Tropisms are
directional growth responses to external stimuli. Common tropisms include
phototropism and geotropism—both of which we discussed in biology class.
Phototropism is positive for plant stems and negative for roots—that is, the
stem will lean towards sunlight as the plant grows while the roots will grow
away from the light. The opposite is true for geotropism—growth in response to
gravity—as the stems grow counter to the force of gravity, and the roots grow
in the direction gravity is acting, which is down. These tropisms seem
reasonable enough. Plants use sunlight for photosynthesis so it makes sense
that they would grow towards it in order to maximize the amount of sunlight
they are exposed to. But what of parasitic plants without the ability to
photosynthesize?
Cuscuta
pentagona—an orange
vine with small white flowers—lacks chlorophyll and therefore is unable to
convert light energy into chemical energy and make its own food. It would not
benefit from phototropism, which leads plants to the light energy source they
need to produce sustenance. Sure, the Cuscuta grows its roots into the
dirt and its vines upward, like other plants. However, it attains the nutrients
it needs by attaching itself to a host plant and sending “microprojections”
into the tomato’s phloem - through which the plant transports sugars (mostly
sucrose, but also glucose), amino acids, plant hormones, and mRNA—to siphon off
the host plant’s photosynthesized sugar supplies. Without a host to live off
of, a young Cuscuta plant will inevitably die. To prevent this from
happening, the Cuscuta has a plant-detecting mechanism by which it
locates the target host plant it will attack through smell.
Cuscuta
pentagona commonly
latches onto tomato plants. As a Cuscuta pentagona seedling grows, it
probes its surroundings, growing and rotating its shoot tip until it finds a
tomato (or other) plant in its vicinity, then wraps around this new host’s stem
in order to have access to the plant’s phloem. Consuelo De Moraes hypothesized
that the Cuscuta or dodder plant detected its host through “chemical
signaling.” She noticed that, despite variable conditions—light or shade, in
the presence of other plants (specifically, wheat), empty pots, or pots with fake plants—dodder vines always grew
towards the tomato plants! To test her hypothesis, she synthesized a tomato
perfume, or “eau de tomato”, with extracts from the stem of a tomato plant. She
then soaked cotton swabs with this perfume and placed them on sticks in a pot
next to the dodder. As she hypothesized, the dodder indeed grew towards the
cotton that gave off the scent of a tomato plant. Dodders grow towards tomato
plants because tomato plants contain beta-myrcene, in addition to two other
chemicals. All three of these chemicals are easily turned into gases and give
off odors which attract dodders.
This is not
the only case in which non-tactile plant communication occurs. University of
Washington scientists David Rhoades and Gordon Orians, observed that willow
trees neighboring those that were plagued by caterpillars were less likely to
be attacked by caterpillars. Upon investigation, Rhoades found that the leaves
of trees found next to, but not
touching, the infested willows contained phenolic and tannin—chemicals that
repel the insects which feed on them. These chemicals were absent in healthy
willows which were not surrounded by caterpillar-plagued willows. Since neither
the roots nor branches of the damaged and healthy trees had contact with each
other, Rhoades proposed that, via pheromones, the infested trees sent a warning
message to healthy trees so that they could defend themselves against the
impending insect attack.
Plants
aren’t as altruistic as they seem, however, as Martin Heil, a Mexican
scientist, found. Heil posited that the infected plants didn’t intend on
warning their neighbors at all—the pheromones released by damaged leaves were
directed at the remaining healthy leaves of the infested plant. Neighboring
plants simply detected these pheromones and reacted to protect themselves. To test
his hypothesis, Heil isolated damaged leaves in a sealed plastic bag to
determine whether this disrupts pheromone communication. As he had suspected, the
damaged leaves were unable to transmit their warning message to healthy leaves
on the same plant, and these healthy leaves remained the same. Heil concluded
that the chemicals released by damaged leaves are necessary for the rest of the
plant to defend itself from further attacks.
Of course,
plants don’t smell in the same way that we do—with nerves that send signals to
our brain and tell us of what scents are around us—but they are most
definitely capable of detecting and responding to chemicals in the air around
them. This makes a lot of sense. We all know that plants, flowers especially,
give off odors and aromas which attract pollinators and vectors, like nocturnal
animals, for seed transport. But, just as humans speak and can hear, plants
give off odors and can detect scents.
I found this
really eye-opening and intriguing. It shows that we can still discover new
things about even seemingly simple organisms with which we are familiar. Although we don’t have enough information to
conclude definitively the nature of pheromone communication, Heil’s experiment
doesn’t rule out the existence of a warning system. The chemicals released by damaged leaves may
be necessary for the survival of the healthy leaves, but this fact alone
doesn’t preclude the intention of the infected plant to warn neighboring plants
of its plight. It would be interesting
to explore the details of pheromone communication, how it varies, if at all,
among different plant species. Pheromones could be the key to giving us a
better understanding of plant communication. Another thing I found ironic is
that humans, although thought to be one of the most complex organisms with
conscious thought, aren’t capable of understanding the pheromones we release,
whereas plants can. Maybe we aren’t as superior to other organisms as some
might think.
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