Incompatibility in angiosperms

 Since Darwinian times considerable knowledge has accumulated on the distribution, physiology and genetics of self-incompatibility (SI) in higher plants. In the second half of this century the first attempts were made to identify the biochemical bases of SI. These included thediscovery that cutinase enables pollen tube penetration at the surface* of the stigma in Cruciferae, sorting of segregation pollen S-phenotypes by serological techniques, a lock-and-key model of the SI reaction, the first detection and characterisation of SI proteins and the discovery of the role of the tapetum in the determination of pollen phenotypes in homomorphic sporophytic SI. This pioneering work was followed by a worldwide effort to identify and understand the cellular and molecular processes which lead to the recognition and rejection of SI pollen. The present review article summarizes briefly the current state of knowledge in areas essential for the understanding and exploitation of SI and outlines new information that has become available during recent years. Received: 14 March 1997 / Revision accepted: 10 June 1997     

Dichogamy in angiosperms

We obtained information on dichogamy and other aspects of the biology of over 4200 species of angiosperms from several hundred published and unpublished sources. We used this information to describe patterns of occurrence of dichogamy and to test specific hypotheses relating dichogamy to other characteristics of plants or their environments. Protandry was more common than protogyny at the intrafloral level, but the reverse was true at the interfloral level. Patterns of dichogamy varied significantly among major taxa, with protogyny more common among monocotyledons and primitive dicotyledons, and protandry expecially common in the Asteridae. Arctic species tended to be less dichogamous and more protogynous than temperate and tropical species. Aquatic and alpine species were especially protogynous. Patterns of dichogamy varied among sexual systems, with gynomonoecious and gynodioecious species especially protandrous, and monoecious species highly protogynous. Autogamous and self-compatible species were disproportionately protogynous. Flowers of intraflorally dichogamous species were slightly larger than those of adichogamous species, owing to the presence of many autogamous species in the latter group. Species with interfloral protogyny bore much smaller flowers than did species with interfloral protandry. Early-blooming species in north-temperate and polar regions were disproportionately protogynous. Sexual structures that abscised, shriveled or moved after completion of their function tended to be presented first, and those that facilitated the other sexual function were presented second. A negative association existed between type of intrafloral and interfloral dichogamy in diclinous species. Most animal-pollinated flowers were protandrous, except beetle-pollinated and refuge and trap blossoms. Wind pollination was markedly associated with protogyny. Vertical inflorescences visited by upwardly-moving vectors were protandrous.     

The leaf size/number trade-off in herbaceous angiosperms

Aims In this study, we examined the extent to which between-species leaf size variation relates to variation in the intensity of leaf production in herbaceous angiosperms. Leaf size variation has been most commonly interpreted in terms of biomechanical constraints (e.g. affected by plant size limitations) or in terms of direct adaptation associated with leaf size effects in optimizing important physiological functions of individual leaves along environmental gradients (e.g. involving temperature and moisture). An additional interpretation is explored here, where adaptation may be more directly associated with the number of leaves produced and where relatively small leaf size then results as a trade-off of high 'leafing intensity'—i.e. number of leaves produced per unit plant body size.Methods The relationships between mean individual leaf mass, number of leaves and plant body size were examined for 127 species of herbaceous angiosperms collected from natural populations in southern Ontario, Canada.Important findings In all, 88% of the variation in mean individual leaf mass across species, spanning four orders of magnitude, is accounted for by a negative isometric (proportional) trade-off relationship with leafing intensity. These results parallel those reported in recent studies of woody species. Because each leaf is normally associated with an axillary bud or meristem, having a high leafing intensity is equivalent to having a greater number of meristems per unit body size—i.e. a larger 'bud bank'. According to the 'leafing intensity premium' hypothesis, because an axillary meristem represents the potential to produce either a new shoot or a reproductive structure, high leafing intensity should confer greater architectural and/or reproductive plasticity (with relatively small leaf size required as a trade-off). This greater plasticity, we suggest, should be especially important for smaller species since they are likely to suffer greater suppression of growth and reproduction from competition within multi-species vegetation. Accordingly, we tested and found support for the prediction that smaller species have not just smaller leaves generally but also higher leafing intensities, thus conferring larger bud banks, i.e. more meristems per unit plant body size.     

Late-acting self-incompatibility in angiosperms

In most self-incompatible (SI) plants, pollen tube growth in self-pollinated flowers is inhibited on the stigma or in the style. SI systems that operate in the ovary have been assumed to be extremely rare. Evidence from many plant species is presented to show that the SI barriers in the ovary, described here as late-acting SI systems, are quite common. The late-acting SI systems are divided into four categories: (1) ovarian inhibition of incompatible pollen tubes before the ovule is reached; (2) prefertilization inhibition in the ovule; (3) post-zygotic rejection of the embryo, and (4) ovular inhibition for which the cytological details have not been established. Whether or not post-zygotic incompatibility systems can be distinguished from inbreeding depression depends upon the assumptions underlying the genetic models of self-incompatibility. However, four approaches are outlined that could distinguish between active uniform rejections that are presumably evolved responses to inbreeding depression and the passive, variable failures that are commonly understood to be expressions of typical inbreeding depression. Possible advantages of late-acting SI include an extended period of time over which pollen genotypes may be evaluated by the maternal parent and greater flexibility in the choice of male parents. Due to a paucity of data regarding the genetics and physiology of lateacting SI systems, little can be said at this time about the possible diversity of such systems of their evolutionary relationships with classical gametophytic and sporophytic SI. An hypothesis for the operation of post-zygotic SI is described whereby maternal resources to developing embryos are terminated if the embryo (and/or endosperm) fall below a threshold level of heterosis. This hypothesis is a modification of one first proposed by Westoby and Rice in 1982 to explain variable maternal resource allocation to developing embryos.     

Anther plastids in angiosperms

In the anther of angiosperms, all types of plastids are found in the course of pollen development. They are located in the different cell layers of the microsporangium and have various functions that contribute to the formation of the functional male gametophyte. This includes photosynthesis, stomata opening, sugar storage and/or mobilization, lipid synthesis and secretion for pollenkitt formation, as well as serving as a physiological buffer under stress conditions. They are also involved in plastid inheritance, but to different extents, according to the species. The plastid is a semi-autonomous organelle. Plastid division in the anther is synchronous with cell division, except in the vegetative cell during pollen maturation. Furthermore, recent data seem to show that plastids are affected by programmed cell death and DNA degradation, which occur in the whole anther throughout pollen development. However, the timing of plastid disappearance fluctuates in the different cell layers and also depending on species. In vitro, following androgenesis, plastids that originate in the microspore are responsible for the occurrence of albino plantlets in Poaceae. This trait reflects the relative independence of the plastid genome when compared with that of the nucleus. In this family, microspore plastids may become so involved in programmed cell death that they are unable to follow the alternative sporopohytic program. The different pathways of plastid differentiation in neighboring anther cell layers require an accurate regulation of cell development that remains widely unknown in the anther.     

Stomatal patterning in angiosperms

My thesis is that understanding stomatal patterning requires a holistic perspective. Since stomata are structures critical to the survival of terrestrial plants, they need to be viewed in relation to their function and their interface with other structural components. With this outlook, I begin by discussing pattern types, means of measuring them, advantages of each type of measurement, and then present patterning from evolutionary, physiological, ecological, and organ views. I suggest areas where I believe profitable studies might enable us to better understand stomatal patterning. The final sections of the paper review stomatal patterning on angiosperm leaves and present a theory of patterning. With the abundance of molecular information, and coming genomic sequences and new tools, an opportunity exists to dissect the process of how cells are selected to become different from their neighbors and assume a fate critical to plant survival. Understanding this biological process at the molecular level requires comprehending the broad base on which stomatal patterning rests.     

5-hydroxyconiferyl aldehyde modulates enzymatic methylation for syringyl monolignol formation, a new view of monolignol biosynthesis in angiosperms

S-Adenosyl-L-methionine-dependent caffeate O-methyltransferase (COMT, EC 2.1.1.6) has traditionally been thought to catalyze the methylation of caffeate and 5- hydroxyferulate for the biosynthesis of syringyl monolignol, a lignin constituent of angiosperm wood that enables efficient lignin degradation for cellulose production. However, recent recognition that coniferyl aldehyde prevents 5-hydroxyferulate biosynthesis in lignifying tissue, and that the hydroxylated form of coniferyl aldehyde, 5-hydroxyconiferyl aldehyde, is an alternative COMT substrate, demands a re-evaluation of the role of COMT during monolignol biosynthesis. Based on recombinant aspen (Populus tremuloides) COMT enzyme kinetics coupled with mass spectrometry analysis, this study establishes for the first time that COMT is in fact a 5-hydroxyconiferyl aldehyde O-methyltransferase (AldOMT), and that 5-hydroxyconiferyl aldehyde is both the preferred AldOMT substrate and an inhibitor of caffeate and 5-hydroxyferulate methylation, as measured by K(m) and K(i) values. 5-Hydroxyconiferyl aldehyde also inhibited the caffeate and 5-hydroxyferulate methylation activities of xylem proteins from various angiosperm tree species. The evidence that syringyl monolignol biosynthesis is independent of caffeate and 5-hydroxyferulate methylation supports our previous discovery that coniferyl aldehyde prevents ferulate 5-hydroxylation and at the same time ensures a coniferyl aldehyde 5-hydroxylase (CAld5H)-mediated biosynthesis of 5-hydroxyconiferyl aldehyde. Together, our results provide conclusive evidence for the presence of a CAld5H/AldOMT-catalyzed coniferyl aldehyde 5-hydroxylation/methylation pathway that directs syringyl monolignol biosynthesis in angiosperms.     

Aperture pattern ontogeny in angiosperms

Pollen grains display a wide range of variation in aperture number and arrangement (pattern) in angiosperms. Apertures are well-defined areas of the pollen wall surface that permit pollen tube germination. For low aperture numbers, aperture patterns are characteristic of the major taxonomic divisions of angiosperms. This paper presents a developmental model that explains most of the aperture patterns that are recorded in angiosperms. It is based on the analysis of the different events that occur during meiosis and lead to microspore differentiation. It demonstrates that variation occurring during meiosis in angiosperms is sufficient to produce the core morphological set of the most commonly observed pollen morphologies.     

Potential NH4+ and NO3− uptake in seven Sphagnum species

The rate of nitrogen uptake by seven Sphagnum species, which from a gradient from hummock to hollow and from ombrotrophic to minerotrophic conditions, was measured as the decrease in the concentrations of NH₄⁺ and NO₃⁻ from solutions in which capitula were grown under laboratory conditions.
The highest uptake rate was by individuals of each species with large capitula and a high number of ion exchange sites, i.e. lawn species ( S. pulchrum , S. fallax , S. papillosum and S. magellanicum ). On a dry-mass basis, the most effective species were the hummock species ( S. fuscum and S. rubellum ), even though these species have a low dry mass. Hummock species, which occur in high densities and have high potential N-uptake rates on a dry-mass basis, were the most effective species in retaining available nitrogen.     

Root-nodule formation in non-leguminous angiosperms

Summary The world-wide survey under the IBP of root-nodule formation in non-leguminous Angiosperms is progressing reasonably satisfactorily, and it is anticipated that when all the results have been collated a useful body of new data will be yielded. In recent studies, also forming part of the IBP, in the author's laboratory, the nodules of further species in the genera Alnus, Myrica, Ceanothus, Coriaria and Dryas have been examined for nitrogen-fixing properties, with positive results. Also the extent to which the nodule endophytes from species of Alnus and Myrica respectively are able to symbiose satisfactorily with other host species in the same genus has been investigated, and the conclusion reached that especially in Myrica there is very considerable specialisation among the endophytes. A marked diurnal variation in the rate of fixation of nitrogen in the nodules of non-legumes growing in a glasshouse lit by daylight has been found, with maximal rates being attained around midday. The implication is that this is the period of the maximal availability of carbohydrates in the nodules, but actual analyses have so far failed to reveal this. Analyses of the amino acid composition of the nodules in several genera have shown that except in Alnus, where citrulline is prominent, asparagine is in most cases the dominant amino acid.     

Asymmetrical crossing barriers in angiosperms

Patterns of reproductive isolation between species may provide insight into the mechanisms and evolution of barriers to interspecific gene exchange. We used data from published interspecific hybridization experiments from 14 genera of angiosperms in order to test for the presence of asymmetrical barriers to gene exchange. Reproductive isolation was examined at three life-history stages: the ability of interspecific crosses to produce seeds, the viability of F1 hybrids, and the fertility of F1 hybrids. Statistically significant asymmetries in the strength of reproductive isolation between species were detected in all genera and at each of the three life-history stages. Asymmetries in seed production may be caused by a variety of mechanisms including differences in stigma/style lengths, self compatibility and differential fruit abortion. Asymmetries in post-zygotic isolation are probably caused by nuclear-cytoplasmic interactions. Asymmetrical reproductive isolation between plant taxa may have important implications for the dynamics of hybrid zones, the direction of genetic introgression and the probability of reinforcement.     

Half tetrad analysis in angiosperms

A biochemical technique involving analysis of endosperm is proposed for detecting meiotic crossovers and for gene mapping in angiosperms with bisporic embryo sacs. In bisporic embryo sac development two spores resulting from meiosis-II division of the same meiosis-I daughter cell contribute two thirds of the total genetic information to the triploid endosperm nucleus, the other third coming from a sperm in the fertilizing pollen grain. In controlled crosses where the marker gene codes for allozymes with phenotypes sensitive to gene dosage, the maternal meiotic contribution to the endosperm nucleus may be determined, thereby allowing crossovers between the marker locus and the centromere to be detected.     

Cold resistance in Antarctic angiosperms

Deschampsia antarctica Desv. (Poaceae) and Colobanthus quitensis (Kunth) Bartl. (Cariophyllaceae) are the only two vascular plants that have colonized the Maritime Antarctic. The primary purpose of the present work was to determine cold resistance mechanisms in these two Antarctic plants. This was achieved by comparing thermal properties of leaves and the lethal freezing temperature to 50% of the tissue (LT50). The grass D. antarctica was able to tolerate freezing to a lower temperature than C. quitensis. The main freezing resistance mechanism for C. quitensis is supercooling. Thus, the grass is mainly a freezing‐tolerant species, while C. quitensis avoids freezing. D. antarctica cold acclimated; thus, reducing its LT50. C. quitensis showed little cold‐acclimation capacity. Because day length is highly variable in the Antarctic, the effect of day length on freezing tolerance, growth, various soluble carbohydrates, starch, and proline contents in leaves of D. antarctica growing in the laboratory under cold‐acclimation conditions was studied. During the cold‐acclimation treatment, the LT50 was lowered more effectively under long day (21/3 h light/dark) and medium day (16/8) light periods than under a short day period (8/16). The longer the day length treatment, the faster the growth rate for both acclimated and non‐acclimated plants. Similarly, the longer the day treatment during cold acclimation, the higher the sucrose content (up to 7‐fold with respect to non‐acclimated control values). Oligo and polyfructans accumulated significantly during cold acclimation only with the medium day length treatment. Oligofructans accounted for more than 80% of total fructans. The degrees of polymerization were mostly between 3 and 10. C. quitensis under cold acclimation accumulated a similar amount of sucrose than D. antarctica, but no fructans were detected. The suggestion that survival of Antarctic plants in the Antarctic could be at least partially explained by accumulation of these substances is discussed.     

Differential Na2SO4 tolerance in tobacco plants regenerated from Na2SO4-grown callus

Abstract The regenerated shoots from sodium sulphate (Na₂SO₄) grown callus of tobacco (Nicotiana tabacum L. cv. Wisconsin 38) were evaluated for Na₂SO₄ tolerance based on shoot proliferation and rooting in vitro, and seed germination in vivo in response to Na₂SO₄. An increase in Na₂SO₄ concentration resulted in significantly decreasing shoot fresh weight, number of shoots, shoot length and leaf size, and increasing per cent shoot dry weight of both control and Na₂SO₄-grown cultures. In rooting, shoots of Na₂SO₄-grown cultures exhibited the highest per cent rooting (85%) in the presence of 1% w/v Na₂SO₄. However, per cent rooting, root number per rooted cutting and root fresh weight decreased significantly with increasing Na₂SO₄ concentration when shoots were transferred to the medium in the absence of Na₂SO₄ for 4-monthly passages. Following acclimatization of the rooted shoots of Na₂SO₄-grown cultures, phenotypic variation was observed during growth and development. There were 13.2% sterile plants. Fertile plants were sorted into normal (N), tolerant (T), and sensitive (S) categories and the respective percentages of plants were 31.6, 44.7 and 10.5, based on per cent germination, germination velocity index and seedling survival to Na₂SO₄. The response of N, T and S types to Na₂SO₄ in subsequent shoot proliferation was similar to that of seed germination.