Desmidiales Classification Essay
Desmidiales, commonly called Desmids (Gr.desmos, bond or chain), are an order in the Charophyta, a division of green algae that forms a sister group to the land plants (Embryophyta).
The desmids belong to the class Zygnematophyceae. Although they are sometimes grouped together as a single family Desmidiaceae, most classifications recognize three to five families, either within the order Zygnematales or as their own order Desmidiales.
The Desmidiales comprise around 40 genera and 5,000 to 6,000species, found mostly but not exclusively in fresh water. Many species may be found in the fissures between patches of sphagnum moss in marshes. With a pH level of approximately 4.0, sphagnum peat provides the ideal environment for this flora.
The structure of these algae is unicellular, and lacks flagella. Although most desmids are unicellular, the species Desmidium swartzii forms chains of cells resembling the algae genus Spirogyra. However, these filaments are arranged in a helix pattern.
The cell of a desmid is often divided into two symmetricalcompartments separated by a narrow bridge or isthmus, wherein the spherical nucleus is located. Each semi-cell houses a large, often folded chloroplast for photosynthesizing. One or more pyrenoids can be found. These form carbohydrates for energy storage. The cell-wall, of two halves (termed semicells), which, in a few species of Closterium and Penium, are of more than one piece, has two distinct layers, the inner composed mainly of cellulose, the outer is stronger and thicker, often furnished with spines, granules, warts et cetera. It is made up of a base of cellulose impregnated with other substances including iron compounds, which are especially prominent in some species of Closterium and Penium and is not soluble in an ammoniacal solution of copper oxide.
Desmids assume a variety of highly symmetrical and generally attractive shapes, among those elongated, star-shaped and rotund configurations, which provide the basis for their classification. The largest among them may be visible to the unaided eye.
Desmids possess characteristic crystals of barium sulphate at either end of the cell which exhibit a continuous Brownian type motion.
Many desmids also secrete translucent, gelatinous mucilage from pores in the cell wall that acts a protecting agent. These pores are either, as in Micrasterias, uniformly distributed across the cell-wall but always appear to be absent in the region of the isthmus, or, in highly ornamented forms, as many genera of Cosmarium, grouped symmetrically around the bases of the spines, warts and so on with which the cell is provided.
In the inner layer of the wall the pore is a simple canal, but in the outer, except in Closterium, the canal is surrounded by a specially differentiated cylindrical zone, not composed of cellulose, through which the canal passes. This is termed the pore-organ. The canals are no doubt in all cases occupied by threads of mucilage in process of excretion. At the inner surface of the wall they terminate in lens- or button-shaped swellings, while from the outer end of the pore-organ there sometimes arise delicate radiating or club-shaped masses of mucilage through which the canal passes and which appear to be more or less permanent in character. In most cases, however, these are absent or only represented by small perforated buttons.
Desmids commonly reproduce by asexual fission, however, in adverse conditions, Desmidiales may reproduce sexually, through a process of conjugation, which are also found among the closely related Zygnematales.
Classification of the families and genera in the Desmidiales:
The family Gonatozygaceae is sometimes included within the Peniaceae, reducing the number of families from four to three. A fifth family Mesotaeniaceae was formerly included in the Desmidiales, but analysis of cell wall structure and DNA sequences show that the group is more closely related to the Zygnemataceae, and so is now placed together with that family in the order Zygnematales.
- Survey of Clare Island 1990 - 2005, noting the Desmidiales recorded. Ed. Guiry, M.D., John, D.M., Rindi, F. and McCarthy, T.K. 2007. New Survey of Clare Island. Volume 6: The Freshwater and Terrestrial Algae. Royal Irish Academy. ISBN 978-1-904890-31-7
- ^Gontcharov AA, Marin BA, Melkonian MA (January 2003). "Molecular phylogeny of conjugating green algae (Zygnemophyceae, Streptophyta) inferred from SSU rDNA sequence comparisons". J. Mol. Evol. 56 (1): 89–104. doi:10.1007/s00239-002-2383-4. PMID 12569426.
- ^Kanetsuna, Y. (2002). "New and interesting desmids (Zygnematales, Chlorophyceae) collected from Asia". Phycological Research. 50 (2): 101–113. doi:10.1046/j.1440-1835.2002.00263.x.
- ^"Zygnematales". World Register of Marine Species. Retrieved 10 September 2017.
- ^ abcBrook, A. J.; Williamson, D. B. (2010). A Monograph on Some British Desmids. London: The Ray Society. ISBN 0-903874-42-3.
- ^Van den Hoek, C., D. G. Mann, & H. M. Jahns, 1995. Algae:An Introduction to Phycology, page 468. (Cambridge: Cambridge University Press). ISBN 0-521-30419-9
- ^Brook, Alan J., 1981. The Biology of Desmids, page 1. (Berkeley: University of California Press). ISBN 0-520-04281-6
- ^ abhttp://www.microscopy-uk.org.uk/mag/indexmag.html?http://www.microscopy-uk.org.uk/mag/wimsmall/algdr.html
- ^Proceedings of the Royal Society -Biological Minerals Formed from Strontium and Barium Sulphates. II. Crystallography and Control of Mineral Morphology in Desmids
- ^Kapraun DF (April 2007). "Nuclear DNA content estimates in green algal lineages: chlorophyta and streptophyta". Ann. Bot. 99 (4): 677–701. doi:10.1093/aob/mcl294. PMC 2802934. PMID 17272304.
- ^Guiry, M. D.; Guiry, G. M. (2016). "AlgaeBase". National University of Ireland. Retrieved 2016-09-18.
- ^Speer, B. R. "Desmidiales: The desmids". University of California Museum of Paleontology. Retrieved 10 September 2017.
Auxins are a class of plant hormones with morphogen-like characteristics that regulate the rate of cell division, cell elongation and expansion, ethylene biosynthesis, apical dominance, organ differentiation and some other essential processes in plant growth [1,2].
In the late 19th century, Charles Darwin investigated “the power of movement in plants” . However, it took nearly a century before, in 1926, Fritz Went opened a new field of study by successfully isolating “the plant growth substance” auxin [4,5]. Later he introduced the avena curvature test enabling many experimenters to be active in auxin research [6,7]. Now it is known that auxin is locally synthesized in (land) plants, mainly in shoot tissues, from where it is distributed unevenly throughout the whole plant . Concentration differences of auxin can cause different responses in plant development. During the early 20th century, the traditional “cut-off and stuck-on” tests and later the use of 14C-indoleacetic acid showed that auxin is distributed through the plant by energy-requiring polar transport, with three important characteristics: chemically highly specific, oxygen-requiring and pH dependent .
The most abundant naturally occurring auxin is indole-3-acetic acid (IAA) which shows characteristic “polar transport” throughout the whole plant [9,10,11,12]. Mid 1970s, based on the known properties of auxin movement (saturable, energy- and protein synthesis-dependent and unidirectional), the “chemiosmotic hypothesis” was formulated as a classic model to explain the mechanism of polar auxin transport (PAT) [10,13]. According to this model, IAA can easily enter the cell cytoplasm in an undissociated lipophilic form (IAAH) when in a slightly acidic extracellular environment (pH 5.5). While in the cytosol, with a neutral pH of about 7, most of the IAA will be dissociated into anions (IAA−) and, therefore, trapped inside the cell. To aid the exit of IAA− active auxin anion efflux carriers were proposed, and the asymmetric distribution of such carriers was postulated to attribute to the directionality of auxin transport . This hypothesis later has been proven to be suitable to describe in general auxin transport in land plants .
Although auxins are shown to be present in early-diverged plant lineages such as mosses, liverworts and algae [15,16], research on auxin transport and signaling remains mainly focused on seed plants, predominantly in the model plant Arabidopsis thaliana . However, algae share many specialized characteristics with the land plants while evolutionally earlier and of simpler structures. Hence, they may provide unique features that may help to unravel (new) auxin working mechanisms, auxin transport characteristics and functions. In this paper, we look into auxin as a plant hormone in algae, and specially focus on auxin transport.
2. The Roles of Auxin in Algae
Algae belong to a very large and diverse group of simple, typically autotrophic organisms. Increasingly data appears that all land plants (embryophytes) diverged from ancestral Charophycean algae, a class of green algae, about 400–500 million years ago, Table 1 [18,19]. Table 1 also shows that the red algae and brown algae are more distant from the streptophytes. In this respect, green algae especially members in the class of Charophyceae are promising candidates to study evolutionary aspects, (new) functionalities and cellular physiology of auxin.
Bioassays, high-performance liquid chromatography, mass spectrometry and some other physicochemical analysis, together with other circumstantial evidences, proof the existence of auxin or auxin-like compounds in many species of algae [17,20,21]. The measured concentrations in these studies are highly variable. The presence and action of auxins have been shown both in unicellular and multi-cellular algae [9,17,22,23,24]. For example, in red algae (e.g., Grateloupia dichotoma, Gracilaria vermiculophylla, Agardhiella subulata) and green algae (e.g., Chlorella pyrenoidosa, Micrasterias thomasiana) auxin stimulates cell division and cell enlargement [25,26,27,28,29,30] and affects the development and growth of rhizoids as well [31,32,33]. Although the full-value of plant hormone systems in algae is still under debate, the aforementioned studies about auxins on algal growth and development indicate that its functions likely correspond to its activity in higher land plants [20,34,35]. As studies so far concentrated on land plant corresponding functions other, new and unexpected, roles for auxins in algae may have been overlooked and remain to be discovered.
Table 1. Partial classification of plants and brown algae.
|Zygnematales, Coleochaetales, Charales, |
Desmidiales, Klebsormidiales, Mesostigmatales
(land plants:mosses lato sensu, lycophytes, ferns and horsetails, seed plants)
3. Transport of Auxin in Algae
Auxin acts both as hormone and morphogen. The role of auxin in apical dominance can be regarded as a classical example of hormonal integration based on hormone distribution in land plants. Similar phenomena have also been described in various seaweeds, suggesting that in algae similar auxin distribution systems and carriers supporting PAT may be present [34,36].
The events in auxin signaling as established in seed plants do not only involve the sensing of auxins at the level of the target cells and their responses but also auxin biosynthesis and metabolism, intracellular compartmentalization, and directional transport through cells facilitated by specific transporters [37,38].
Auxin transport in seed plants is characterized by its polarity, directionality, distance, and transporting cells [17,39]. Polar auxin transport (PAT) is facilitated by influx carriers (AUX1/LAX proteins) and efflux carriers (PIN proteins), together with some other transport proteins (ABCB/PGP transporter family) [11,13]. The asymmetric distribution of efflux-carriers, mainly related to the plasma-membrane-localized PINs, aids to the gradient of auxin concentration through the whole plant, while the differences of auxin concentration in turn regulate the number and location of efflux-carriers [14,40,41,42]. The endoplasmic reticulum (ER)-localized PIN proteins (like PIN5, PIN8) are thought to charge the intracellular compartmentalization of auxin and homeostasis, in cooperation with members of the recently-found auxin carrier family PIN-LIKEs (PILS) [43,44,45].
To understand this whole system better, splitting it into several parts and searching them back in the evolutionarily earlier organisms of green or brown algae lineages will be helpful.
In view of some of the apparent morphological similarities between algae and land plants one might ask if the ability of auxin polar transport is required for such differential development? Studies in the large coenocytic (multi-cellular structures without cross walls) green alga Caulerpa paspaloides show different results. Although these algae show characteristics like a leaf-like assimilator which grows up, rhizoid clusters that grow down, and a rhizome that grows horizontally, auxin displays an uniform and non-polar distribution, which might be caused by diffusion and cytoplasmic streaming [22,46,47]. This suggests that auxin polar transport and auxin gradients do not participate in, at least, the later development and maintenance of these three different organs. In multi-cellular algae the existence of PAT has also been investigated. In Chara, a branched, multi-cellular green alga, a specific carrier system is suggested to mediate the transmembrane auxin transport . This carrier lacked the inhibition by 1-N-Naphthylphthalamic acid (NPA, a phytotropin) which is typical for inhibiting efflux carriers in land plants . In the same study, such specific auxin carriers were not found in the simple unicellular green alga Chlorella vulgaris . On the other hand, Klämbt and coworkers reported that in growing rhizoids of Chara, auxin efflux showed NPA-sensitive activation . These contradictory results are thought to be due to some additional effects of NPA, unrelated to its ability to increase intracellular IAA levels by blocking IAA efflux . Recent experiments, showing NPA sensitive polar transport of radioactive labeled auxin, provide more direct proof of the presence of PAT in Chara and suggest the presence of specific auxin efflux carriers as in land plants . In addition, PAT has been shown to exist in brown algae species like Fucus distichus and Ectocarpus siliculosus [31,49].
Comparing studies in unicellular (micro or coenocytic) green algae with multi-cellular green algae (Chara), Dibb-Fuller and Morris proposed that “the appearance of specific auxin carrier systems in the Chlorophyta may have been functionally associated with the evolution of multi-cellularity, rather than with the evolution of a plant body which is characterized by distinctly different morphological regions” . This conclusion seems in accordance with the early hypothesis raised by others, that after the development of multicellular organisms, simple diffusion of IAA would not be efficient enough, the movement across cell membranes is required for polar transport of IAA [8,47,50]. Although the relatively lipid-soluble IAA could account for very slow diffusive transport through membranes, for more effective longer distance transport and gradients in tissue, membrane bound auxin transporters and channels are required.
Since PAT in seed plants is largely dependent on the asymmetric distribution of PIN proteins, the PIN proteins seem a key in the investigation of PAT mechanisms. Though the PILS carrier family is quite conserved throughout the evolution of plants and can be found from unicellular algae to highly developed seed plants, the PIN exporter families at the plasma membrane are comparatively young in evolutionarily perspective [37,51]. The plasma membrane-localized PIN proteins are thought to exist only in land plants, while the endoplasmic reticulum (ER)-localized PIN proteins (like PIN5, PIN8 in Arabidopsis) are evolutionarily older, and can be traced back to an origin in Streptophyta algae .
By using a basic local alignment search tool (BLAST), partial PIN protein sequences can be identified in several species of green algae like Spirogyra, Penium, and the evolutionarily even earlier lineage Klebsormidiophyceae [51,52]. However, the above results are based on the expressed sequence tag (EST) database, since the nuclear genomes sequences database is barely described in the multicellular green algae of the Streptophyta group. The scant annotated genomes are almost all from the unicellar algae of the Chlorophyta group, and there is not yet evidence of the existence of PIN protein sequences [37,51]. With the growing information on nuclear genomes of multicellular green algae groups, a clearer picture is soon expected on the presence of PIN proteins.
Although evidence for functional PIN proteins supporting PAT in algae is (still) scarce this does not rule out the (functional) existence of PAT in algae. In the brown algae Ectocarpus siliculosus PAT has been shown while this species lacks any PIN homolog . The possibility of other types of auxin (polar) transporter mechanisms rather than PIN proteins cannot be excluded. Besides, the presence of plasmodesmata or other active (e.g., vesicle based) transport systems may exist. Researches have shown that there exists a special plasma membrane invagination-structure named charasomes in Chara species, but not in Nitella species. Interestingly, PAT also shows up in Chara corallina, but not in Nitella [48,54,55,56,57]. Charasomes are thought to be closely related to the ability of endocytosis in Chara and the origin of vesicles . Although even if it is possible for auxin to be transported in vesicles, the driving force of these vesicles in these algae species remains a mystery: the block of cytostreaming by cytochalasin H could not stop the auxin transport in Chara while the normal microtubule system seems not fast enough to reach the observed speed of auxin transport. This suggests the existence of an unknown amplification mechanism [48,58]. Taking everything into consideration, it is still a challenge to identify possible PIN protein independent polar auxin transport mechanisms that may have emerged in the early period of evolution, and may or may not have been eliminated during the evolution of land plants. Algae form an attractive model to investigate these potentially early systems and learn more about the basic properties of auxin transport and auxin function.
At present, our knowledge on the role of auxin and its transport in algae is still very limited. However, we are still able to draw some conclusions based on the available studies. With regard to (polar) auxin transport, Figure 1 illustrates a summary and comparison between land plants and multi-cellular algae, like Chara corallina. In both models PAT is present based on auxin influx carriers and NPA sensitive efflux carriers, and a chemiosmotic mechanism. Although the presence of NPA sensitive PAT in Chara in combination with our understanding of PAT in land plants justifies this hypothetical model, it must be emphasized that the hypothetical presence of these specific transport proteins in algae is only based on indirect evidence. In addition, the membrane potential difference across the plasmalemma should also be taken into consideration as this plays a significant role in the chemiosmotic mechanism, and complicates the auxin transport model [59,60,61]. Compared with our knowledge the model system of seed plants—Arabidopsis, studies on algae are still facing many challenges, especially with regard to the still insufficient genetic background database and possibilities for application of molecular biology tools.
Figure 1. (A) Auxin transport in seed plants; (B) The hypothetical auxin transport system in Chara.
Figure 1. (A) Auxin transport in seed plants; (B) The hypothetical auxin transport system in Chara.