Effect of Gamma Irradiation on 2-Acetyl-1-pyrroline Content, GABA Content and Volatile Compounds of Germinated Rice (Thai Upland Rice).

Sompong Sansenya,Kannika Phasai,Chanun Sricheewin,Yanling Hua,Saowapa Chumanee

doi:10.3390/plants6020018

Abstract

Aroma intensity in rice is related to the level of 2-acetyl-1-pyrroline (2AP). The accumulation of 2AP in rice has been synthesized via l-proline metabolism by inactive betaine aldehyde dehydrogenase enzyme (BADH2), which activates 2AP accumulation. Meanwhile, active BADH2 inhibits 2AP accumulation but activates γ-aminobutyric acid (GABA) accumulation. The improvement of 2AP content in rice has been reported under certain conditions, such as high salinity, water treatment, and reduction of high intensity solar exposure. In this study, we conducted the effects of gamma irradiation on 2AP content, GABA content and volatile compounds of germinated rice (Thai upland rice). Our results showed that the GABA content was highest when rice seeds germinated within a 24-h. The 2AP content of irradiated rice (germinated within a 24-h duration) was higher than non-irradiated rice for all gamma doses, particularly at 20 Gy, which showed a 23-fold higher level of 2AP than non-irradiated rice. On the other hand, the reduction of the GABA content of irradiated rice was caused by an increase in the gamma dose. At 300 Gy, irradiated rice had a GABA content approximately 2.6-fold lower than non-irradiated rice. Moreover, we observed that a reduction of volatile compounds occurred when increasing gamma dose. However, some volatile compounds appeared in the irradiated rice at gamma doses of 60 Gy, 80 Gy, 100 Gy and 300 Gy. Furthermore, we observed that the level of Octanal, which is the compound most related to aroma intensity, of irradiated rice was stronger than that of non-irradiated rice. Our results demonstrate for the first time that 2AP and GABA contents are sensitive to gamma irradiation conditions. Moreover, the results indicate that the gamma irradiation technique can be used to improve the aroma intensity of rice.

Highlights

Rice aroma is formed by a mixture of a volatile compounds and more than 100 other compounds [1,2,3]
The optimized germination time that produced the highest GABald to form γ-aminobutyric acid (GABA) content was further used in experiments to test the effect of gamma irradiation on GABA content, 2AP content and volatile compounds
Our observation shows that the GABA content of the irradiated rice significantly (p < 0.05) decreased with all gamma doses compared with the non-irradiated rice

Summary

Introduction

Rice aroma is formed by a mixture of a volatile compounds and more than 100 other compounds [1,2,3]. 2-acetyl-1-pyrroline (2AP) as characteristic compound of fragrant rice, and one of the most abundant aroma compounds found in fragrant rice is reported to be Octanal [3,4,5]. (Badh2) inhibits 2AP synthesis, while the recessive betaine aldehyde dehydrogenase gene (badh). Plants 2017, 6, 18 activates 2AP accumulation [6,7]. The recessive badh gene encoding the inactive betaine aldehyde dehydrogenase (BADH2) catalyzes the substrate γ-aminobutyraldehyde (GABald) to ∆1 -pyrroline, and acetylates 2AP. The dominant Badh gene encoding the active BADH2 catalyzes the substrate GABald to form γ-aminobutyric acid (GABA) [7,8]. GABA accumulation in plants has been reported via two pathways. The active BADH2 enzyme catalyzes GABald to form

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