There were still new fungi to find when I visited the gardens in November – I found leathery black Bulgaria iniquinans, also known as Black bulgar or Black jellydrops, growing on and decomposing dead oak and ash wood alongside plenty more of the Turkeytail fungus I found in October. The Rosy bonnet fungus, Mycena rosea, which I found growing on leaf litter on the woodland floor is another decomposer. M. rosea is a delicate-looking species but its pretty pink colour is a warning that it contains the same toxic muscarine compound as its cousin, the red and white Fly Agaric.
The other thing to catch my eye this month were fallen oak leaves, still with vivid green patches (“green islands”) amidst the gold and brown. These are the result of plant growth hormones known as cytokinins, secreted by leaf mining insects (Englebrecht et al., 1969). Cytokinins play many roles in the development of leaves, maintaining the ability of meristem cells to keep dividing and controlling the number and size of cells in a leaf (Wu et al., 2021). They are also important in regulating when a leaf starts to senesce, or die, reducing sugar accumulation, increasing chlorophyll synthesis and prolonging the time for which the leaf can photosynthesise. Boosting cytokinin levels is one way in which the leaf ageing caused by adverse environmental conditions such as water stress can be postponed, and crop yields potentially improved.
Leaf miners are the larval stages of a wide range of insects (mostly flies, moths and sawflies) which live and grow inside leaves. The larvae get some protection from predators and a ready source of food, usually avoiding the parts of the leaf with the most indigestible cellulose or high levels of tannins. Most mining species are restricted to a limited range of plants but identifying the miners I found is way beyond my skill set!
By prolonging the photosynthesising life of the leaves, the leaf miners maintain their food supply for a little longer, to their obvious advantage. The larvae secrete the cytokinins which produce this effect, though it turns out that the cytokinins may actually be produced by bacterial symbionts living within the larvae and are probably being produced even by larvae mining leaves which are not on the point of senescence. Mélanie Body and colleagues found that leaf-mining caterpillars of the micromoth, Phyllonorycter blancardella, can only produce significant amounts of cytokinins if they harbour endosymbiotic Wolbachia bacteria (Body et al., 2013). A little like our growing understanding of the wide-ranging effects of our own gut microbial flora on health, a whole new field of enquiry starts to open up if bacterial symbionts play an important role in plant-insect interactions….
Body, M. et al. (2013) Leaf-Miners Co-opt Microorganisms to Enhance their Nutritional Environment. J Chem Ecol., 39, pp 969–977. DOI 10.1007/s10886-013-0307-y
Engelbrecht, L., Orban, U. and Heese, W. (1969) Leaf-miner Caterpillars and Cytokinins in the “Green Islands” of Autumn Leaves, Nature (London), 223(5203), pp. 319–321. Available at: https://doi.org/10.1038/223319a0.
Wu, W., Du, K., Kang, X. et al. (2021) The diverse roles of cytokinins in regulating leaf development. Hortic Res 8, 118 . https://doi.org/10.1038/s41438-021-00558-3