2023-06-16 19:00:00
The world of senses It is extensively studied in animals. Apart from the human senses —which are more than five—, we recognize senses in other animals that are totally foreign to us: some perceive radiation outside the visible spectrum, like bees with ultraviolet, or snakes with respect to infrared; sharks sense changes in the electromagnetic field; the fire beetle perceives a fire at a long distance; octopuses and cuttlefish can see polarized light; ants perceive and recognize pheromone trails; and bats can ‘hear’ textures.
Clearly, we accept that many animals have senses, similar to or very different from ours. But out of the animal kingdom We find living beings that lack a nervous system. And therefore, from the anthropocentric conception of what is considered ‘senses’, these living beings cannot have them.
But if we understand the term ‘sense’ as the physiological process that involves the reception and recognition of stimuli —external or internal—, and the generation of appropriate responses, then it is not unreasonable to think that living organisms without brains may have them. After all, receiving stimuli and responding to them is a property of living beings. And plants are no strangers.
No brain, but with phytohormones
It is evident that plants have no nervous system, but it does not mean that they do not perceive or react to events that happen around them or to themselves. Plants have a very limited ability to move; they cannot flee from their predators, and therefore, they have developed another type of evolutionary strategies for their defense. One of the most peculiar is related to biochemistry.
In addition to primary metabolism, —the one that allows living beings to stay alive—, plants have a varied and very complex secondary metabolism. Evolution has endowed plants with amazing abilities when it comes to chemical synthesis —metabolites– what they are practically natural laboratories. And they use those metabolites for many functions.
Instead of neurons and nerve impulses, plants use signaling molecules to give instructions from one cell to another. Are the phytohormones, which control all aspects of plant growth and development, defense against pathogens and herbivores, or stress tolerance. Unlike animals, in which hormones are produced exclusively in certain tissues —glands—, all cells in a plant can produce phytohormones.
There are several types of phytohormones: cytokines, brassinosteroids, gibberellins, sallicylate, ethylene, strigolactones or abscisic acid. Each one has its behavior, its properties and its functions. And among them, the senses of plants.
Phototropism, growth toward light. -Pat_Hastings(/iStock
Tropisms: growing in the right direction
A common experiment to check the ability of plants to perceive stimuli and respond appropriately is to germinate a bean or a lentil, and once the seedling develops its first leaves and its first root, change your position. It doesn’t matter how she’s oriented, or if the ground is tilted, the root always points downward. And it does not matter in which direction the position of the seedling is changed by reorienting the root in another direction, it won’t take long to curve to head back down.
This movement is called gravitropismo, growth in favor of gravity; a mechanism mediated by phytohormones, and specifically, by the auxins. These phytohormones are synthesized at the stem apex and are distributed throughout the plant, being transported from cell to cell, according to a gravitational gradient. In general, auxins regulate growth differently depending on the tissue on which they act. In the case of the root, a higher concentration of auxins reduces the elongation of the cells. In this way, when the root is oriented downwards, following gravity, the distribution of auxins is homogeneous, and growth is uniform. But if the root is tilted, the side facing down accumulates more auxins, inhibiting its elongation; the upper side then grows faster, and the root redirects itself until it finds its way in favor of gravity.
Another simple experiment is place the newly germinated seedling in an opaque box with a hole that allows light to enter. No matter how the plant is placed indoors, which will grow into the hole. In fact, if the experiment is complicated by making a maze inside the box, and the plant is placed at the opposite end of the hole where the light enters, the stem will solve the mazethrough growth, without error.
This growth directed toward the light source is called phototropismand it is also regulated by auxins, due to the effect of proteins called phototropins, which allow or prevent the diffusion of auxins depending on the light. When the light falls on the apex of the stem directly, the auxin is distributed evenly, and the stem grows evenly; however, when light falls from one side, phototropins favor an uneven distribution of auxin, which tends to accumulate on the shadow side.
In the stem, auxins have the opposite effect than in the root.: favor cell elongation. Thus the shaded side grows taller, and thus curves the stem towards the light. Plants’ perception of light is so sensitive that they can distinguish minute differences in intensity. That is why they can solve these mazes; the light bounces off the walls and the plant always chooses the brightest path.
Chemical detection
One of the most fascinating senses of plants is the chemoception or ability to perceive chemical substances, analogous to our sense of taste or smell. In this case, the plants use certain phytohormones of a volatile or semi-volatile nature, such as terpenes.
These biomolecules are synthesized and released by plants when faced with certain stimuli, such as aggression or the presence of pathogens, and diffuse them into the atmosphere, transporting them through the air to be received by neighboring plants.
At the receiving plant, these molecules sensitize different signaling pathways and trigger defensive or immune responses with which to anticipate attacks. It is, therefore, a kind of alarm signal that is activated from some plants to others, and constitutes one of the most curious forms of communication between living beings.
Carnivorous plants have ‘touch’. – Markgoddard(/iStock
the touch of plants
Plants also enjoy excellent sense of touch. In some cases, touch triggers rapid but temporary and reversible movements. They are the ones found in some carnivorous plants, which close their leaves as if it were a trap with which to lock up insects, or that of the mimosa, which folds its leaves before contact, to protect them. This movement is called nastiaand is produced by sudden changes in the turgor of some cells of the leaf.
But in addition, plants are capable of recognizing tactile signals from a solid surface, and grow accordingly. This phenomenon is called thigmotropismoand it has two aspects, the positive thigmotropism, which translates into a growth in favor of that surface, and frequently, a strong grip on it, using it as a support regardless of its shape or inclination —a very evident characteristic among climbers—; and the negative thigmotropismwhich occurs in the roots, and seeks to avoid contact with those surfaces, for example, dodging rocks that it could not cross in its growth, and looking for cracks or recesses through which to grow.
Positive thigmotropism, growth in favor of solid surfaces. – Martin Barraud(/iStock
The process by which plants recognize tactile stimuli is not well known, but a recent publication in the scientific journal Nature Plants It has opened the doors of knowledge a little more. According to the results of the experiments carried out by Alexander H. Howell and colleagues, from Washington State University in Pullman, United States, plant cells respond to tactile stimuli sending calcium signals to its neighboring cells, in a kind of waves, slow but constant, that are transmitted through the plant. It is surprising to researchers how finely sensitive plant cells can be in discriminating when something is touching them and when it is not.
One more event, which continues to surprise researchers, in living beings that are very capable of perceiving the environment that surrounds them, without the need for a nervous system.
References:
- Folta, K. M. et al. 2001. Unexpected roles for cryptochrome 2 and phototropin revealed by high-resolution analysis of blue light-mediated hypocotyl growth inhibition. The Plant Journal, 26(5), 471-478. DOI: 10.1046/j.1365-313x.2001.01038.x
- Howell, A. H. et al. 2023. Pavement cells distinguish touch from letting go. Nature Plants, 1-6. DOI: 10.1038/s41477-023-01418-9
- Ku, Y.-S. et al. 2018. Plant Hormone Signaling Crosstalks between Biotic and Abiotic Stress Responses. International Journal of Molecular Sciences, 19(10), 3206. DOI: 10.3390/ijms19103206
- Rosenkranz, M. et al. 2021. Volatile terpenes – mediators of plant-to-plant communication. The Plant Journal, 108(3), 617-631. DOI: 10.1111/tpj.15453
- Sharif, R. et al. 2022. Hormonal interactions underlying parthenocarpic fruit formation in horticultural crops. Horticulture Research, 9uhab024. DOI: 10.1093/hr/uhab024
- Ullah, A. et al. 2018. Phytohormones enhanced drought tolerance in plants: a coping strategy. Environmental Science and Pollution Research, 25(33), 33103-33118. DOI: 10.1007/s11356-018-3364-5
#science #perception #plants #senses