EXPRESSION OF ABSCISIC-ACID RESPONSIVE ELEMENT-BINDING PROTEIN IN SALT-TOLERANT INDICA RICE (ORYZA-SATIVA L. CV POKKALI)

As the products of abiotic stress and ABA inducible genes are predicte d to play an important role in the mechanism of salt tolerance, the ex pression of transcription factor that recognizes abscisic acid-respons ive element (ABRE) is likely to be regulated when plants are exposed t o abiotic stress. Northern analysis of total RNA from control and salt -treated 10-day-old Pokkali (salt tolerant) rice plants was performed to find out the level of transcripts homologous to wheat cDNA (GC19) f or EmBP-1 (bZIP class factor), a transcription factor that recognizes ABRE. Salinity stress (72 h)-induced accumulation of two transcripts, of 2.0 kb (r2.0) and 1.5 kb (r1.5), in roots was detected. Both transc ripts were detectable even after 6 h of salt or abscisic acid treatmen t, whereas sheath and lamina showed constitutive levels of r1.5 transc ript. When P-32-labeled DNA containing ABRE was used in a gel mobility shift assay, a low level of complex formation by binding factor was d etected from the nuclear extract of lamina of control rice plants. Qua ntitative enhancement of complex formation was found with the nuclear extract prepared from the lamina of plants treated with 200 mM NaCl fo r 26 h over control nuclear extract, suggesting a step of regulation o f expression of ABRE-binding protein in response to salinity stress. S outh-western blot analysis of equal amounts of nuclear proteins of lam ina showed binding of P-32-labeled ABRE-DNA with two polypeptides (22- 28 kDa) present at constitutive levels in control or NaCl-treated plan ts. Preincubation of the laminar nuclear extract of control plants, wi th spermidine or proline at 5 mM concentration showed quantitative enh ancement of ABRE binding activity. Kinetics of spermidine stimulation showed gradual increase of complex formation from 5 mM concentration. Similarly, addition of GTP to the control nuclear extract also showed quantitative enhancement of complex formation and heparin was found to inhibit GTP activated complex formation by about 25%. Results may sug gest the presence of ABRE binding protein in presynthesized and inacti ve form in control plants and GTP mediated activation is probably one of the way to regulate the expression of ABRE-binding factor.