Dr Cara Daly Writes about the Fascinating Biology of Plant Fungal Invaders

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Dr Cara Daly details the biology of plant fungi and the interesting processes of a plant’s internal defence system

When it comes to biological warfare in the garden, plants are like sitting ducks. Rooted firmly in the ground, they have no choice but submit to a constant barrage of weather-related stress, insect, and fungal attacks. However plants can defend themselves; they often have thick waxy cuticles, can taste bitter, and may be covered in sticky, smelly hairs to repel nibbling pests. They have an internal defence too which acts like an immune system and is known as the ‘hypersensitive response’.

Fungal Invader  

The photo below shows that immune system in action in a Rubus leaf. Those brown blotches on the leaf are sections of dead plant tissue and their formation has probably been triggered by a fungal invader. If I asked you to hypothesize as to what might have happened in those dead regions of the leaf, you might suggest that the fungal invader has killed the plant cells.

In reality, it is likely that the plant has killed its own cells. How? Well, plant cell biochemistry changes when a pest invades – in an effort to save the plant, a concert of plant biochemistry kicks into action ultimately instructing the plant cells in that region to activate a death programme in the plant’s own cells! These suicidal plant cells die by a well-studied process called Programmed Cell Death. In doing so, they kill themselves, but crucially, they also kill the invader and the plant gets to live another day. Good news for the plant, but I can see how those brown spots might not be so attractive to a gardener!

Fundamental Drives

Many vegetable gardeners will agree when I say that diseases like mildews, blights and rusts are enemy number one. Fungal diseases thrive in Ireland’s mild damp climate and their success lies in their unique biology. Fungi are so different to plants or animals that they are categorised in their own kingdom but like all life on earth, they have two linked fundamental drives: 1) to get nutrition, and 2) to send their genes into the next generation. Fungi is particularly good at the latter. An infected leaf will produce millions of microscopic spores containing the genetic code to establish the next generation.

The spores are usually wind-blown, or parachuted onto a plant in a water droplet. The fungal spore immediately glues itself to the plant, and then, much like a new root emerging from a germinating seed, the spore produces a germ tube which bores down into the plant. At this stage, some plants perceive the fungal invader and quickly activate Programmed Cell Death to kill the cells (and invader) but sometimes, the germ tube grows so rapidly that it quickly forms a branched network of fungal threads (called hyphae) which completely overwhelms the plant’s ability to fight the infection. And the fungal invader has a sting in its tail -  the hyphae emit their digestive enzymes into the plant tissue which break down the plant tissue into the nutrients the fungus needs, and the fungus reabsorbs the goodies; bad news for our tomato plants, but good news for the fungus which quickly produces more spores to continue the disease cycle.

New Adhesives

Plant scientists are kept busy trying to understand the ‘Hows and Whys’ of all these processes. For instance, the impressively strong glue which attaches the fungal spore to the leaf has been studied in an effort to develop new adhesives. Interestingly, fungi are one of the few organisms that can digest plant material which is actually crucial to the fact that humans evolved at all; without a means of breaking down and recycling ancient dead forests, life on Earth as we know it, would never have evolved at all.

This is the first in the series  "Wow Moments in Plant Science" written by Horticulture (Kildalton College) course leader Dr Cara Daly. The series is published in the quarterly magazine of the Royal Horticultural Society of Ireland (RHSI) who have kindly allowed us to reproduce it here.

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