Potato
blight
Sporangia
Reproduction
Figure 1 shows how potato blight reproduces. When both mating
strains (labelled male and female in Figure 1) meet, they combine to
produce non-motile spores (oospores). These spores have thick walls
and are capable of being carried by wind to plants some distance
away. This feature is shared by many fungal pathogens. But potato
blight has another trick that most fungi lack. It can produce motile
spores. What is more, it can do so without needing both mating
strains to be present, as the process is asexual. These spores are
called zoospores, and they swim through films of water (e.g. on the
surface of leaves, or in the soil), attracted by substances that diffuse
from plant parts (this sort of movement is called chemotaxis). Infected
plants produce millions of these tiny spores, quickly infecting whole
fields full of crops. By the time a farmer notices the tell-tale signs of
discoloured leaves, fruit or tubers, it is usually too late to save the
crop. Most farmers therefore spray their crops with fungicide early in
the year, especially if cool, wet weather is forecast.
Plant destroyer
Phytophthora — literally meaning ‘plant destroyer’ — is a genus
with 30 species, all of which can cause devastating infections of
plants. Farmers most fear Phytophthora infestans — the scourge of
popular and economically important crops including potatoes and
tomatoes. The Irish potato famine (1845–49) was a period of mass
starvation and emigration due to the failure of the potato harvest
in Ireland, caused by potato blight. Part of the problem was the
heavy dependence of the Irish on a single crop, but the weather
played a huge role in the severity of the disease. The summers
of those years were particularly cool and wet — conditions that
favoured the production and spread of blight spores.
Sporangium
Oospore
germinates and
produces
sporangium
Zoospores released
Zoospores and/or
sporangia germinate
on surface
Infected
seedling
Oospore
Infected
tuber
Hyphae
Germ
penetrate leaf
tube
Hyphae emerge
from leaf and
produce sporangia
Asexual
Infected
plant
Figure 1 Reproduction in Phytophthora
Sexual
What sort of organism is Phytophthora?
Leaf surface
Everyone agrees that Phytophthora is a pathogen — an organism that can cause disease (Greek pathos = disease).
Most biologists would agree that it is in the phylum Heterokontophyta (Greek ‘different’ + ‘pole’ + ‘plant’ — more
about this later). But people disagree on other descriptions. In some textbooks it is described as a fungus, in others
as a protoctist, and my favourite — a protoctist that shares some features with fungi (e.g. ‘it has hyphae, like the
mould fungi: the walls of the hyphae are made of cellulose and not chitin…’). Happily there is little need to worry
about these differences as everyone will agree that it is a eukaryote, and if you study biology at university, you will
probably find that the only three groups of organisms people will agree on will be prokaryotes, eukaryotes and
archaeans.
So what was all that about ‘different’, ‘pole’ and ‘plant’? Here you need to have a close look at a zoospore
(Figure 2). Each has two ‘poles’ — flagella (as does the Chlamydomonas ‘plant’ on page 5) but in Phytophthora
they are not identical. The one with fine filaments emerging from it (the tinsel flagellum) pulls the zoospore through
water, the other (the whiplash flagellum) beats to drive it forward. Many brown seaweeds (plants as far as I’m
concerned) produce zoospores that are almost indistinguishable from those of potato blight.
Tinsel
flagellum
Whiplash
flagellum
Figure 2 Phytophthora
zoospore
P. infestans on potato leaf
Hyphae
Spraying potato crops in Norfolk, UK
Further reading
Tomatoes infected with P. infestans
Watch zoospores of Phytophthora swimming and exhibiting chemotaxis as they infect
a root: www.youtube.com/watch?v=PxF8OwDtJh0
The Great Hunger:
these statues are a
memorial to those who
died in the Irish potato
famine 1845–49
Coloured scanning electron
micrograph of potato blight
fungus (×500)
20
Biological Sciences Review February 2019
Protecting the
pinosaur
BiologicalSciencesReviewExtras
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‘Why potato blight strategy will need to change in 2018’, Farmers Weekly, 31 Jan
2018: https://tinyurl.com/y8qmpdhs
Liz Sheffield, University of Liverpool.
21
www.hoddereducation.co.uk/biologicalsciencesreview
Box 1
Genetic bottlenecks
The first bottle in Figure 1.1 represents
a population of individuals, such as the
pinosaurs alive in Cretaceous times. The
different colours represent individuals
with alleles that differ from those of
other members of the population. The
second illustration shows the population
being subjected to an influence, such
as a pathogen or an environmental
catastrophe, which kills many individuals.
Such events will remove some genetic
variation, and in the case of the pinosaur,
this was so severe that only a single
genotype survived.
Discovery of a plant
dinosaur
The story starts in 1994, when a national parks
and wildlife ranger noticed an unusual-looking tree
in a remote canyon in Wollemi National Park in the
Blue Mountains of New South Wales, Australia. He
took a sample to a plant expert, who was excited
to confirm that it was a conifer that was thought
to have died out at least 2 million years ago. These
trees can reach over 30 m in height and were very
successful up to and including the Cretaceous
period (along with many species of dinosaur).
Although they are not pines (they are most closely
related to monkey puzzle — Araucaria — species),
these beautiful trees are affectionately referred to
as pinosaurs (and Wollemi pines).
‘Brutality of Cork’s famine years: “I saw hovels crowded with the sick and dying in
every doorway”’, Irish Examiner, 8 May 2018: https://tinyurl.com/ycrm6w8x
Figure 1.1
Original
population
Bottlenecking
event
Surviving
population
Disaster averted
Sadly, in 2005, it became clear that
the original trees were infected with
Phytophthora (see also B iological
S cienceS R eview , Vol. 31, No. 3,
pp. 20–21). It is not known how the
pathogen arrived, but suspicion fell
on unauthorised visitors who failed
to take the precautions necessary to
avoid contamination.
Happily, the foresight of the
conservationists had paid off and
trees were growing all over the world
in parks and gardens by then. Now
an ‘insurance population’ of 191
trees is thriving at a secret Australian
location. This area is sufficiently
distant from the original site that it
should avoid fire, predators or disease
striking the other grove. This insurance
population is growing ‘like Weetabix
kids’, at 30 cm per year instead of the
1 cm of those in the original (much
darker) location, according to Dr Heidi
Zimmer, a senior scientist with NSW
Environment. This year, for the first
time, some of this population has
produced fertile seeds, so the future of
this iconic living fossil looks bright.
Pinosaur bark — an attractive
feature likened to CocoPops
or bubbling chocolate
Pinosaur cones. Left is a male cone, releasing
a cloud of pollen. Right is a female cone,
which, when pollinated, will produce seeds
Vulnerability
The discovery triggered massive media interest and intensive research, which quickly showed
that there were only 100 or so individuals in one area of the park, and that they were genetically
identical. This means that the species must have gone through a severe genetic bottleneck (see
Box 1). The researchers realised that this made the plants extremely vulnerable, as a disease
that affected one tree would necessarily affect them all. They kept the location of these rarities
secret, took extensive measures to avoid transporting in pathogens, and did something completely
unprecedented. Using cuttings and seeds, they cultivated new plants just as fast and furiously as they
were able. They hoped that, by making the plants commercially available, the original trees would
avoid unwanted attention, especially from plant poachers.
Young pinosaurs growing in a commercial
glasshouse. When planted out, the trees
are tolerant of full sun and deep shade,
below freezing to 40 o C, so are popular with
gardeners all over the world
Further reading
For more on this story see: www.youtube.com/watch?v=LQuhXLDXI-0
For more about living fossils, including animals, see: www.youtube.com/watch?v=uX1Qbztryds
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BIOLOGY DEPARTMENT
TEACHERS
66
THE CLAPPER 2018 - 2019
The Wollemi pine
Biological Sciences Review April 2019
Liz Sheffield, University of Liverpool
www.hoddereducation.co.uk/biologicalsciencesreview
Head of Department
Bige SÖZEN KILIÇ
Teachers
Özge KEŞAPLI CAN
Tessa SIEBRITS
21