**3. Biochemical basis of fragrance**

Generally, the aromatic rice cultivars are enriched with large volatile and semi volatile compounds *viz.,* alcohols, aliphatic aldehydes, alkane, alkene, aromatic aldehydes, aromatic hydrocarbon, carboxylic acid, ester, furan, ketone, N-heterocyclic, phenol, and terpenes [14–17].

### **3.1 Structure and chemistry of 2AP**

Among the different volatile compounds, 2-acetyl-1-pyrroline (2AP) with popcorn-like aroma and lowest odor threshold is reported to be the potent biochemical compound to impart fragrance in rice [18]. The chemical structure of 2AP is an N-heterocyclic compound containing 1-pyrroline ring in which the hydrogen at position 2 is replaced by an acetyl group with a methyl ketone group. 2AP content in scented rice varieties include 0.04–0.09 ppm, whereas non-aromatic varieties have 10x less (<0.006–0.008 ppm) [19].

#### **3.2 Biosynthetic pathway for 2AP**

There are many contradictions and views regarding the biochemical pathway for 2AP synthesis and it is still being explored. It was reported that *L-*proline was the precursor for the production of 2AP [20]; and is involved in polyamine degradation pathway which is the main enzymatic pathway and there are some other non-enzymatic pathways reported having an influencing action on 2AP concentration. In the enzymatic polyamine pathway, arginine, ornithine, spermidine, putrescine, etc. are degraded into GAB-ald which spontaneously cyclises *Rice Aroma: Biochemical, Genetics and Molecular Aspects and Its Extraction… DOI: http://dx.doi.org/10.5772/intechopen.98913*

#### **Figure 1.**

*2AP biosynthetic pathway in rice. (a) Enzymatic (BADH2-dependant) 2AP synthesis [21, 22] (b) non enzymatic (BADH2-independant) 2AP synthesis [23].*

to Δ<sup>1</sup> -pyrroline, an immediate precursor of 2AP biosynthesis [21]. The nonfunctional badh2 enzyme (encoded by *osbadh2* gene) inhibits the conversion of the γ-aminobutyraldehyde (GAB-ald) to γ-aminobutyric acid (GABA) thereby allowing the formation of Δ<sup>1</sup> -pyrroline and ultimately the 2AP in scented rice whereas the reverse happens in non-scented rices (functional BADH2 enzyme coded by *OsBadh2* gene inhibits 2-AP formation) [22].

Some non-enzymatic direct pathways had also been described by many scientists and researchers. Glutamate produces proline and the proline accumulated during stress is converted to Δ<sup>1</sup> -Pyrroline-5-carboxylic acid (P5C) by the enzyme Δ1 -Pyrroline-5- carboxylate synthase (P5CS). This P5C combines directly with methylglyoxal without involving any enzymes or might be converted to Δ<sup>1</sup> pyrroline and thereby enhancing the 2AP concentration [23]*.* In normal plants the methylglyoxal produced from glycolysis is detoxified by glyoxalase enzymes and their concentration was kept low. It was speculated that 2AP is a generative volatile compound to detoxify methylglyoxal in rice plant [24] (**Figure 1**).

2AP concentration differs in different plant parts of rice. The concentration is more in grains and flag leaf than in any parts of the plant [25–27]. Glutamate, ornithine and proline are important amino acids that serves as nitrogen (N2) source in the ring of Δ<sup>1</sup> -pyrroline [11]. The high aroma content in grains is mainly from the larger availability of N2 from the soil. So, the aroma concentration may vary depending upon the nitrogen availability to the plants [28]. Advanced researches are essential in correlating the genetic and biochemical aspects of scented rice varieties, particularly with regard to the nitrogen and acetyl group donor in 2AP in order to reveal the key enzymes that are involved in the biosynthetic pathway of aroma in rice.
