Overexpression of cytosolic ascorbate peroxidase in tomato confers tolerance to chilling and salt stress

Ascorbate peroxidase (APX) plays an important role in the metabolism of hydrogen peroxide in higher plants, affording them protection against oxidative stress. We studied the effect of overexpressing a cytosolic ascorbate peroxidase (cAPX) gene-derived from pea (Pisum sativum L.)-in transgenic tomato plants (Lycopersicon esculentum L.). Transformants were selected in vitro using kanamycin resistance and confirmed by polymerase chain reaction (PCR) and northern analyses. An APX native-gel assay indicated that, in the absence of stress, APX activity in transgenic plants was several times greater than that measured in wild-type (WT) plants. Several independently transformed lines were propagated and evaluated for resistance to chilling and salt stress. After placing seeds at 9 degrees C for 5 weeks, percent germination was greater for seeds obtained from transgenic lines (26% to 37%) compared to the WT (3%). Plants from transgenic lines also had lower electrolyte leakage (20% to 23%) than WT (44%) after exposure to 4 degrees C. Visual assessment of transgenic and WT lines exposed to salinity stress (200 or 250 mm) confirmed that overexpression of APX minimized leaf damage. Moreover, APX activity was nearly 25- and 10-fold higher in the leaves of transgenic plants in response to chilling and salt stresses, respectively. Our results substantiate that increased levels of APX activity brought about by overexpression of a cytosolic APX gene may play an important role in ameliorating oxidative injury induced by chilling and salt stress.