Overexpression of HARDY, an AP2/ERF gene from Arabidopsis,improves drought and salt tolerance by reducing transpirationand sodium uptake in transgenic Trifolium alexandrinum L

Abstract Trifolium alexandrinum L. was transformed

with the Arabidopsis HARDY gene that belongs to the

stress-related AP2/ERF (APETALA2/ethylene responsive

element binding factors) superfamily of transcription factors.

The fresh weights of the transgenic lines L2 and L3

were improved by 42 and 55% under drought stress and by

38 and 95% under salt stress compared to the wild type,

respectively. The dry weights were similarly improved.

Overexpression of HARDY improved the instantaneous

water use efficiency (WUE) under drought stress by

reducing transpiration (E) and under salt stress by

improving photosynthesis (A), through reducing Na?

accumulation in leaves, and reducing E. However, HARDY

improved the growth of drought-stressed transgenic plants

as compared to the wild type by delaying water depletion

from soil and preventing rapid decline in A. L2 and L3 had

thicker stems and in case of L3, more xylem rows per

vascular bundle, which may have made L3 more resistant

to lodging in the field. Field performance of L2 and L3

under combined drought and salt stress was significantly

better than that of the wild type in terms of fresh and dry

weights (40%, 46% and 31%, 40%, respectively). The

results provide further evidence for the efficiency of

overexpression of a single gene in improving tolerance to

abiotic stress under field conditions.