lower leaves and/or in roots. After rehydration, vast elevation of bioactive CKs in leaves was observed.