Indirect defences

Question 1

Ants in the plants?

Answer 1

Plant-ant interactions are common - they include seed dispersal, weeding, pollination and anti-herbivore
defence.
Once attracted to a plant, ants defend it against herbivory, but how does a plant entice ants in the first place?

Question 2

Bed and Breakfast

Answer 2

Structures called domatia provide nests.
Extra Floral Nectaries (EFNs) (ser] secrete carbohydrates/sugars or lipid Bodies provide food

Question 3

How are domatia created

Answer 3

Domatia are formed by the hypertrophy (enlargement) of plant tissues to create internal cavities.
The wall at a particular spot in the swelling is thin allowing ants to gnaw an entrance. Queen ant may live inside of hollow thorn structure.

Question 4

Something to Eat

Answer 4

In Cecropia (south american plants) a pad of dense trichomes (the trichilium) sits at
the base of each petiole. From these sprout 1-2mm long
glycogen-containing beads called Műllerian bodies. This is the Azteca ants’ primary food source

Question 5

Something to Drink

Answer 5

Globally & taxonomically widespread - 93 Angiosperm and 5
fern families have EFNs and found in fossils dating to 35 mya. EFNs are found on leaves, twigs, or external parts of flowers – they have no role in pollination. EFNs are particularly common on both Old and New world Acacias.

Question 6

Is it effective?

Answer 6

Dan Janzen was the first to show ant-plant defence in Mexican Acacia.
Acacias produce Beltian Bodies, EFNs, and swollen thorns
Mexican Acacias are inhabited by Psuedomyrmex ferruginea (ant)

Question 7

Janzen’s experiments

Answer 7

Plots containing 25 A. cornigera plants All trees cut to 60cm height.
Acacia crowns removed from the exclusion plot and burned
Acacia crowns left next to stumps in the ant plot (allows re-colonisation)
After only 45 days without ants, barely any leaves
however for the plots that were cut shown and where the ants were allowed to recolonise, there were much more leaves.

Question 8

Ant defence work!

Answer 8

Now well demonstrated for lots of different plant-ant interactions across the globe
But is it just invertebrate herbivores that are deterred?

Question 9

The ant and the elephant

Answer 9

Goheen & Palmer (2010) examined whether Crematogaster ants associated with African Acacia drepanolobium influenced elephant browsing.

One year after ant removal (100%, 60% or 30% of colony),
elephant damage had increased on unprotected trees.
Negative relationship between ant abundance (activity) and elephant damage

Question 10

But even ants aren’t fool proof!

Answer 10

Eubanks et al (1997) examined use of A. cornigera by
Polyhymno (Lepidoptera) larvae.

Polyhymno use silk to build sealed shelters from acacia leaves. These shelters greatly reduce detection by P.
ferruginea.
Polyhymno larvae thus have access to an otherwise
poorly defended plant and feeding reaches levels which
can kill the host plant

Question 11

And never trust an ant!

Answer 11

Male et al (2012) showed how ant exclusion increased
flower bud retention in Hirtella physophora.
Ants partially castrate their host by destroying +60% of its
buds; they also deter pollinators. Thus ants reduce
reproduction, diverting plant resources to growth, including
domatia

Question 12

But don’t trust an Acacia either!

Answer 12

Palmer et al (2008) examined EFN production in A.
drepanolobium following exclusion of large herbivores declined following herbivore exclusion. The number of trees occupied by ants also declined

Question 13

Some other indirect defences

Answer 13

Self-Crypsis e.g hakea= produces two forms of leaf so helps hide fruit. Seed pod looks like broad leaves.
Mimicry (of other plants) perhaps look like stinging nettles
Mimicry (of environment)= stone plant
Thigmonasty / seismonasty mimosa looks dead after being touched
Tolerance= good at recovering
Phenology=spatial/ temporal avoidance, some plants may grow and flower at times of year where they may be fewer herbivores around.

Question 14

A cry for help?

Answer 14

Herbivore activity is unpredictable, so ants might waste a lot of time patrolling host plants in the search for herbivores
Rapid communication between ants and damaged plants, could elicit defensive responses and direct workers to sites of herbivore activity. So the plant may produce volatile cues to communicate a particular part in the plant where there are herbivores.

Question 15

Volatile (ants)?

Answer 15

Bruna et al (2008) looked at the response of ants to host plant VOCs.
The ant Pheidole minutula is a host specialist on Maieta guinensis in Brazil
The primary host for sympatric Azteca ants is Tococa bullifera, but they are more generalist in their host preferences

Question 16

Variable volatility

Answer 16

Both ant species responded quickly to extracts
taken from their main host plant But Pheidole responded most strongly to extracts from its host plant while Azteca responded to extracts taken from other plants

Question 17

Signalling – beyond ants

Answer 17

Ant response to host plant VOCs released during herbivory is common, but ants seldom use the herbivore as a source of food

Question 18

SOS Signalling

Answer 18

Tri-trophic interactions – attraction of predators to plants subject to herbivory suspected for decades.
Kessler & Baldwin (2001) was the first field study to
demonstrate the so-called ‘SOS’ effect in nicotine.
They first established that wild tobacco plants release VOCs
when attacked by three herbivorous insect species

Question 19

VOCs affect the herbivores

Answer 19

Herbivores avoided laying their eggs in response to
previous attack (hornworm) and application of either
MeJA or linalool

Question 20

But is it predators?

Answer 20

They only observed one predatory insect during the entirety of the study can we be sure it’s a predation effect?
They glued five moth eggs to leaf undersides on 105 plants and then applied various VOC combinations to the plants.
Egg survival much reduced on plants treated with MeJA and
cis-3-hexene-1-ol (1), linalool (4) &cis-αbergamotene. So Kesler & Baldwin (2001) concluded that VOCs attracted predators and reduced herbivore loads.

Question 21

An easy meal

Answer 21

VOCs known to attract a range of herbivore enemies, including insects, predatory mites, nematodes and even birds.
Exploitation of VOC cues for prey location likely to bring significant fitness benefits for the predator.

Question 22

The parasitoid conundrum

Answer 22

Parasitoids are also attracted to VOCs - a good way of finding host insects. (some wasp species)
Koinobiont parasitoids allow their host to continue development before emergence As a consequence, the host larvae may be stimulated to eat more plant tissue
How does this benefit the plant ?

Question 23

Is the enemy of my enemy really my friend?

Answer 23

Defence - my definition….
Any morphological, biochemical or structural characteristic
that reduces the likelihood of herbivory or its subsequent
impact on plant fitness.
Despite evidence for VOC-mediated attraction of predators and parasitoids to damaged plants, there is no clear proof that this increases plant fitness and so cant actually be considered a defence.