Here’s how wild grass turned into domesticated rice

A team of plant biologists at a Bengaluru has thrown light on a tiny molecule which played a key role in the domestication of rice, the dietary staple for half the world’s population.

The scientists led by Padubidri V Shivaprasad at the National Centre for Biological Sciences (NCBS) in Bengaluru found that the sacrifices made by this biomolecule, a molecular regulator, would have been critical in transforming semi-aquatic plants -- the wild relatives of domesticated rice -- into plants bearing abundant grains.

During its long history of cultivation that dates back to at least 4000 BC, rice plants, with ample help from pre-historic plant breeders, have acquired and accumulated traits that would help increase yield (such as highly branched flowering structures) and weeded out those undesirable ones that reduced yield. Scientists have been able to identify many genes that are linked to some of these changes that place during rice domestication by comparing genomes of cultivated rice plants and their wild relatives.

In the latest work, which appeared in the journal Plant recently, the NCBS scientists sought to prove that there was more than these altered genes. Shivaprasad and his colleagues hypothesised that certain regulatory molecules named small RNAs (sRNAs) might have also contributed to domestication-associated changes. sRNAs are tiny messenger molecules that dictate which genes can make proteins as well as when and where within the organism.

Rice varieties

For this, the Bengaluru scientists compared sRNAs found in high-yielding rice varieties with those in hundreds of land-races (rice varieties that were grown before green revolution) and in their wild progenitors. Their work helped them zero in on one particular class of sRNAs called miR397. They found that while miR397 was abundantly present in wild rice varieties, their levels were barely detectable in cultivated varieties, including the traditional ones.

Further scrutiny by the scientists revealed that miR397 is responsible for silencing members of a gene family called laccase that are key for producing lignin, the woody tissue which provides mechanical strength to plant stems, helping them bear more grains.

As a next step, when the Indian scientists introduced the gene responsible for the expression of miR397 in domesticated rice plants, the progeny plants were more similar to their wilder cousins. They had long, spindly stems, narrow short leaves, few flowering structures and hardly any rice grains.

“The important finding of our study is that when rice accumulates increased levels of lignin, it gives additional strength to the plant to bear more grains. Our finding gives an opportunity to our breeders to efficiently improve the rice yield by reducing miR397 or by elevating laccase levels,” said Swetha Chenna, the first author of the study.

“More importantly, this has given as a good molecular marker for breeding high-yielding varieties,” said Shivaparasad. “We have found that there are at least 26 genomic regions previously linked, but still uncharacterised, to rice yield,” he added.