Plants and animals can also live in marriage

When we think about the life of plants it is difficult to imagine without interaction with the animals, as they establish different symbiotic relationships day after day. These symbiotic relationships include all the herbivores, or in the contradictory way, all the carnivorous plants. But there are many other super important interactions between plants and animals, such as the relationships that allow them to help each other and to live together. So, this time I want to present mutualism between plants and animals.

And, what is mutualism? it is the relationship established between two organisms in which both benefit from living together, i.e., the two get a reward when they live with the other. This relationship increase their biological effectiveness (fitness), so there is a tendency to live always together.

According to this definition, both pollination and seed dispersal by animals are cases of mutualism. Let’s see.

POLLINATION BY ANIMALS

Many plants are visited by animals seeking to feed on nectar, pollen or other sugars they produce in their flowers and, during this process, the animals carry pollen from one flower to others, allowing it reaches the stigma in a very effective way. Thus, the plant gets the benefit of fertilization with a lower cost of pollen production, which would be higher if it was dispersed through the air. And the animals, in exchange, obtain food. Therefore, a true relationship of mutualism is stablished between the two organisms.

 “Video:The Beauty of Pollination” – Super Soul Sunday – Oprah Winfrey Network (www.youtube.com)

The extreme mutualism occurs when the species evolve depending on the other organism, i.e., when there is coevolution. We define the coevolution such as these evolutionary adaptations that allow two or more organisms to establish a deep relationship of symbiosis, due that the evolutionary adaptations of one specie influence the evolutionary adaptations of another organism. For example, this occurs between various orchids and their pollinators, as is the well- known case of Darwin’s orchid. But there are many other plants that also have co-evolved with their pollinators, as a fig tree or cassava.

In no way, this should be confused with the trickery produced by some plants to their pollinators, that is, when they do not obtain any direct benefit. For example, some orchids can attract their pollinators through odours (pheromones) and their curious forms that resemble female pollinator, stimulating them to visit their flowers. The pollinators will be impregnated with pollen, which will be transported to other flowers due to the same trickery.

Bee orchid (Ophrys apifera) (Autnor: Bernard DUPONT, flickr).

SEED DISPERSAL BY ANIMALS

The origin of seed dispersal by animals probably had occurred thanks to a co-evolutionary process between animals and mechanisms of seed dispersal in which both plants and animals obtain a profit. The most probably is that this process began in the Carboniferous (~ 300MA), as it is believed that some plants like cycads developed a false fleshy fruits that could be consumed by primitive reptiles that would act as seed dispersers. This process could have intensified the diversification of flowering plants (angiosperms), small mammals and birds during the Cretaceous (65-12MA).

The mutualism can occur in two ways within the seed dispersal by animals.

The first case is carried out by animals that eat seeds or fruits. These seeds or some parts of the fruits (diaspores) are expelled without being damaged, by defecation or regurgitation, allowing the seed germination. In this case, diaspores are carriers of rewards or lures that result very attractive to animals. That is the reason why fruits are usually fleshy, sweet and often have bright colours or emit scents to attract them.

For example, the red-eyed wattle (Acacia cyclops) produces seeds with elaiosomes (a very nutritive substance usually made of lipids) that are bigger than the own seed. This suppose an elevated energy cost to the plant, because it doesn’t only have to produce seeds, as it has to generate the award too. But in return, the rose-breasted or galah cockatoo (Eolophus roseicapillus) transports their seeds in long distances. Because when the galah cockatoo eats elaiosomes, it also ingest seeds which will be transported by its flight until they are expelled elsewhere.

On the left,  Galah  cockatoo (Eolophus roseicapillus) (Autnor: Richard Fisher, flickr) ; On the right, red-eyed wattle’s seeds (black) with the elaiosome (pink) ( Acacia cyclops) (Autnor: Sydney Oats, flickr).

And the other type of seed dispersal by animals that establishes a mutualistic relationship occurs when the seeds or fruits are collected by the animal in times of abundance and then are buried as a food storage to be used when needed. As long as not all seed will be eaten, some will be able to germinate.

A squirrel that is recollecting som nuts (Author: William Murphy, flickr)

But this has not finished yet, since there are other curious and less well-known examples that have somehow made that both animals and plants can live together in a perfect “marriage.” Let’s see examples:

Azteca and Cecropia

Plants of the genus Cecropia live in tropical rain forests of Central and South America and they are very big fighters. The strategy that allow them to grow quickly and capture sunlight, avoiding competition with other plants, resides in the strong relationship they have with Azteca ants. Plants provide nests to the ants, since their stems are normally hollow and with separations, allowing ants to inhabit inside. Furthermore, these plants also produce Müllerian bodies, which are small but very nutritive substances rich in glycogen that ants can eat. In return, the ants protect Cecropia from vines and lianas, allowing them to success as a pioneer plants.

Ant Plants: Cecropia – Azteca Symbiosis (www.youtube.com)

Marcgravia and Bats

Few years ago, an interesting plant has been discovered in Cuba. This plant is pollinated by bats, and it has evolved giving rise to modified leaves that act as satellite dish for echolocation performed by these animals. That is, their shape allow bats to locate them quickly, so they can collect nectar more efficiently. And at the same time, bats also pollinate plants more efficiently, as these animals move very quickly each night to visit hundreds of flowers to feed.

Marcgravia (Author: Alex Popovkin, Bahia, Brazil, Flickr)

In general, we see that the life of plants depends largely on the life of animals, since they are connected in one way or another. All the interactions we have presented are part of an even larger set that make life a more complex and peculiar one, in which one’s life cannot be explained without the other’s life. For this reason, we can say that life of some animals and some plants resembles a marriage.

REFERENCES

• Notes from the Environmental Biology degree (Universitat Autònoma de Barcelona) and the Master’s degree in Biodiversity (Universitat de Barcelona).
• Bascompte, J. & Jordano, P. (2013) Mutualistic Networks (Chapter 1. Biodiversity and Plant-Animal Coevolution). Princeton University Press, pp 224.
• Dansereau, P. (1957): Biogeography: an Ecological Perspective. The Ronald Press, New York., pp. 394.
• Fenner M. & Thompson K. (2005). The Ecology of seeds. Cambridge: Cambridge University Press, 2005. pp. 250.
• Font Quer, P. (1953): Diccionario de Botánica. Editorial Labor, Barcelona.
• Izco, J., Barreno, E., Brugués, M., Costa, M., Devesa, J. A., Fernández, F., Gallardo, T., Llimona, X., Parada, C., Talavera, S. & Valdés, B. (2004) Botánica ªEdición. McGraw-Hill, pp. 906.
• Murray D. R. (2012). Seed dispersal. Academy Press. 322 pp.
• Tiffney B. (2004). Vertebrate dispersal of seed plants through time. Annual Review of Ecology, Evolution and Systematics. 35:1-29.
• Willis, K.J. & McElwain, J.C. (2014) The Evolution of Plants (second edition). Oxford University Press, pp. 424.
• National Geographic (2011). Bats Drawn to Plant via “Echo Beacon”. http://news.nationalgeographic.com/news/2011/07/110728-plants-bats-sonar-pollination-animals-environment/

Communication among plants: allelopathy

As always have been said, plants are unable to speak. But, even if they don’t speak, this does not mean they do not communicate with each other. Relatively few years ago, during the period from 1930 to 1940, it was discovered that plants also transmit certain stimuli to others. But, what kind of communication exist among them? What are their words and how are pronounced? And what involves this interaction?

INTRODUCTION

In 1937, Molisch introduced the term allelopathy referring to the two Latin words “Allelon” and “Pathos”, which mean “another” and “suffering”, respectively. But, the actual meaning of the word was determined by Rice in 1984. Allelopathy now means any effect that a plant transmits to another directly or indirectly through production of different metabolism compounds, causing either a positive or negative effect on the other organism. These compounds are called allelochemicals.

The allelochemicals are released on the environment by plants. But, they are not directly aimed to the action site, thus it is a passive mechanism. To be effective, allelopathic interaction needs that these substances are distributed along the ground or the air and that they reach the other plant. Once inside the recipient plant, this one may have defense and degradation mechanisms of the compounds while avoiding the effect, or conversely, it will suffer a pathological effect.

Allelopathy (Adapted image of OpenClips)

ROUTES OF RELEASE

The release of allelochemicals can be 4 main ways:

• Leaching: the aerial part of the plant lets go substances by rain effect. Then, they can fall on other plants or on the ground. Therefore, it can be direct or indirect effect, depending on whether they falls on another plant or not. Although, in principle, it is considered indirect.
• Decomposition: the plants drop their leftovers on the ground, where they decomposed under the microorganisms action, which help the release of the compounds. The plant leftovers range from leaves to branches or roots. The substances found there may be inactive until coming into contact with moisture or microorganisms, or can be active and then be inactivated by the microorganisms activity or by being retained on the ground. So, it is an indirect way. The decomposition is very important because the most of allelochemicals are released this way.
• Volatilization: the substances are released by the stomata (structures that allow the exchange of gas and transpiration). These are volatile and water-soluble, thus can be absorbed by other plant’s stomata or be dissolved in water. Commonly, plants using these pathways occur in temperate and warm climates. It is considered a direct route.
• Exudation: the plants can also release allelochemicals directly by live roots. The exudation system depends especially of roots state, of the kind of roots and of their growing level (if they are growing or not).

The 4 main pathways of allelochemical releasing: volatilization (V), leaching (L), descomposition (D) and root exudation (E). (Adapted image of OpenClips)

REGULATORY FACTORS

Factors influencing the release of allelochemicals are normally abiotic, such as high radiation, low humidity, unsuitable pH, ultraviolet light, temperature, nutrient deficiency, pollution or contamination (including pesticides ). The higher is the stress caused by this factors to the plant, highest is the allelochemicals amount released from secondary metabolic routes.

• This is important for research and pharmacy: for generating relevant oils many plants are grown under stressful conditions, as it is thanks to the production of these secondary metabolites that they can survive.

Furthermore, biotic factors also take part, such as insects, herbivores or competition with other plant species. These activate the plant defenses and then the organism is stimulated to secrete bitter substances, or substances that harden the tissues, that are toxic or give off unpleasant odors, etc.

Finally, each plant has its own genome and this makes synthesize those or other substances. But, they are also determined by the phenology (life stages) and the development (if the size of the plant is bigger, it can release more allelochemicals).

ACTION MODE

The allelochemicals are very diverse and, therefore, it’s difficult to establish a general action model; since it depends on the compound type, the receiving plants and how it acts.

When we talk about how the allelochemicals can act at internal level, there is a large number of physiological parameters that can be affected. They have action on the cellular membrane, disrupt the activity of different enzymes or structural proteins or alter hormonal balance. They can also inhibit or reduce cellular respiration and chlorophyll synthesis, leading to a reduction in vitality, growth and overall development of the plant. Furthermore, these substances can also reduce seed germination or seedling development, or affect cell division, pollen germination, etc.

On the other hand, at external level, the allelochemicals may be related to the release or limitation of nutrients that are found in the soil. Others act on microorganisms, leading to a perturbation on the symbiotic relationships they establish. In addition, these substances have great importance into the generations succession, as they determine certain competition tendencies and also act on the habitat ecology. Even so, it is a successive competition, as they do not directly compete to obtain the main resources.

EXAMPLES

One of the best known allelochemicals is the juglone, produced by the Eastern black walnut (Juglans nigra). Juglone, once released to soil, can inhibit the other plants growth around the tree. This allows the issuing organism to get more resources, avoiding competition.

Eastern black walnut  (Juglans nigra) (Photo taken by Hans Braxmeier)

A very curious case is that of the acacias (Acacia). These plants synthesize a toxic alkaloid that migrates to the leaves when the body is attacked by a herbivore. This substance’s toxicity is high, because it damages with the contact and ingestion, becoming deadly even for large herbivores.In addition, this alkaloid is volatile and transferred by air to other nearby acacias, acting as an alarm. When the other acacias receive this signal, this component is segregated to leaves, making them darker. Even so, the effect is temporary. This makes animals like giraffes have to constantly move to eat a few leaves of each acacia, and always against the wind, to avoid toxicity.

Acacias (Acacia) (Photo taken by Sarangib)

REFERENCES

• A. Aguilella & F. Puche. 2004. Diccionari de botànica. Col·leció Educació. Material. Universitat de València: pp. 500.

• A. Macías, D. Marín, A. Oliveros-Bastidas, R.M. Varela, A.M. Simonet, C. Carrera & J.M.G. Molinillo. 2003. Alelopathy as a new strategy for sustainable ecosystems development. Biological Sciences in Space 17 (1).
• J. Ferguson, B. Rathinasabapathi & C. A. Chase. 2013. Allelopathy: How plants suppresss other plants. University of Florida, IFAS Extension HS944
• Notes of Phanerogamae, Applied Plant Physiology and Analisi of vegetation, Degree of Environmental Biology, UAB