RHIZOIDS: FUNCTIONS, IN BRYOPHYTES, FUNGI AND ALGAE - BIOLOGY - 2024

The rhizoids are structures that resemble the roots of vascular plants, and perform similar functions to them. They are located in the lower portion of some organisms, such as algae, mosses and fungi.

The presence of rhizoids has also been reported in sea sponges and other simple animals that live anchored to a substrate. The rhizoid is responsible for fixing the vegetative body of the organism to the substrate, and is capable of mediating the absorption of nutrients.

Rhizoids help mosses to anchor themselves in their substrate. Source: pixabay.com

The structure of the rhizoids varies considerably depending on the study group. Some have complex branching, while others are flexible and bulbous.

In fungi, rhizoids are found on the thallus and are very similar to true roots. In mosses, the rhizoids are responsible for anchoring the gametophyte to the substrate, facilitating the absorption of mineral salts and water.

Rhizoids are key elements that, thanks to their varied morphology, allow differentiating between species, being an important taxonomic element in fungi.

Rhizoid functions

Currently, there are few studies related to rhizoid function. However, the evidence that we have indicates that rhizoids are particularly important in fixing the body to the substrate and in taking up nutrients from the soil.

Fixing to the substrate

In the literature, the rhizoids are attributed functions of anchoring to the substrate. The rhizoids of several liverworts form a kind of discs or branches at the ends of the structure that, when in contact with solid particles, adhere strongly.

A similar pattern of branching and sticking has been evidenced at the tips of rhizoids in mosses and some ferns when they come into contact with hard surfaces.

Other facts support this fixing role. In some mosses, rhizoids are more abundant and much more branched in individuals that grow on hard substances (such as rocks, for example), compared to those that grow in soils.

Water and nutrient intake

Rhizoids have also been attributed functions related to the uptake and transport of water. Many bryophytes are ectohydric - they lack a thick cuticle and absorb water over the entire surface of the body.

In these species, rhizoids are not necessary for direct water intake, but several of these mosses produce a thick layer of rhizoids.

Other species of bryophytes are endohydric and have an internal water transport system. In these species, the rhizoids have a fundamental function and mediate the entire water intake.

In some species of algae, it has been detected that rhizoids act directly in the uptake of inorganic nutrients. In these species, the rhizoids exhibit a concentration of minerals greater than the concentration of the water in which they are immersed.

Rhizoids in Bryophytes

Bryophytes include a series of terrestrial plants that are found in humid environments. Some can occupy aquatic habitats, while others can develop as epiphytic plants.

Regarding its external characteristics, the gametophyte of the bryophytes can end up in different size ranges, from a few millimeters to almost 50 centimeters.

Some thalli are called foliose thalli and these differ in a main axis with "leaves" and the opposite portion with rhizoids.

This terminology is attributed to the organs of these plants because they are of the non-vascular type, that is, they do not have the vascular system of higher plants. Therefore, it is not correct to use the terms leaves and roots, properly speaking.

The rhizoids of bryophytes can be unicellular or multicellular. Those of the unicellular type can present smooth walls or be tuberculated. The latter have plug-shaped invaginations.

Both types of rhizoids are located in the crown region of the ventral surface of the thallus. With the exception of the genus Anthoceros and other thaloid bryophytes, whose smooth-walled rhizoids are scattered throughout the ventral surface.

Mosses, on the other hand, are characterized by having multicellular rhizoids with oblique septa.

Rhizoids in fungi

In the kingdom of fungi, rhizoids appear in the forms that grow attached to a substrate and need some structure to assist their anchoring. These extensions can be of the simple or branched type.

In fungi, rhizoids are filamentous substrate-fixing organs that differ from hyphae in two main characteristics.

First, they decrease in size as the branching process occurs. In this way, each successive branch is smaller than the branch that gave rise to it. As a result of this process, an extended branched system is obtained.

Second, in contrast to cells of the thallus, those of the rhizoid lack a nucleus. These characteristics explain why rhizoids have limited capacity for growth.

Rhizoids in algae

Rhizoids develop in the haploid phase of some algae, such as the genera Chara and Spirogyra. As in the previous groups, the rhizoid can be of the unicellular or multicellular type, and this characteristic depends on the group.

For example, in Zygnematales the rhizoids are of the unicellular type. This group of algae are also called Conjugales. They are green algae that inhabit bodies of fresh water, they are characterized by branching filaments and relatively thick cells.

In contrast, in the Charales group of macroalgae - their development exceeds 60 centimeters in length in many cases - the rhizoids are multicellular. Like Zygnematales algae, caral algae are green and freshwater. From a phylogenetic point of view, they are considered close to terrestrial plants.

Rhizoids versus root hairs

There is an obvious resemblance between the rhizoids and the root hairs of vascular plants. Although they perform similar functions, it is argued that both organs are analogous and not homologous, since there is no correspondence between their structures and they are produced in different phases of the life cycle.

It is possible that the similarity between the rhizoids and the root hairs is the result of a convergent evolutionary process.

References

1. Curtis, H., & Schnek, A. (2006). Invitation to Biology. Panamerican Medical Ed.
2. Griffin, DH (1996). Fungal physiology. John Wiley & Sons.
3. Jones, VA, & Dolan, L. (2012). The evolution of root hairs and rhizoids. Annals of botany, 110 (2), 205-212.
4. Moore, R., Storey, R., & Uno, G. (2001). Principles of botany. McGraw-Hill.
5. Newton, AE, & Tangney, RS (2007). Pleurocarpous mosses: systematics and evolution. CRC Press.