EXPRESSION OF ARGININE DECARBOXYLASE IN SEEDLINGS OF INDICA RICE (ORYZA-SATIVA L) CULTIVARS AS AFFECTED BY SALINITY STRESS

The effect of salinity stress on the activity of arginine decarboxylas e (ADC, EC 4.1.1.19), the first enzyme in biosynthesis of polyamines ( PA) from arginine, as well as its transcript level has been compared i n salt-sensitive (M-1-48) and salt-tolerant (Pokkali) rice cultivars. Treatment of 72 h grown seedlings either with increasing concentration s of NaCl or with 150 mM NaCl for different time periods, showed a gra dual increase of activity in Pokkali. In M-1-48 an immediate increase followed by sharp decrease was observed on prolonged treatment beyond 6 h or above 150 mM NaCl. To generate a DNA probe for ADC, the polymer ase chain reaction was used with oat genomic DNA and sequence-specific primers. A region of oat genomic DNA containing a coding sequence for 166 amino acids of the C-terminal part of the ADC enzyme was amplifie d and called OAD1. Southern analysis of EcoRI- or BamHI-cut genomic DN As from different cultivars of rice with OAD1 as the probe revealed st rong hybridization with one DNA fragment of rice and restriction fragm ent length polymorphism (RFLP) was noticed. Northern analysis of total RNA of rice with OAD1 as the probe revealed hybridization with a tran script of similar size to the ADC transcript in oat. While in Pokkali, at least a 20-fold accumulation of OAD1 homologous transcript was det ected after treatment with 200 mM NaCl, only a seven-fold increase in transcript level was found in M-1-48 after 150 mM NaCl treatment. Resu lts suggest that in the salt-tolerant rice cultivar Pokkali ADC enzyme activity increases and its transcript also accumulates during the pro longed salinity stress, this mechanism is absent in the salt-sensitive rice cultivar M-1-48 where a prolonged period of salinity stress down -regulates both ADC activity and its transcript level.