Basmati Smells Like That Because The Gene Is Broken
Basmati smells incredible because a gene broke. Humans then spent centuries breeding for the damage.
A working copy of the gene BADH2 makes an enzyme that consumes the precursor to 2-acetyl-1-pyrroline, the compound behind the smell of basmati and jasmine rice. Working normally, it keeps that compound from building up. You get rice that smells like rice.
In fragrant varieties the gene carries an eight-base-pair deletion in exon 7. That introduces a premature stop codon and truncates the enzyme. With nothing consuming the precursor, the aroma compound accumulates.
So the smell is a loss of function. It is also recessive, which is how it hides in a population until someone crosses the right two plants. Shared mutations across fragrant varieties point back to a single ancestral break, which people then selected for, repeatedly, across centuries, with no idea what they were selecting — only that one plant in the field smelled better than its neighbours.
Researchers have since done it on purpose. CRISPR edits to BADH2 in non-fragrant varieties raised the aroma compound without changing amylose content or gelatinization temperature. The smell arrived; the texture stayed put.
Caveats. BADH2 is the major locus but not the only one, and documented exceptions to the exon-7 deletion exist, including a second null allele with a seven-base-pair deletion in exon 2.
We are also declining to repeat the very popular claim that this is the same compound responsible for pandan leaves and bread crust. It is everywhere. We could not verify it. This is a library.