Plantae

Introduction

This section grows out some comments on the main plant section, now called Chlorobionta land plants and green algae) from Chris Taylor, one of our regular contributors. He objected that the "plants" go deeper than that, to include the Glaucophyta and red algae (Rhodophyta). As usual, he was correct. Fortunately, having used the term Chlorobionta for land plants and green algae, we had accidentally freed up the vaguer "Plantae" to use for a more inclusive group.

Its hard to give Plantae a reasonable phylogenetic definition. There are three, and possibly four, living groups which diverge from the base of the Plantae: the red algae, the glaucophytes, and the green plants (plus green algae). Just possibly, the Cyanidiales, usually placed at the base of the red algae, represent a fourth basal branch. But see, e.g., Ciniglia et al. 2004). All of these groups share the incorporation of a cyanobacterium as an organelle, i.e., a chloroplast. There is cautious general agreement that this critical bit of indigestion probably happened only once in this group, so that all of the Plantae descend from a single common ancestor. What we don't know is the nature of the beast before it started cultivating house plants.

We also are very uncertain what the sister group of the Plantae might be. Our working hypothesis is that it is the Alveolata + Chromista group. However, many would disagree, and it is not a strongly-supported hypothesis. One significant cause of difficulty is the bizarre origin of the chloroplasts of Chromista. It appears that, just as some ancestral plant first acquired a chloroplast by failing to digest a cyanobacterium, the ancestral chromist acquired a chloroplast by failing to digest a plant. The resulting hybrid organism, with its potential for three-way lateral gene transfer, seems to have been designed by some malicious deity with the specific intent to make accurate phylogenetic analysis impossible.

So, who are we to quarrel with divine providence? Bowing to the ineffable, we will, for the moment at least, leave Plantae as "things with primary chloroplasts." This kind of apomorphy-based definition almost always leads to grief in the long run, but we have no good alternatives.

Image: Glaucocystis from the Protist Information Server.

ATW050128. Text public domain. No rights reserved.

Plant Phylogeny

The basal phylogeny of plants is simplicity itself, largely because very little survives near the base of the Plantae. Undoubtedly, a good many interesting plant types evolved in the 1-3 Gy since the first plant acquired its chloroplast. However the fossil record is essentially non-existent for all but a very few types. What we have today are the Glaucophyta, Rhodophyta, and Chlorobionta. Of these three, the glaucophytes are plainly the most unspecialized. As the image of Glaucocystis shows, this is a rather plain vanilla organism, largely a sack full of chloroplasts. The chloroplasts themselves are also primitive. They retain a "cell wall" of peptidoglycan which ought to dispel any doubts about their bacterial origin. Further, the thylakoid membranes of the chloroplasts are not stacked, suggesting a significantly different, and perhaps more primitive, evolutionary path.

Likely prasinophyte basal chlorobiont) remains are known from close to 1500 Mya. Javaux et al. (2004). If these have been correctly identified, the division between red and green algae must also be extremely ancient. However, the two algal groups have enough in common to be virtually certain that their divergence post-dates the origin of the Glaucophyta.

Image: Microcladia from the UCLA OceanGLOBE site.

ATW050131. Text public domain. No rights reserved.

Plant Diversity

Glaucophyta

This group is also known as the Glaucocystophyta, for the reason that the type genus is Glaucocystis -- to which we respond, "so what?" The type species of the Rhodophyta is probably not Rhodis, or algal taxonomy is in very deep trouble. Rhodis is the rhododendron, and not known to be planktonic. Accordingly, we will retain the traditional name for this little taxon of three genera: Glaucocystis, Cyanophora, and Gloeochaete. Other genera have been included, at times, based largely on the presence of cyanelles primitive chloroplasts). However these appear, on closer inspection, to be unrelated. In particular, Paulinella is not a glaucophyte, but an amoeba which apparently acquired its cyanelle by independent primary endosymbiosis. McFadden 2001).

Glaucophytes are rather widely-distributed in fresh water, but are never found in large numbers in any one place. Glaucophytes have a motile stage with two unequal flagellae of the usual eukaryote type. The flagellae bear two rows of "hairs," but are morphologically dissimilar to the mastigonemes of the Alveolata and Chromista. Glaucocystis, but probably not the other genera, has a cellulose cell wall. The cell membrane is reinforced by flat vesicles and microtubules, much like the cortical alveoli found in many alveolates. Like the red algae, glaucophytes reserve carbohydrates as starch, outside the chloroplast. The mitochondria are conventional, with flattened cristae. Glaucophytes undergo open mitosis and lack centrioles associated with the centrosomes.

The glaucophyte chloroplast has the main feature of interest. It retains a number of cyanobacterial features which have been lost in the chloroplasts of red and green algae. In particular, the cyanelles retain a peptidoglycan wall like a eubacterium. The thylakoid membranes, in which photosynthesis takes place, are not stacked, as in green plants, but have a concentric organization. The thylakoids bear clusters of accessory pigments, phycobilisomes, just as cyanobacteria do, with characteristic phycobilin pigments bound to proteins in the same manner as cyanobacteria, as well as other accessory pigments similar to those in bacteria: such as β-carotene, and the carotenoids zeaxanthin and β-cryptoxanthin. The main photosynthetic pigment is always chlorophylla. Finally, also like cyanobacteria, glaucophyte cyanelles contain carboxysomes, polyhedral structures which stockpile RuBisCO, the enzyme responsible for fixing carbon dioxide. Bhattacharaya et al. (1995); Katz et al. (2004).

Images: Cyanophora from the Phycological Images site of Prof. Isao Inouye, University of Tsukuba. Phycobilisome from the website of Dr. Frank J. Jochem, Florida International University.

ATW050131. Text public domain. No rights reserved.

Rhodophyta

There has been a concerted effort in the semi-popular literature to avoid the word "algae," or to put it into quotation marks, because the algae are not a monophyletic group. We have two objections to this practice. First, although the "algae" are not monophyletic, the red algae, brown algae, etc., are monophyletic groups. That is, they each include their common ancestor and all of its descendants. Second, although the "algae" are not monophyletic, their chloroplasts are monophyletic. That is, all Cyanobacteria ("blue-green algae") and algal chloroplasts are, together, all of the descendants of some unique common ancestral cyanobacterium. Thus, it makes perfectly good phylogenetic sense to speak of algal chloroplasts; and it is reasonable to call their hosts and prokaryotic free-living forms algae. The only real problem is that the algae, thus defined, also include the Embryophyta, i.e., the land plants. However, we whisk this minor embarrassment under the rug with the observation we are at least consistent with our discussions elsewhere. From the point of view of phycology, oak trees are just a peculiar form of green algae. So, other than this explanation, we make no apology for speaking of Rhodophyta as the red algae.

The rhodophytes are an ancient group -- maybe. Some red algae excrete calcium carbonate, so we may have a fossil record to work with. Remains suggestive of red algae have been identified from the Mesoproterozoic. However, these are single-celled forms. Almost all living rhodophytes are multicellular. Supposedly definitive fossils of multicellular red algae are known from the Furongian under the name Solenopora and related forms. We hasten to add that none of these fossil forms appears to be a member of the extant coralline red algae. Worse, a number of recent papers apparently challenge the identification of just about all of the Paleozoic forms. They may be calcareous sponges, cyanobacteria, or stromatolites. On the other hand, the recent identification of Late Proterozoic (or Early Cambrian?) Rhodophyta from superbly preserved phosphate beds in China seems to be a rather sure bet.

The corallines are still important, arguably the most important, reef formers today. However, the Rhodophyta are now better known as the source of nori, and of various gums, gels, glops and goos used as additives to control the texture and consistency of processed foods, and to prevent the resulting viscous slurry from separating into its noxious component fluids. If you've ever wondered what carrageenan and agar really are, this is it. Red algae were probably the first multicellular organisms. In fact, few living rhodophytes are unicells. Over their very long evolutionary lifetime, they seem to have experimented with a large number of ways to get cells to stick together. One of the these methods is to blanket everyone in a paralyzing, gelid mass of sulfonated sugar polymers -- like institutional tapioca pudding. It works, after a fashion; but we, as metazoa, are fortunate to have evolved in a different direction.

Individual cells are also supplied with cell walls, usually of cellulose or xylan another polysaccharide) fibers. Individual cells are similar to glaucophytes. Sugars are stored as floridean starch -- glycogen, more or less -- which accumulates in the cytoplasm. Rhodophytes, like glaucophytes, lack chlorophyll b but carry a complement of phycobilins arranged in phycobilisomes on unstacked thylakoids. The chloroplasts do not, however, have "cell wall" structures. This system seems to be unusually efficient, as red algae are able to live and photosynthesize at considerable depths, sometimes more than 200m below the surface. Rhodophytes are typically found fixed to substrate in coastal marine environments of almost any kind.

Image: Solenopora from the Kentucky Paleontological Society web site. Solieria from a virtual tour of Chek Jawa.

Links: Divisional Characteristics and Background of Rhodophyta; Introduction to the Rhodophyta; Rhodophyta - The Red Algae; Rhodophyta. The web has many resources on Rhodophyta. But, be warned. For some reason, the proportion of sites offering misinformation of various kinds is unusually high for this taxon.

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Chlorobionta

This is the lineage of land plants and green algae. As these have their own section elsewhere, we will not say too much about them here. The Chlorobionta have chlorophyll b and various carotenoid accessory pigments,  but lack phycobilins and phycobilisomes. Their cell walls are cellulose, and their storage material is starch, which accumulates inside the chloroplasts. These chloroplasts have stacked thylakoids.

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