The culprit, in that case, is ethylene, a phytohormone. Yes, just like animals, plants have hormones that control growth, development and stress perception! Ethylene is a simple molecule composed of 2 carbons attached via a double bond and sandwiched between 4 hydrogens (H2C=CH2).
|Ethylene chemical structure Public Domain, Ben Mills posted on Wikimedia|
In most fruits, ethylene is the primary hormone that guides ripening. In the case of our banana, they are actually picked green and then exposed to large quantities of ethylene gas to ripen them artificially so they all ripen at the correct time to go to market. Inside the banana's cells, ethylene binds to specialized receptors (ETR1) that are found on the cell membranes.
While one often thinks about gene regulation being based on positive interactions, a hormone binds and turns something on, ethylene binding to the receptor actually deactivates this receptor! Without ethylene, the receptor is keeping CTR1 activated so that it keeps the downstream proteins inactive. When ethylene binds, it turns the receptor off so that CTR1 is no longer activated and EIN2 can then stabilize the EIN3 transcription factor which activates ethylene responsive genes.
|Ethylene regulatory network, personal illustration, based on Shi et al 2012 model|
For our banana, going from a hard, green, bitter unripe banana to a soft, yellow, sweet ripe banana requires a plethora of genes from many pathways. In fact, a study showed that over 100 pathways were involved in ripening. These genes are responsible for softening the tissue, changing starch to sugar, production of aromatics, and more. Next time you take a bite of your ripe banana, remember the importance of ethylene and how many genes were involved to make this perfect bite!
Alexander and Grierson, 2002 J. Exp. Bot. 53 (377): 2039-2055. doi: 10.1093/jxb/erf072 http://jxb.oxfordjournals.org/content/53/377/2039.fullAsif, M., et al, 2014. BMC Plant Biol. 14: 316. doi: 10.1186/s12870-014-0316-1 http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4263013/
Shi Y, et al., 2012. Plant Cell 24 (6):2578-2595. doi:10.1105/tpc.112.098640 http://www.plantcell.org/content/24/6/2578
Wikipedia. Ethylene https://en.wikipedia.org/wiki/Ethylene