Vachellia collinsii

Vachellia collinsii
V. collinsii in Guanacaste, Costa Rica.
Scientific classification
Kingdom:	Plantae
Clade:	Tracheophytes
Clade:	Angiosperms
Clade:	Eudicots
Clade:	Rosids
Order:	Fabales
Family:	Fabaceae
Subfamily:	Caesalpinioideae
Clade:	Mimosoid clade
Genus:	Vachellia
Species:	V. collinsii
Binomial name
Vachellia collinsii (Saff.) Seigler & Ebinger
Synonyms
Acacia collinsii Saff.

Vachellia collinsii is a species of flowering plant native to Central America and parts of Africa. It grows in secondary succession in seasonally dry ecosystems in southern Central America, with preferences to Savannah-like climate. The Vachellia collinsii, also known as acacia collinsii, can grow upwards of 40 feet tall. The tree grows relatively straight with thorns generously distributed across the branches. The small, pinnate leaves grow opposite from each other similar to a Mimosa. Acacia’s like full sun and most likely would not be found among the trophic understory common to that of jungles. Acacia’s can thrive in higher humidities, such as above 70% humidity. The Acacia collinsii has a wide distribution across the world compared to other acacias as well as a wide ecological distribution as it can grow at sea level to 1000 meters elevation. Since this species has a diverse geological and ecological distribution, it has a wider range of morphological traits. Nonetheless, differences between other ant-acacias can be seen through elongated cylindrical inflorescences, somewhat small stipular spines, terete—cylindrical—spines that wrap around a cross section, 3-5 petiolar glands commonly dome-shaped, a lack of rachis glands, and leaflets with lateral veins.

Most trees typically have auxin that grow downward, suppressing branch growth on the sides. From top to bottom of the tree, there is less suppression of lateral growth, thus, allowing for more and fuller branches near the base of the tree creating a pyramid shape with a strong central trunk. This is not the case for the Vachellia Collinsii as it lacks a strong apical meristem resulting in branching throughout the entire length of the tree. There are green bumps called extrafloral nectaries that make sugar for the ants. They are extrafloral because the actual yellow flowers don't produce nectar and produce a different kind of nectar. When the plant is starved, they begin to produce Beltian bodies suggesting a feedback loop. Ants protect the plant typically against herbivores. If the plant has enough nutrients, it doesn't need to make as many Beltian bodies.

It exhibits a symbiotic relationship with several species of ant (Pseudomyrmex spinicola, Pseudomyrmex ferruginea). The ant-Vachellia system involving this species has been studied by ecologists like Daniel Janzen in Palo Verde National Park and Santa Rosa National Park, both in Guanacaste Province, Costa Rica. The ants chew holes in the tips of the hollow stipular thorns, known as domatia, so that they can enter, and create their colony inside. A single ant colony may span several V. collinsii trees. Medium sized herbivores are deterred by the thorns but the ants help protect the trees from attack by other smaller or larger animals such as caterpillars and elephants. When a predator brushes and shakes the plant’s thorns in an attempt to feed, the ants will become disturbed and run outside and release alarm pheromones to alert other ants. All ants that come in contact with the volatile become aggressive and attack the animal by biting and stinging. Some ant species even cut down vegetation on the ground surrounding their trees and trim the encroaching branches of other plants, which allows the V. collinsii trees to thrive. In exchange, V. collinsii not only provides the ants with hollow thorns in which to live, but also produces lipid- and protein-rich food bodies, known as Beltian bodies, on the tips of new leaflets, which are consumed by the ants and are critical for larval growth. If the acacia has enough nutrients, it does not produce as many beltian bodies which suggests the presence of a feedback loop. Vachellia collinsii also provides the ants with sugar-rich nectar from extrafloral nectaries located at the leaf petiole. Since there are several species of ants that may occupy an acacia collinsii, there have been observations of intraspecific interactions between these species of ants, especially between Pseudomyrmex spinicola and Crematogaster brevispinosa where C. brevispinosa may take over trees occupied by P spinicol. C brevispinosa has also been seen to invade trees occupied by P. Nigrocienta. Ants such as the Crematogaster brevispinosa will occupy trees that are dying or heavily damaged and many others will occupy trees that were previously inhabited. Furthermore, a hypothesized benefit to hosting ant colonies is that the acacia may have receptors within the domatia for ant feces that triggers absorption pathways for additional nutrient uptake at the extremities of the plant's stem tissue. When ants defecate in the domatia, their feces contain nutrients from their food which could be good for the plant. This allows the plant to obtain additional nutrients and in a shorter period of time as the nutrients do not need to travel all the way up starting from the roots.

Diving deeper into why the acacia collinsii produce traits of the “swollen thorn syndrome,” the mechanism and pathways are still unknown but there have been experiments and strong evidence related to a change in gene expression of miR156/miR157 and SPL transcription factors, in different environmental conditions. The production of food bodies that are high in proteins and lipids as well as extrafloral nectaries is very costly so the plant must have some indication of when to start producing those traits. Generally, an acacia will not produce these traits immediately after germinating so it is age dependent and the extrafloral nectaries will be produced first around 50–75 days of age, then swollen and elongated stipules, followed by full beltian bodies. Along with the production of these traits, are declines in miR156/miR157 genes. The miR156-SPL pathway has been known in many plants to coordinate when the plant flowers as well as plant development combined with stress tolerance. In an Arabidopsis, the miR156 will keep the plant juvenile, then become suppressed when it is in the right conditions in order to further develop adult traits. When put in low light conditions, there is higher miR156/miR157 as well as a delay in swollen thorn syndrome traits. The other way around is also true in that when put in well lit conditions such as the natural environment, there is a low expression of miR156/miR157 genes when the plant is producing extrafloral nectaries, swollen stipules, and beltian bodies. Although it is unknown how the expression of the traits are linked to the miR156/miR157 genes, hypotheses include temporal coordination or regulation where the miR156/157 turn on and off in a pattern in order to turn on those genes for the swollen thorn syndrome(which are still unknown). There is also evidence that these traits are part of their defense mechanism and that nectar secretion from the extrafloral nectaries depend on jasmonic acid but the mechanism is unknown as jasmonic acid could have a different function here than in a typical plant. The developmental timing of the “Swollen Thorn Syndrome” can also be influenced by natural selection as well. More research needs to be done on what developmental constraints and factors that may have influenced the later development of these traits.

Vachellia collinsii Thorns (domatia)

References

1. Cui LG, Shan JX, Shi M, Gao JP, Lin HX. The miR156-SPL9-DFR pathway coordinates the relationship between development and abiotic stress tolerance in plants. Plant J. 2014 Dec;80(6):1108-17. doi: 10.1111/tpj.12712. Epub 2014 Nov 20. Erratum in: Plant J. 2015 Jun;82(5):901. PMID 25345491.
2. Ebinger, John and Seigler, David S. , 1995, “Taxonomic Revision of the Ant-Acacias (Fabaceae, Mimosoideae, Acacia, Series Gummiferae) of the New World'', Annals of the Missouri Botanical Garden 82, pp. 117-138: 123-124
3. Ewing, Doug. UW Greenhouse (Redmond Location) Excursion Communication, 23 May 2021.
4. Janzen, Daniel H. (1966). Coevolution of Mutualism Between Ants and Acacias in Central America. Evolution, 20(3), 249-275. doi:10.2307/2406628
5. Leichty, Aaron R, and R Scott Poethig. “Development and evolution of age-dependent defenses in ant-acacias.” Proceedings of the National Academy of Sciences of the United States of America vol. 116,31 (2019): 15596-15601. doi:10.1073/pnas.1900644116
6. Paiva, É. A., Ballego‐Campos, I., & Gibernau, M. (2020). True nectar or stigmatic secretion? Structural evidence elucidates an old controversy regarding nectaries in Anthurium. American Journal of Botany, 108(1), 37–50. https://doi.org/10.1002/ajb2.1595
7. Suarez, Andrew V., et al. “Defense of Acacia Collinsii by an Obligate and Non Obligate Ant Species: The Significance of Encroaching Vegetation.” Biotropica, vol. 30, no. 3, 1998, pp. 480–482. JSTOR, www.jstor.org/stable/2389133. Accessed 24 May 2021.