Banana aroma is a result of acetohydroxyacid synthase and isopropylmalate synthase alternative isoforms that bypass feedback inhibition

Comment from a daily banana eater: Everything you ever wanted to know about the aroma of bananas! 😊

From the significance and abstract:

"Significance

During banana fruit ripening, the two feedback-inhibited, rate-limiting enzymes of the valine and leucine biosynthetic pathway are simultaneously alternatively spliced, effectively losing their feedback regulation and promoting carbon flux into the pathway for the accumulation of precursors to branched-chain amino acids and volatile branched-chain esters. The fact that the two reactions are positioned sequentially in the pathway may help explain why the production of significant levels of “banana” flavor compounds is so rare in nature. Understanding this phenomenon is timely given the current threat to global Cavendish production due to the spread of Fusarium wilt. Coordinated, dual alternative splicing of rate-limiting enzymes is a means to metabolic dysregulation to enable unidirectional, committed processes like aroma formation during fruit ripening.

Abstract

The distinctive aroma of banana fruit (Musa spp.) results from the upregulation of a pathway that is, paradoxically, understood to be feedback limited. The primary character impact compounds in banana are branched-chain esters with 3-methylbutyl moieties. Recent work has established that these esters, as well as prominent “fruity” 2-methylpropyl and butyl esters, are derived from the ripening-dependent de novo synthesis of the branched-chain amino acids valine and leucine, and their respective α-ketoacid precursors.

The biosynthetic pathway possesses two sequential, rate-limiting, and feedback-inhibited enzymes: acetohydroxyacid synthase and isopropylmalate synthase. We found these enzymes to be alternatively spliced in ripening banana fruit pulp. Unripe fruit and nonfruit tissues had only trace levels of alternative splicing. Revealingly, the domains corresponding to the allosteric inhibitory binding of valine and/or leucine were truncated in the altered isoforms. During ripening, the expression and frequency of the shorter splice forms increased concomitantly with production of branched-chain esters and accumulation of their α-ketoacid precursors. Purified proteins of the modified isoforms were active but relieved of feedback inhibition. Transient expression of the shortened isoforms in Nicotiana benthamiana led to a greater accumulation of iso-branched-chain metabolites than the full-length isoforms. Results indicate that a developmentally dependent and simultaneous dysregulation in two sequential steps of the branched-chain amino acid synthetic pathway enables the biosynthesis of banana fruit’s unique character-impact esters."

Banana aroma is a result of acetohydroxyacid synthase and isopropylmalate synthase alternative isoforms that bypass feedback inhibition | PNAS (no public access)

Your Bananas Will Get More Expensive Soon. Here's Why with Palki Sharma

Recommendable! Except for this climate change nonsense! I noticed lately that the bananas at my grocery store had become smaller than usual. India is the largest grower of bananas in the world.

Researchers have gone bananas over banana’s complex ancestry

Amazing stuff!

"... An extensive genetic analysis of more than 100 varieties of wild and cultivated bananas unpeels the fruit’s tangled history of domestication and reveals the existence of three previously unknown—and possibly still living—ancestors. ...
About 7000 years ago, bananas were not the seedless, fleshy fruits we know today. The flesh was pitted with black seeds and nearly inedible. Instead, people ate the banana tree’s flowers or its underground tubers. ...
Scientists do know the banana’s predominant wild ancestor is a species named Musa acuminata, which occurs from India to Australia. Most researchers agree that Papua New Guinea is where domesticated bananas as we know them first appeared. Today, there are many banana varieties—more than 1000 at last count. ...
Complicating matters, some bananas have the usual two sets of chromosomes, whereas others have three sets or more, suggesting at least some modern bananas are hybrids that resulted from the interbreeding of two or more varieties, or even different species. ...
There’s good reason to try to tap into the modern banana’s deep historical gene pool: The $8 billion banana industry, which produces 100 billion bananas annually, is threatened by diseases such as Panama disease and banana bacterial wilt. ..."

Researchers have gone bananas over this fruit’s complex ancestry | Science | AAAS