Do we know the biological purpose of Polyphenol oxidases

[Cikomyr2]

I was seeing a facebook post that railed against people who denounce GMO food. The point made "people dont mind a thousand generation of manipulation to breed a 5-meter pumpkin, but remove one enzyme that makes fruits brown and people lose their mind".

And i was thinking.. why do fruits brown?

Like, i get the factual explanation that give details about ***what causes the browning***. There's enzymes named Polyphenol oxidases that, when exposed to oxygen, cause the browning.

Okay, cool. But what's the point of that enzyme? If its an enzyme shared across multiple species of plants that have fruits, is it.. important? Purposeful? Are we taking out something from the fruit we don't even understand the consequences of it missing beyond the browning?

[NichG]

I don't know about other fruit, but tomato seeds have a protective coat that requires the fruit around it to ferment in order to remove it. In nature, that would be responsible for among other things timing the propagation of the plant's offspring so it doesn't just start growing new plants right as it's getting colder. That sort of thing in other seeds (maybe tomatoes? dunno) can be a mechanism to require the seeds to pass through a digestive tract, which means the seeds range further and don't compete with the parent plant.

So a lot of this stuff can be more relevant to the plant controlling it's own spread, in a way that's less relevant given farming practice.

[Jay R]

I suspect that the purpose is for the fruit to start rotting faster, to fertilize the seed.

[Obviously, this does not apply to fruits that were eaten by animals. Those seeds are dropped on the ground surrounded by good fertilizer.]

[Cikomyr2]

I suspect that the purpose is for the fruit to start rotting faster, to fertilize the seed.

[Obviously, this does not apply to fruits that were eaten by animals. Those seeds are dropped on the ground surrounded by good fertilizer.]

That would make a good explanations. I had another theory that browning discourage animals from taking just bites of the fruit and leave the core, while animals who eat the whole fruit (and thus spread seeded droppings) arent inconvenienced by it.

But ultimately, id like to have an answer or proper theory on the utility of this enzyme, just so i feel reassured about removing it. If its to facilitate natural propagation then yeah its not important to our needs.

I just dislike us meddling with biological features we do not understand. The meddling is not the problem - GMO can be awesome - but i just feel we are one greedy corporate executive away from doing a big oopsie that destroys pillars of monoculture food production.

[NichG]

I'd be more bothered by the way things are engineered to be second-generation-sterile than engineering aspects of the fruit then.

I don't see making some varietals that lack a benefit for us or don't do well as really particularly risky - if it's a maladaptive change then gene escape can't go very far, and if somehow losing PPO were good for the plant but bad for us (so that removing it creates an invasive), it's unlikely that we'd see it all over the place in nature already. Losing a gene is much easier than discovering a new function after all.

But making it the norm to grow staple crops that can't be locally propagated is a much bigger risk of destroying a pillar of food production since, if that company collapses or it's factories have some problem or the actual fertile base stock is lost, those crops would suddenly become unavailable.

So I guess put me down for opensource GMO and making it normal to hack plants at the individual farm scale.

[Cikomyr2]

I'd be more bothered by the way things are engineered to be second-generation-sterile than engineering aspects of the fruit then.

I don't see making some varietals that lack a benefit for us or don't do well as really particularly risky - if it's a maladaptive change then gene escape can't go very far, and if somehow losing PPO were good for the plant but bad for us (so that removing it creates an invasive), it's unlikely that we'd see it all over the place in nature already. Losing a gene is much easier than discovering a new function after all.

But making it the norm to grow staple crops that can't be locally propagated is a much bigger risk of destroying a pillar of food production since, if that company collapses or it's factories have some problem or the actual fertile base stock is lost, those crops would suddenly become unavailable.

So I guess put me down for opensource GMO and making it normal to hack plants at the individual farm scale.

Oh, absolutely. But that's deliberate greed, and it's easier to regular than "accidental greed" where some people just push for unsafe practices in order to improve the bottom line, without understanding the consequence of their action. We see that plenty in the construction business, or the financial sector.

Compare to what you describe, which is deliberately creating a time bomb that your company have a monopoly on the key.

[Bohandas]

Indeed. That's not a problem with GMO food, that's a problem with Monsanto and the like.

[Eldan]

Luckily, I have access to the literature, so I could look it up.

A few quotes:

PPOs are induced in response to both biotic and abiotic stresses in plants and have been implicated in several functional processes, including plant defence, regulation of plastidic oxygen levels and the phenylpropanoid pathway, among others

Thirteen years ago, the acclaimed review “Defensive roles of polyphenol oxidase in plants” by Constabel and Barbehenn (2008) summarized the progress of the understanding of PPO involvement in plant defence against pests and pathogens and proposed the potential defence mechanisms based on previous evidence.
...
Polyphenol oxidases (PPOs) catalyse the oxidation of phenols to highly reactive o-quinones, after which non-enzymatic secondary reactions result in the formation of melanin polymers and cross-linked polymers with protein functional groups.
...
PPOs are induced in response to both biotic and abiotic stresses in plants and have been implicated in several functional processes, including plant defence, regulation of plastidic oxygen levels and the phenylpropanoid pathway, among others.

From:
Recent advances in polyphenol oxidase-mediated plant stress responses, Zhang and Sun 2021, Phytochemistry.

And from there to others:

The enzyme polyphenol oxidase (PPO) catalyzes the oxidation of phenolic compounds into highly reactive quinones. Polymerization of PPO-derived quinones causes the postharvest browning of cut or bruised fruit, but the native physiological functions of PPOs in undamaged, intact plant cells are not well understood.
...
Silencing of PPO caused major alterations in the metabolism of phenolic compounds and their derivatives (e.g. coumaric acid and catechin) and in the expression of phenylpropanoid pathway genes.
...
Overall, these results suggest that PPO plays a novel and fundamental role in secondary metabolism and acts as an indirect regulator of cell death in walnut.
...
Best characterized is a role of some PPO genes in plant defense against insects and pathogens. As previously discussed, many PPO genes are up-regulated upon pathogen challenge, and overexpression of a potato (Solanum tuberosum) PPO in tomato (Solanum lycopersicum) resulted in decreased susceptibility to Pseudomonas syringae pv tomato
...
it appears likely that different PPOs within a gene family may play a variety of roles in the physiology and development of a single plant.

(Novel Roles for the Polyphenol Oxidase Enzyme in Secondary Metabolism and the Regulation of Cell Death in Walnut, Araji et al. 2014)


And then...

The biological role of PPO is unclear, although the enzyme has been implicated in pigment formation [ 17], oxygen scavenging in the chloroplast [ 18 ], and plant defence [ 18 ].

(An apple polyphenol oxidase cDNA is up-regulated in wounded tissues, Boss et al. Plant Molecular biology, 1995)


Anyway, summing up what I can find: PPOs have a lot of effects, but they all group vaguely under "stress". Stressed plants of all kinds ramp up their chemical metabolism. PPOs are upregulated in response to a) herbivorous insects and b) mechanical wounding and c) oxygen inside the plant, where it shouldn't be. They start producing toxins against herbivores and they "catch" reactive oxygen, before it can damage the plant.

I suspect (and most authors seem to agree, though they can't really prove it) that the browing reaction is an oxygen reaction. The fruit notices it is damaged (by plucking) and reacts defensively. There's oxygen where there shouldn't be oxygen, and oxygen is dangerous, so reacting with the oxygen is itself the number one priority, because uncontrolled oxygen causes enormous amounts of damage everywhere.

Anyway, you really don't want to remove something that is stuck this deeply in the defence pathways. You'd have herbivores all over the plant, eating it (thus you'd need a lot more pesticides, which have off-target side effects) and if it also works against reactive oxygen, you'd probably have everything from weird tasting fruit to massive spikes in plant cancer.

[Cikomyr2]

Luckily, I have access to the literature, so I could look it up.

A few quotes

[CUT FOR SPACE]

Anyway, you really don't want to remove something that is stuck this deeply in the defence pathways. You'd have herbivores all over the plant, eating it (thus you'd need a lot more pesticides, which have off-target side effects) and if it also works against reactive oxygen, you'd probably have everything from weird tasting fruit to massive spikes in plant cancer.

Wow.

Thank you! Thank you very much! Half of that I didn't understood but your summary was very well put!

This is great and informative!!