Despite not accumulating glycine betaine, rice (Oryza sativa) possesses two genes encoding betaine aldehyde dehydrogenase (BADH, EC 1.2.1.8) (BADH1 and BADH2). A BADH2 allele which codes for a truncated BADH2 enzyme is responsible for the elevated level of 2-acetyl-1-pyrroline (2AP) which gives Jasmine and Basmati style fragrant rices their distinct aroma.
In this study the transcript levels of the genes which encode BADH1 and BADH2 were studied using quantitative reverse-transcriptase PCR (qRTPCR) in multiple fragrant and non-fragrant rice varieties. In non-fragrant rices, BADH2 transcript levels were significantly higher than those of BADH1 in leaf and mature seed, whilst in developing seed BADH1 and BADH2 transcript levels were similar. This indicates that the loss of BADH2 activity due to the deletion in the gene in fragrant rice is likely to result in a significant reduction in total functional BADH.
BADH2 transcript levels in non-fragrant rice varieties were significantly higher than the levels in fragrant rice varieties in all tissues at all developmental stages, this 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 indicating that the loss of function of BADH2 does not lead to an increase in BADH1 expression.
In response to salt treatment, BADH1 gene transcript levels and the ratio of BADH1/BADH2 transcripts increased significantly in leaf tissue from both non-fragrant and fragrant rice varieties, whilst no relationship between BADH2 transcript levels and salt treatment was observed. This suggests that BADH1, but not BADH2, is involved in the salt stress response in rice.
Increased levels of 2AP have been reported by farmers where fragrant rice varieties are exposed to drought and salt stress. However, no significant relationship between leaf tissue 2AP levels and salt treatment was detected, conflicting with these anecdotal reports.