Embryo and cotyledon development in other species have not been extensively studied at the molecular level, only on a morphological descriptive basis Wardlaw, Embryos of monocots and eudicots are basically morphologically indistinguishable at the early embryo stages and differences only become apparent at the globular stage when, in eudicots, the outgrowth of the cotyledons results in a heart-stage embryo and, in monocots, the embryo is more elongated as the cotyledon defines the primary axis Burgher, Demonstrating the orthologous function of Arabidopsis genes involved in cotyledon development in monocots is difficult, due to morphological differences.
PIN1 orthologues have also been isolated from maize Carraro et al. The DRN homologue in maize, ZmDRN is expressed in the prospective scutellum Zimmermann and Werr, , thereby showing an analogous expression pattern to that in Arabidopsis , where DRN expression pre-patterns cotyledon development in the Arabidopsis globular embryo, and suggesting an orthologous function.
There is, therefore, clear conservation of many transcription factor hierarchies controlling cotyledon development in Arabidopsis in divergent plant species, and involving auxin Table 1. The function of these genes in monocot cotyledon development is often hard to elucidate, and even if they have no role in cotyledon development, their further characterization by heterologous complementation, together with the isolation of more heterologous genes will reveal the repertoire of ancestral pathways affecting cotyledon development and those which have sub-functionalized or diverged.
An evolutionary developmental biology perspective should help to address this question. Studies on cotyledon development in gymnosperms are few and are mainly related to cotyledon number Harrison and Von Aderkas, and limited to somatic embryos and morphological descriptions. However, a comparison of expressed sequence tags from loblolly pine embryos with those of Arabidopsis revealed a large conservation with angiosperm embryogenesis-related genes Cairney et al.
The variation in cotyledon number within a given gymnosperm species correlates with embryo size, which alters from year to year Butts and Buchholz, There is a greater degree of variation in cotyledon number in somatic embryos compared with zygotic embryos.
The ring-shaped initiation of multiple cotyledons in gymnosperms means that multiple axes of bilateral symmetry are established by cotyledon initiation and not just two axes as in eudicots. An abnormally large number of cotyledons within a species has been referred to in the literature as pleiocotyly or polycotyly.
Pleiocotyly in natural populations has not been extensively studied, although widespread examples of spontaneous tricotyledony have been reviewed by Holtorp and Haskell in Populus Rajora and Zsuffa, , Prosopis cieraria Nagesh et al. The genetic basis for some of these examples is not known, but a tricotyledonous mutant of Catharanthus roseus was found to be controlled by a single recessive mutation Rai and Kumar, In tomato, two independent loci causing polycotyledony were characterized: the polycotyledon poc mutation was mapped Madishetty et al.
The defective embryo and meristems dem mutant also lacks a functional SAM and is caused by the mutation of a gene encoding a novel protein of unknown function Keddie et al. In pea, three independent single cotyledon sic loci when mutated, give rise to single or fused cotyledons Liu et al. Paternal half-sibling families were compared to assess whether the low phenotypic penetrance was due to stabilizing selection or a lack of genetic variance for the trait.
These alternatives could not be distinguished due to the small samples and low number of phenotypic plants. The variable numbers of cotyledons in gymnosperms has been addressed in clonal populations of somatic embryos of hybrid larch and was found to correlate directly with the diameter of the apical embryo surface Harrison and Von Aderkas, The possibility exists that additive variance in natural populations of some plants could allow the selection of pleiocotyly as a trait, although the potential adaptive value of variable cotyledon numbers has not been well studied and is dependent on understanding the genetic basis of natural variation in cotyledon number.
The possible agricultural benefits of pleiocotyly or manipulating cotyledon size are greater yield and quality of oils in oil seed plants such as rape or sunflower, where oil is stored in the cotyledons rather than the endosperm.
Additional cotyledons may even confer an enhancement in seedling establishment; a tricotyledonous sunflower mutation had no detrimental effect on growth and development and led to three leaves per node, increasing photosynthetic surface area and therefore potential productivity Hu et al.
The Arabidopsis developmental paradigm has elucidated many gene hierarchies regulating cotyledon development and has demonstrated the key involvement of auxin. This genetic robustness and plethora of parallel pathways presumably reflects evolutionary adaptation to ensure buffering against negative mutations at a critical point in plant survival, thereby ensuring that the embryonic patterning programme proceeds successfully. It will be interesting to discover how conserved this redundancy is amongst other eudicots for which Arabidopsis is not representative.
Despite the enormous diversity of cotyledon morphology and development within and between the monocots and eudicots, the existence of many cotyledon development genes and regulatory pathways are conserved, such as those involving CUC genes, STM , pathways involving auxin, although in many cases no monocot mutants are available to demonstrate how they affect cotyledon development.
The increased ease of isolating gene homologues by heterologous cloning and applying the information to evolutionary developmental biology will increasingly reveal the extent of this conservation across the plant kingdom and distinguish pathways which have divergently evolved. It will also inform the debate on the evolution of cotyledon states.
The similarities in the genetic regulation and auxin-dependent initiation and outgrowth of cotyledons and leaves demonstrates the homology between the structures. However, open questions such as what distinguishes leaf founder cells from cotyledon precursor cells and how cotyledon and leaf development differ, including determinants of phase change and the relationship of cotyledon initiation to the SAM, remain tantalizing foci of research.
Natural variation in cotyledon number is under-researched and understanding the genetic basis of pleiocotyly could enable breeding agronomically important seed oil crops for increased productivity based on this trait.
Google Scholar. Google Preview. Oxford University Press is a department of the University of Oxford. It furthers the University's objective of excellence in research, scholarship, and education by publishing worldwide. Sign In or Create an Account. Sign In. Advanced Search. Search Menu. Article Navigation. Close mobile search navigation Article Navigation. Volume Article Contents Abstract. Cotyledon diversity. The evolution of cotyledony. Homology between cotyledons and leaves.
Do cotyledons arise from the SAM? Cotyledon morphogenesis in Arabidopsis. The hormonal control of cotyledon development. Cotyledon development in other eudicots and monocots. Cotyledon development in gymnosperms. Pleiocotyly and natural variation in cotyledon number.
Breeding for cotyledon number. Cotyledon organogenesis. Chandler John W. E-mail: john. Oxford Academic. Revision received:. Cite Cite John W. Select Format Select format. Permissions Icon Permissions. Abstract The cotyledon represents one of the bases of classification within the plant kingdom, providing the name-giving difference between dicotyledonous and monocotyledonous plants.
Arabidopsis , auxin , cotyledon , dicots , evolution , monocots , natural variation , phylogeny. Table 1. Open in new tab. Open in new tab Download slide. Regulation of shoot epidermal cell differentiation by a pair of homeodomain proteins in Arabidopsis. Google Scholar Crossref. Search ADS. Google Scholar PubMed. Genes involved in organ separation in Arabidopsis: an analysis of the cup-shaped cotyledon mutant. The polycotyledon mutant of tomato shows enhanced polar auxin transport.
Production of phenocopies of the lanceolate mutant in tomato using polar auxin transport inhibitors. Overexpression of Arabidopsis ESR1 induces initiation of shoot regeneration.
Formation of the shoot apical meristem in Arabidopsis thaliana : an analysis of development in the wild type and in the shoot meristemless mutant. KNAT6: an Arabidopsis homeobox gene involved in meristem activity and organ separation. Local, efflux-dependent auxin gradients as a common module for plant organ formation.
Morphogenesis in pinoid mutants of Arabidopsis thaliana. Asymmetric leaves1 mediates leaf pattering and stem cell function in Arabidopsis. Expressed sequence tags from loblolly pine embryos reveal similarities with angiosperm embryogenesis. Seed plant phylogeney inferred from all three plant genomes: monophyly of extant gymnosperms and origin of Gnetales from conifers. Auxin synthesized by the YUCCA flavin monooxygenases is essential for embryogenesis and leaf formation in Arabidopsis.
Mechanisms of constraints; the contributions of selection and genetic variance to the maintenance of cotyledon number in wild radish. Mutations of Arabidopsis thaliana that transform leaves into cotyledons.
Upcoming changes in flowering plant family names: those pesky taxonomists are at it again! Lentibulariaceae , with special attention to embryo evolution.
Plant development is regulated by a family of auxin receptor F box proteins. Comparative anatomical observations on the dicotylous and tricotylous seedlings of Raphanus sativus L.
Developmental basis of homeosis in precociously germinating Brassica napus embryos: phase change at the shoot apex. Auxin distribution and transport during embryonic pattern formation in wheat.
Illustrations of polycotyledony in the genus Persoonia , with some reference to Nuytsia. Morphology of fruits, seeds and embryos of Argentinian Capparis L. Efflux-dependent auxin gradients establish the apical-basal axis of Arabidopsis.
Convergent evolution of shoots in land plants: lack of auxin polar transport in moss shoots. Signalling between the shoot apical meristem and developing lateral organs. Patterns of auxin transport and gene expression during primordium development revealed by live imaging of the Arabidopsis inflorescence meristem. Preliminary observation on a spontaneous tricotyledonous mutant in sunflower. Research Workshop Proceedings. A critical role of sterols in embryonic patterning and meristem programming revealed by the fackel mutants of Arabidopsis thaliana.
Fundamental concepts in the embryogenesis of dicotyledons: a morphological interpretation of embryo mutants. Seedling morphology, and schizocotyly in Hammada salicordia Moq. The number of cotyledons present is one characteristic used by botanists to classify the flowering plants angiosperms.
Species with one cotyledon are called monocotyledonous or, "monocots" and placed in the Class Liliopsida. Plants with two embryonic leaves are termed dicotyledonous "dicots" and placed in the Class Magnoliopsida. Reference Terms. Upon germination, the cotyledon usually becomes the embryonic first leaves of a seedling. Related Stories. A team has now The depth of this sleep is inherited from their mother. Researchers reveal how this maternal imprint Also included here are the types of fruits, fruit dispersal mechanisms, and seed germination.
The distinctions between dicots and monocots, the two major groups of flowering plants, are presented in this tutorial Read More. Skip to content Main Navigation Search.
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