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ENVIRONMENTAL STRESS AND ADAPTATION TO STRESS

Overproduction of Abscisic Acid in Tomato Increases Transpiration Efficiency and Root Hydraulic Conductivity and Influences Leaf Expansion^1,[OA]

Warwick HRI, Wellesbourne, University of Warwick, Warwickshire CV35 9EF, United Kingdom (A.J.T., J.A., B.J.M., J.M.T.M., H.W.H., J.S.H.); Environmental Biology Group, Research School of Biological Sciences, Australian National University, Canberra, Australian Capital Territory 2601, Australia (G.D.F.); and Department of Plant Sciences, School of BioSciences, University of Nottingham, Sutton Bonington Campus, Loughborough, Leicestershire LE12 5RD, United Kingdom (R.C.S., I.R.A.S., C.R.B., I.B.T.)

Overexpression of genes that respond to drought stress is a seemingly attractive approach for improving drought resistance in crops. However, the consequences for both water-use efficiency and productivity must be considered if agronomic utility is sought. Here, we characterize two tomato (Solanum lycopersicum) lines (sp12 and sp5) that overexpress a gene encoding 9-cis-epoxycarotenoid dioxygenase, the enzyme that catalyzes a key rate-limiting step in abscisic acid (ABA) biosynthesis. Both lines contained more ABA than the wild type, with sp5 accumulating more than sp12. Both had higher transpiration efficiency because of their lower stomatal conductance, as demonstrated by increases in ¹³C and ¹⁸O, and also by gravimetric and gas-exchange methods. They also had greater root hydraulic conductivity. Under well-watered glasshouse conditions, mature sp5 plants were found to have a shoot biomass equal to the wild type despite their lower assimilation rate per unit leaf area. These plants also had longer petioles, larger leaf area, increased specific leaf area, and reduced leaf epinasty. When exposed to root-zone water deficits, line sp12 showed an increase in xylem ABA concentration and a reduction in stomatal conductance to the same final levels as the wild type, but from a different basal level. Indeed, the main difference between the high ABA plants and the wild type was their performance under well-watered conditions: the former conserved soil water by limiting maximum stomatal conductance per unit leaf area, but also, at least in the case of sp5, developed a canopy more suited to light interception, maximizing assimilation per plant, possibly due to improved turgor or suppression of epinasty.

² Present address: Duchy College, Rosewarne, Camborne, Cornwall TR14 OAB, UK.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Andrew J. Thompson (a.j.thompson{at}warwick.ac.uk).

^[OA] Open Access articles can be viewed online without a subscription.

www.plantphysiol.org/cgi/doi/10.1104/pp.106.093559

^* Corresponding author; e-mail a.j.thompson{at}warwick.ac.uk; fax 44–(0)24–7657–4500.

Received November 22, 2006; accepted January 24, 2007; published February 2, 2007.

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