A guest post by Debarati Chakraborty
The power of aroma compounds to offer physical, mental and spiritual delight is one of the oldest enchanting tales. Aroma stimulates the limbic system, one of the primitive parts of the human brain in evolutionary terms – steering our emotions, behaviour and long-term memory. This urge towards enjoying aroma may be the vital driving force that inspired the selection of hundreds of landraces i.e. traditionally cultivated, locally adapted varieties, of aromatic rice, having varying amounts of aroma, throughout our Asia continent.
Aromatic rice in South-East Asian cultures
Scented rice is an integral part of religious and cultural traditions throughout Asia. Mention can be made of rituals like Nabanna, Laxmi puja, Eid al-Fitr, Eid al-Adha of India, Bangladesh and Pakistan; Modsurung, Kaamatan of Malayasia; Long Kaek of Thailand, Chuseok of Korea which include special offerings made of aromatic rice. In China, an age-old tradition of using aromatic rice crackers for celebrating the engagement of daughter exists.
The magic of aromatic rice even received royal patronage. Rice varieties like Mahasali, Bara/Peshawari Basmati, Tapovan Basmati, Mushkin were cooked in the imperial kitchens of the Indian Emperors like Harshavardhana, Akbar.
Several ancient historical texts of India like Charaka Samhita, 700 BCE; Susruta Samhita 400 BCE etc. provide names of various aromatic rice landraces. Some of the most famous aromatic rice of Asia are Basmati of India and Pakistan; Kataribhog of Bangladesh; Sadri of Iran; Jasmine of Thailand; Bahra of Afghanistan; Malagkit Sungsong of Philippines; Lua Thom of Vietnam. But apart from these, there exist several other locally cultivated iconic aromatic rice landraces. Distinctive properties of these rice have given cultural identities to the area cultivated. Examples include Jakou Mochi of Japan; Ikkikoku of China; Gobindobhog, Joha, Chakhao of India; Bansphul, Balam of Bangladesh and many others.
The chemistry of rice scent
But what chemistry is underlying this well-loved rice aroma? The major source of rice aroma is 2-Acetyl-1-pyrroline (or short, 2-AP). This volatile is a potent odourant. It has alow odour threshold. This means that it can excite the tiny hair-like cilia of the olfactory receptor neuronal cells in the nasal cavity at an extremely low concentration of 0.02 ng/L in the air!!! To compare, the odour threshold of Geraniol, a volatile found in rose oil, citronella, citrus fruits, lemongrass, lavender is 4000ng/L, i.e. a much greater amount needed to be detected by our senses. 2-AP also has high odour activity value, the ratio between the concentration of 2-AP in a sample to its odour threshold value.
The gene ruling the aroma formation is betaine aldehyde dehydrogenase 2/badh2. It encodes an enzyme which processes a biochemical called Gamma-aminobutyraldehyde/GABald to produce Gamma-Aminobutyrate/GABA. This biochemical can save the plant in several stressful conditions like drought, cold, heat, salinity or waterlogging, oxygen deficiency, excess carbon di-oxide, fungal and bacterial infections. If this badh2 gene is non-functional due to mutation and the enzyme is not produced, GABald accumulates and changes to form 1-pyrroline, which further changes to generate 2-AP aroma. The most common mutation conferring this recessive trait is badh2.1. It was reported in several aromatic rice collected throughout Asia. But apart from this, there are several other mutations of badh2 which can trigger scent in rice.
Origin of rice aroma
So why is the plant producing aroma instead of GABA? How did this aromatic rice originate? To find the answer, we have to plunge into the vibrant history of Asian cultivated rice. We also need to deduce the roles of early farmers and their cultural preferences which caused fixation of this trait.
Domestication entails taming the wild progenitor into today’s cultivated form. The decades of quest based on archaeological and genetic shreds of evidence to untangle the riddle of rice domestication have distinguished Asian cultivated rice into three major groups: indica, japonica and aus.
The three centres of rice domestication include Southern China and the Yangtze valley for japonica, Indochina and the Brahmaputra valley for indica, and central India or Bangladesh for aus. Aromatic rice is not confined, but rather well segregated among all these three groups. However, previous studies on aromatic rice are mostly focused on long-grained Basmati type and some high yielding varieties. Very little emphasis has been given on traditional landraces to decipher the mysteries of their origin, domestication and underlying cause of aroma.
Indian traditional rice aroma genetics
I have sequenced the seventh exon of badh2 to check the presence of badh2.1 allele with 84 Indian traditional rice landraces of which 55 were aromatic. I got 11 aromatic landraces without badh2.1 and 3 non-aromatic with badh2.1. All the obtained DNA sequences were submitted to a special bank (yes, there is a bank for DNA sequences too) called NCBI Genbank, so that anyone can use this data.
The findings refuted the previous view that badh2.1 solely occurs in tropical japonica, a subgroup of japonica, as all the samples were indica. The occurrence of aromatic rice devoid of badh2.1 was even more significant. It proved that badh2.1 is not the only allele responsible for aroma production in traditional Indian aromatic rice landraces. As recent findings proved independent domestication for indica and japonica, the findings re-ignited the possibility of the separate origin of aroma in these landraces as well as in wild rice Oryza rufipogon and Oryza nivara.
Aromatic rices of North-East India – an independent origin of aroma gene in aus subpopulation?
I have further reanalysed the traditional rice landraces of North-Eastern India comprising indica, japonica, and aus samples. Among 171 samples, 107 were aromatic and the remaining 64 were non-aromatic. Their identity i.e. indica, japonica or aus was deciphered by comparing them to the critically acclaimed dataset from Garris et al. 2005.
The results classified scented rice of Assam, Mizoram and Sikkim as aus and 50% of Manipur as indica. Around 40% of aromatic rice from Arunachal Pradesh and all but one from Nagaland were japonica. Whereas rest of the Manipur and Arunachal Pradesh samples, both aromatic and non-aromatic, were a mixture between indica and japonica.
The results also highlight the role of culture towards maintaining agrodiversity, since all these landraces are being conserved by distinct ethnic groups. All these landraces showed medium to a high level of uniqueness when tested at the genetic level. Moreover, 55.1% of the aromatic landraces were aus. Of these, a majority of aromatic rice varieties, 47 out of 59, in aus group were short, 5.50 mm to medium-grained, 5.51–6.60 mm. Whereas japonica aromatic landraces were mostly ruled by long, 6.61–7.50 mm and extra-long, >7.5 mm grains.
The occurrence of so many short and medium-grained aus type aromatic rice is vital. It predicts a probable origin of aroma independently in aus-type short and medium-grained aromatic landraces; independent of long-grained basmati. The detection is reinforced by the following evidence: (a) aus group of rice was separately and independently domesticated with sparse gene flow from japonica and indica, (b) the distinct origin of many domestication traits in aus, and (c) a possibility of the occurrence of aroma in wild rice population.
The role of Silk Routes towards spread of aromatic rices in North-East India
The results indicate the spread of rice-centric culture from their places of domestication. The current scenario places the origin of aus in the Jeypore tract, Odisha but the culture of aus cultivation dwells in a vast area of undivided Bengal. Hence the occurrence of so many aus rice in Assam, Mizoram, Sikkim samples signaled an influx and assimilation of aus into these regions from contiguous regions of Bangladesh, West Bengal and Odisha. The historical background of 1947 partition and migration from Bangladesh attests this conclusion further.
Similarly, the occurrence of japonica type grains can be due to recurring trade, exchange of goods, merchandise and gifts among neighbouring countries of Myanmar, China, Tibet and Nepal. The most probable pathways which may have facilitated these exchanges were the perilous Southern and Southwestern Silk routes. Various goods – tea, spices, spring salt, hemp, silk, horses etc. were carried through these routes since ages.
The Southern Silk route, more famous as ‘Chamadao’, which in the Chinese translates to “the tea and horse road”, connects Sichuan and Yunan of China to Tibet, Nepal and India. It was used since 150 BCE. A division of this route is named Southwestern Silk route, connecting Southern Yunnan of China to upper Myanmar and finally to eastern India, existed since 400 BCE. It is most plausibly through this route that prince Chaolung Sukaphaa, the first Ahom king migrated from Yunan in 1236. It is his migration which initiated the wet rice cultivation in the fertile plains of Assam.
Thus, human migrations to places covering great distances have facilitated the dissemination of rice grains as food, seed, merchandise and gifts. Mixed ancestry may have resulted through exchange and subsequent interbreeding of rice from different groups.
Thus the mystery of aromatic rice domestication has revealed its new colours in the study. Further study involving wild populations from the three domestication centres is the way forward. Also, questions like the influence of environment on aroma, the eco-physiological role of aroma if any, await to be unveiled to usher us into the new aromatic skyline.
About the author
I have recently submitted my doctoral thesis on genetic and biochemical underpinnings of aroma in Indian rice landraces at the University of Kalyani, India. I also regularly use social media (Facebook, Instagram and Twitter) to garner awareness about organic farming, archaeobotany, ecological agriculture and indigenous crop plant varieties. Besides this, singing is my passion.