Water deficits and heat shock effects on photosynthesis of a transgenic Arabidopsis thaliana constitutively expressing ABP9, a bZIP transcription factor

The effects of water deficits (WD), heat shock (HS), and both(HSWD) on photosynthetic carbon- and light-use efficienciestogether with leaf ABA content, pigment composition and expressionsof stress- and light harvesting-responsive genes were investigatedin ABP9 [ABA-responsive-element (ABRE) binding protein 9] transgenicArabidopsis (5P2). WD, HS, and HSWD significantly decreasedphotosynthetic rate (A) and stomatal conductance (g_s) in wild-typeplants (WT). A and g_s of 5P2 transgenic plants were slightlyreduced by a single stress and were hardly modified by HSWD.Although A and electron transport rate (ETR) in 5P2 plants weredepressed under optimal growth conditions (control) in relationto WT, they were enhanced under HS and HSWD. These results indicatethat ABP9 transgenic plants are less susceptible to stress thanthe WT. In addition, the increased ABA contents in both WT and5P2 plants in response to WD and/or HS stresses suggest thatdeclines in A and g_s might have been due to ABA-induced stomatalclosure. Moreover, compared with WT, 5P2 plants exhibited higherABA content, instantaneous water use efficiency (IWUE), Chla/b, NPQ, and lower Chl/carotenoid ratios. Finally, alteredexpression of stress-regulated or light harvesting-responsivegenes was observed. Collectively, our results indicate thatconstitutive expression of ABP9 improves the photosyntheticcapacity of plants under stress by adjusting photosyntheticpigment composition, dissipating excess light energy, and elevatingcarbon-use efficiency as well as increasing ABA content, IWUE,and expression of stress-defensive genes, suggesting an importantrole of ABP9 in the regulation of plant photosynthesis understress. Key words: ABP9, ABA, heat shock, photosynthesis, stress tolerance, water deficits Received 1 September 2007; Revised 19 December 2007 Accepted 21 December 2007