The origin of the sporophyte shoot in land plants: a bryological perspective

Background

Land plants (embryophytes) are monophyletic and encompass four major clades: liverworts, mosses, hornworts and polysporangiophytes. The liverworts are resolved as the earliest divergent lineage and the mosses as sister to a crown clade formed by the hornworts and polysporangiophytes (lycophytes, monilophytes and seed plants). Alternative topologies resolving the hornworts as sister to mosses plus polysporangiophytes are less well supported. Sporophyte development in liverworts depends only on embryonic formative cell divisions. A transient basal meristem contributes part of the sporophyte in mosses. The sporophyte body in hornworts and polysporangiophytes develops predominantly by post-embryonic meristematic activity.

Scope

This paper explores the origin of the sporophyte shoot in terms of changes in embryo organization. Pressure towards amplification of the sporangium-associated photosynthetic apparatus was a major driver of sporophyte evolution. Starting from a putative ancestral condition in which a transient basal meristem produced a sporangium-supporting seta, we postulate that in the hornwort–polysporangiophyte lineage the basal meristem acquired indeterminate meristematic activity and ectopically expressed the sporangium morphogenetic programme. The resulting sporophyte body plan remained substantially unaltered in hornworts, whereas in polysporangiophytes the persistent meristem shifted from a mid-embryo to a superficial position and was converted into an ancestral shoot apical meristem with the evolution of sequential vegetative and reproductive growth.

Conclusions

The sporophyte shoot is interpreted as a sterilized sporangial axis interpolated between the embryo and the fertile sporangium. With reference to the putatively ancestral condition found in mosses, the sporophyte body plans in hornworts and polysporangiophytes are viewed as the product of opposite heterochronic events, i.e. an anticipation and a delay, respectively, in the development of the sporangium. In either case the result was a pedomorphic sporophyte permanently retaining juvenile characters.     

Molecular adaptation and the origin of land plants

The origin and diversification of land plants was one of the most important biological radiations. Land plants are crucial components of all modern terrestrial ecosystems. The first land plants had to adapt to a wide array of new environmental challenges including desiccation, varying temperatures, and increased UV radiation. There have been numerous studies of the morphological adaptations to life on land. However the molecular adaptations to life on land have only recently gained attention. These studies have greatly benefited from the recent advances in our understanding of the phylogenetic relationships between and among the charophycean algae and the basal land plant groups. In this review I summarize the current knowledge of a variety of physiological and biochemical adaptations to land including plant growth hormones, isoprene, phenolics, and heat shock proteins.     

Increased hexose transport in the roots of tomato plants submitted to prolonged hypoxia

We investigated the effects of prolonged hypoxia on the sugar uptake in tomato (Solanum lycopersicum L. var. MP-1) roots. Hydroponic cultures of whole tomato plants were submitted to hypoxic treatment for 1 week, and the roots were analyzed for sugar concentrations, hexose uptake and hexose transporter expression level. Contrary to what has been observed after anoxic shock or short-term hypoxic treatment, we show that sugar concentrations increase and hexose uptake is up-regulated in the roots after 1 week of hypoxic treatment. Increased hexose transport is concomitant with the induction of the hexose transporter gene LeHT2. These responses may be due either to a direct effect of low O₂ supply, or to a secondary effect associated with the increase in sugar concentrations, which, typically, develops in most hypoxic plants.     

Unresolved problems on the origin and early evolution of land plants.

The origin of land plants or embryophytes from the Charophyceae is generally accepted today by the botanists. In fact, numerous morphological, cytological, ultrastructural, biochemical and molecular characters are shared in these organisms. A fundamental problem is still constituted by the evolution of the sporophyte, i.e. the appearance of two different phase cycles (gametophyte/sporophyte alternance), although two theories ("antithetic" and "homologous") try to explain this evolutionary event.However, another phylogenetic dilemma is represented, in my opinion, either by the formation of bryophytes or by the transition from these first land plants to the pteridophytes, considering them at whole organism level.The bryophyte gametophyte is the most elaborate of the land plants. It presents several complex characters, principally the growth developmental form, the appearance of multicellular sex organs, antheridia and archegonia. Also the sporophyte shows a complicated structure that is not found in the other land plants or tracheophytes. The sporangium, in particular, exhibits some intricate morphological traits such as the peristome of true mosses for spore dispersion, the elaters of liverworts and the indeterminate growth in the hornworts.The pteridophytes are represented especially by their dominant sporophyte. This latter has the capacity to produce multiple sporangia and, in many cases, two kinds of spores which develop in male and female gametophyte (heterosporous pteridophytes). Another important characteristic of this sporophyte is its ability to become independent of the gametophyte. However, one of the most innovative character is the formation of true vascular elements (xylem and phloem).All these very large evolutionary jumps are discussed on the basis of the phyletic gradualistic neo-Darwinian theory and the punctuated equilibrium theory of Eldredge and Gould. In this context other genetic evolutionary mechanisms are also considered.Nevertheless, the origin of bryophytes and pteridophytes remain, at the moment, a mystery.     

The effect of streptozotocin-induced diabetes on hepatic hexose transport

3-O-methyl-D-glucose (which is not metabolized in isolated parenchymal cells) was used to characterize the hexose transport process in hepatocytes prepared from 24 h fasted rats. The Vmax and Km obtained were 161 +/- 12 nmol/mg dry wt./min and 39 +/- 4 mM respectively (Europe-Finner GN, 1984, Biosci. Rep. 4, 483-489). Streptozotocin-induced diabetes decreased the Km of the system by 50% to a value of 19 +/- 6 mM without causing any change in the Vmax. Short term insulin treatment of cells prepared from 24 h diabetic rats appeared to partially return the system to normal.     

Sexual conflict and the alternation of haploid and diploid generations

Land plants possess a multicellular diploid stage (sporophyte) that begins development while attached to a multicellular haploid progenitor (gametophyte). Although the closest algal relatives of land plants lack a multicellular sporophyte, they do produce a zygote that grows while attached to the maternal gametophyte. The diploid offspring shares one haploid set of genes with the haploid mother that supplies it with resources and a paternal haploid complement that is not shared with the mother. Sexual conflict can arise within the diploid offspring because the offspring's maternal genome will be transmitted in its entirety to all other sexual and asexual offspring that the mother may produce, but the offspring's paternally derived genes may be absent from these other offspring. Thus, the selective forces favouring the evolution of genomic imprinting may have been present from the origin of modern land plants. In bryophytes, where gametophytes are long-lived and capable of multiple bouts of asexual and sexual reproduction, we predict strong sexual conflict over allocation to sporophytes. Female gametophytes of pteridophytes produce a single sporophyte and often lack means of asexual reproduction. Therefore, sexual conflict is predicted to be attenuated. Finally, we explore similarities among models of mate choice, offspring choice and segregation distortion.     

Quercetin inhibits hexose transport in a human diploid fibroblast

Summary The flavonol quercetin, a phloretin analog, inhibits transport of 2-deoxyglucose and 3-O-methylglucose in a cultured human diploid fibroblast. This inhibition is related to transport itself and not to the reported effects of flavonoids on membrane-bound ATPases. From concentration-inhibition curves at several pH's we conclude that uncharged (acid) quercetin (pK=7.65) is the inhibitory form of the molecule (K _I =10m). Quercetin, unlike phloretin, is rapidly degraded in 0.1n NaOH; the degradation products are weakly inhibitory to hexose transport.     

Molecular data from 27 proteins do not support a Precambrian origin of land plants

Heckman et al. (Science 293: 1129-1133) used sequences obtained from GenBank to infer divergence times in fungi and green plants. They estimated that the crown group of land plants originated in the Precambrian, at 703 ± 45 mya, a date much older than dates implied by the fossils, which are no older than about 450 mya. This paper presents an analysis of an entirely different set of sequence data from 27 plastid protein-coding genes in 10 land plants and a green algal outgroup. It uses a calibration point closer to the origin of land plants and inference methods that do not assume a molecular clock. This leads to estimates ranging from 425 to 490 mya, which brackets the age suggested by the fossil record. Possible explanations for the differing conclusions in the two studies include differences in calibration points and use of single-copy plastid genes rather than nuclear gene families.     

Effects of strontium on calcium-dependent hexose transport in muscle

Hexose transport in isolated perifused rat and guinea pig left atria and in isolated intact rat hemidiaphragms was followed by measuring the tissue/medium distribution of the nonmetabolized glucose analog, 3-O-methyl-D-glucose. Stimulation of 3-methylglucose transport by insulin, hyperosmolar medium, K+-free medium, and ouabain was depressed or absent in Ca2+-free medium. Addition of 2 mM Sr2+ to Ca2+-free media restored the response of transport to the stimulatory factors. Sr2+ also increased basal hexose transport. The Ca2+ dependence and the effect of Sr2 was greatest in guinea pig atria and least in rat hemidiaphragms. It is concluded that Sr2+ plays a Ca2+-like role in the regulation of hexose transport.     

Involvement of hexose transport in myogenic differentiation

A high (HAHT) and a low (LAHT) affinity hexose transport system are present in undifferentiated rat L6 myoblasts; however, only the latter can be detected in multinucleated myotubes. This suggests that HAHT is either down-regulated or modified as a result of myogenesis. The present investigation examined the relationship between HAHT and myogenic differentiation. While myogenesis could be inhibited by the potent hexose transport inhibitor phloretin, it was not affected by phlorizin which had no effect on hexose transport. This relationship was further explored using six different HAHT-defective mutants. All six mutants, altered in either the HAHT transport affinity (Type I mutants) or capacity (Type II mutants), were impaired in myogenesis. Since these mutants were selected from both mutagenized and non-mutagenized cells with different reagents, or with different concentrations of the same reagent, the deficiency in myogenesis was likely due to changes in HAHT properties. This notion was confirmed by the observation that growth of Type I mutants in high D-glucose concentrations could rectify the defect in myogenesis. D-glucose was unlikely to rectify the defect in myogenesis, if this defect was due to a second unrelated mutation that may have arisen during isolation of the mutants. Since both types of mutants were not altered in LAHT, D-glucose should still be taken up into the cells. The fact that the glucose-mediated increase in fusion could not be observed in Type II mutants (deficient in the HAHT transporter) suggested that myogenesis was dependent on the presence of D-glucose or its metabolites in specific HAHT-accessible compartments. It is tempting to speculate that trans-acting regulators involved in myogenesis may be synthesized from the glucose metabolites in these specialized HAHT-accessible compartments.     

Specificity and kinetics of hexose transport in Trypanosoma brucei

Transport of 6-deoxy-D-glucose was studied in Trypanosoma brucei in order to characterise the kinetics of hexose transport in this organism using a nonphosphorylated sugar. Kinetic parameters for efflux and entry, measured using zero-trans and equilibrium exchange protocols, indicate that the transporter is probably kinetically symmetrical. Comparison of the kinetic constants of D-glucose metabolism with those for 6-deoxy-D-glucose transport shows that transport across the plasma membrane is likely to be the rate-limiting step of glucose utilisation. The transport rate is nevertheless very fast and 6-deoxy-D-glucose, at concentrations below Km, enters the cells with a half filling time of less than 2 s at 20 degrees C. Thus the high metabolic capacity of these organisms is matched by a high transport rate. The structural requirements for the trypanosome hexose transporter were explored by measuring inhibition constants (Ki) for a range of D-glucose analogues including fluoro and deoxy sugars as well as epimeric hexoses. The relative affinities shown by these analogues indicated H-bonds from the carrier to the C-3, C-4 and C-5 hydroxyl oxygens and from the C-1 and C-3 hydroxyl hydrogens to the binding site. Hydrophobic interactions are likely at the C-2 and C-6 regions of the glucose molecule. Spatial constraints appear to occur around C-4 indicating that the transport site at this position is not freely open to the external solution as is the case with the mammalian hexose transporter. However, the trypanosome transporter appears to accept D-fructose but the common mammalian (erythrocyte type) hexose transporter does not.     

The origin of land plants: Phylogenetic relationships among charophytes,bryophytes, and vascular plants inferred from complete small-subunit ribosomal RNA gene sequences

Complete nuclear-encoded small-subunit 18S rRNA (=SSU rRNA) gene sequences were determined for the prasinophyte green alga Mantoniella squamata; the charophycean green algae Chara foetida, Coleochaete scutata, Klebsormidium flaccidum, and Mougeotia scalaris; the bryophytes Marchantia polymorpha, Fossombronia pusilla, and Funaria hygrometrica; and the lycopod Selaginella galleottii to get a better insight into the sequential evolution from green algae to land plants. The sequences were aligned with several previously published SSU rRNA sequences from chlorophytic and charophytic algae as well as from land plants to infer the evolutionary relationships for major evolutionary lineages within the Chlorobionta by distance matrix, maximum parsimony, and maximum likelihood analyses. Phylogenetic trees created by the different methods consistently placed the Charophyceae on the branch leading to the land plants. The Charophyceae were shown to be polyphyletic with the Charales (charalean algae) diverging earlier than the Coleochaetales, Klebsormidiales, Chlorokybales, and Zygnematales (charophycean algae) which branch from a point closer to the land plants in most analyses. Maximum parsimony and maximum likelihood analyses imply a successive evolution from charophycean algae, particularly Coleochaetales, to bryophytes, lycopods, and seed plants. In contrast, distance matrix methods group the bryophytes together with the charophycean algae, suggesting a separate evolution of these organisms compared with the club moss and the seed plants. Correspondence to.: V.A.R. Huss     

Effect of chemotactic factors on hexose transport in polymorphonuclear leucocytes

Transport of the nonmetabolizable glucose analogue, 3-O-methylglucose, was assessed in human polymorphonuclear leucocytes with or without the chemotactic peptide N-formylmethionylleucylphenylalanine (fMet-Leu-Phe). The peptide increased entry of labelled 3-O-methylglucose about 5-fold and the intracellular distribution space about 70%. The half-time of equilibration was 3 s in the treated cells. Similar effects were observed with zymosan-treated serum (containing the chemotactic factor C5a), with arachidonic acid, calcium ionophore A23187 and phorbol myristate acetate. However, the chemotactic protein, thrombin, had no effect, even though binding to high-affinity receptors was demonstrated. Km for zero-trans entry of 3-O-methylglucose was about 1 mM and fMet-Leu-Phe increased Vmax from 5 to about 25 amol.s-1.cell-1. Similar values were obtained from incubations for a few seconds with glucose and 2-deoxyglucose. The rate of 2-deoxyglucose uptake (8 min incubations) was limited by the transport step at substrate concentrations lower than approx. 0.1 mM, whereas the phosphorylation step became rate-limiting at higher concentrations. Thus, 2-deoxyglucose uptake can only be taken as a measure of transport at a tracer concentration. It is concluded that chemotactic factors can, but do not necessarily, increase the maximal transport velocity of hexoses entering the polymorphonuclear leucocyte via the glucose transporter.     

The GapA/B gene duplication marks the origin of Streptophyta (charophytes and land plants)

Independent evidence from morphological, ultrastructural, biochemical, and molecular data have shown that land plants originated from charophycean green algae. However, the branching order within charophytes is still unresolved, and contradictory phylogenies about, for example,the position of the unicellular green alga Mesostigma viride are difficult to reconcile. A comparison of nuclear-encoded Calvin cycle glyceraldehyde-3-phosphate dehydrogenases (GAPDH) indicates that a crucial duplication of the GapA gene occurred early in land plant evolution. The duplicate called GapB acquired a characteristic carboxy-terminal extension (CTE) from the general regulator of the Calvin cycle CP12. This CTE is responsible for thioredoxin-dependent light/dark regulation. In this work, we established GapA, GapB, and CP12 sequences from bryophytes, all orders of charophyte as well as chlorophyte green algae, and the glaucophyte Cyanophora paradoxa. Comprehensive phylogenetic analyses of all available plastid GAPDH sequences suggest that glaucophytes and green plants are sister lineages and support a positioning of Mesostigma basal to all charophycean algae. The exclusive presence of GapB in terrestrial plants, charophytes, and Mesostigma dates the GapA/B gene duplication to the common ancestor of Streptophyta. The conspicuously high degree of GapB sequence conservation suggests an important metabolic role of the newly gained regulatory function. Because the GapB-mediated protein aggregation most likely ensures the complete blockage of the Calvin cycle at night, we propose that this mechanism is also crucial for efficient starch mobilization. This innovation may be one prerequisite for the development of storage tissues in land plants.