What might a supposed “tree enzyme” be — injected into a Morton Bay Fig tree?
So eventually I decided to email the City of Fremantle, who put me in contact with their Parks Department, who gave me the name of the company who had handled the tree, who replied with a most informative letter.
And the answer is…

Glutathione


Most animal biochemists, like me, will be more familiar with the importance of glutathione (reducing sulphydryl group highlighted in yellow, above) as a reducing agent in erythrocytes, the constituents of which are subject to oxidation damage.
What was wrong with the tree?

“…the figs in Kings Square… had quite elevated levels of heavy metals Cd, Cr, and metalloids Hg and As.
We traced this back to the colonial past, where in the 1830s – 1840s, when gold had been discovered south east of Fremantle, a battery operated (by the Crown) in Fremantle. In those days, gold was commonly lixiviated using mercury. Kings Square had a pit cut earlier for mining limestone. A good place to dispose of battery mullock is a pit.The earliest gold deposits were mafic ores, so often contained Cd and Cr.”

Why use glutathione to treat the trees?

“Firstly, it stimulates the jasmonate pathway, thereby alters transpiration and root growth.
Secondly, it assists in the isolation and immobilisation of heavy metals.
Thirdly, if assists in the breakdown of triazine herbicides by ring cleavage…”

And my correspondent kindly provided a review by Hossain et al. in the Journal of Botany, which elaborates on the second point.
And why the “tree enzyme” business?
This did not come from the company that handled the treatment, but from someone (unknown or nameless) who was handling the public relations, and who wished to provide reassurance that the treatment was ‘natural’:

“I guess in the world of snake oil, “enzymes” are benign, natural things, rather than usually ephemeral catalytic proteins.”