The effect of salt on betaine aldehyde dehydrogenase transcript levels and 2-acetyl-1-pyrroline concentration in fragrant and non-fragrant rice (Oryza sativa)

Rice (Oryza sativa) possesses two genes encoding betaine aldehyde dehydrogenase (BADH, EC 1.2.1.8) isozymes (BADH1 and BADH2) despite not accumulating glycine betaine. A mutant BADH2 allele (badh2) which codes for a substantially truncated BADH2 enzyme is responsible for the elevated level of 2-acetyl-1-pyrroline (2AP) which gives fragrant rice varieties their characteristic aroma. Increased 2AP concentration has been reported, but not scientifically validated, where fragrant rice varieties have been exposed to drought and salt stress. We studied the transcript levels of the genes which code for BADH1 and BADH2 using quantitative real-time PCR (qRTPCR) in leaf and seed at several developmental time points and in response to salt in a range of fragrant and non-fragrant rice varieties. In all non-fragrant rice varieties, BADH1 and BADH2 transcript levels were very similar in developing seed whilst BADH2 transcript levels were significantly higher than those of BADH1 in leaf and mature seed, demonstrating that the deletion in the gene in fragrant rice results in a very significant reduction in total functional BADH transcript. BADH2 transcripts in non-fragrant rice varieties were significantly more abundant than badh2 transcripts in fragrant rice varieties in leaf and seed tissue at all developmental stages, which may be associated with the loss of BADH2 function in fragrant genotypes. The abundance of BADH1 transcripts was similar in fragrant and non-fragrant rice varieties. In response to salt treatments of increasing concentrations, the level of BADH1 transcript and the ratio of BADH1/BADH2 transcripts increased significantly in leaf tissue from both non-fragrant and fragrant rice varieties, whilst no consistent relationship between BADH2 transcript levels and salt treatment was observed, suggesting that BADH1 but not BADH2 has a role in the response of rice to salt stress. No significant relationship between leaf tissue 2AP levels in Jasmine and Basmati rice varieties and salt treatment was detected indicating little likely impact of salt on the level of 2AP based fragrance in rice.