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Morphological Description of Plants: New Perspectives in Development and Evolution

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Morphological Description of Plants: New Perspectives in Development and Evolution ® International Journal of Plant Developmental Biology ©2010 Global Science Books Morphological Description of Plants: New Perspectives in Development and Evolution 1* 2 Emilio Cervantes • Juana G . de Diego 1 IRNASA-CSIC. Apartado 257. 37080. Salamanca. Spain 2 Dept Bioquímica y Biología Molecular. Campus Miguel de Unamuno. Universidad de Salamanca. Spain Corresponding author : * [email protected] ABSTRACT The morphological description of plants has been fundamental in the history of botany and provided the keys for taxonomy. Nevertheless, in biology, a discipline governed by the interest in function and based on reductionist approaches, the analysis of forms has been relegated to second place. Plants contain organs and structures that resemble geometrical forms. Plant ontogeny may be seen as a sequence of growth processes including periods of continuous growth with modification that stop at crucial points often represented by structures of remarkable similarity to geometrical figures. Instead of the tradition in developmental studies giving more importance to animal models, we propose that the modular type of plant development may serve to remark conceptual aspects in that may be useful in studies with animals and contribute to original views of evolution. _____________________________________________________________________________________________________________ Keywords: concepts, description, geometry, structure INTRODUCTION: PLANTS OFFER NEW reflects a more general situation in Biology, a discipline APPROACHES TO DEVELOPMENT that has matured from the experimental protocols and views of biochemistry, where the predominant role of experimen- The study of development is today a major biological disci- tation has contributed to a decline in basic aspects that need pline. Although it was traditionally more focused in animal to be more descriptive, such as those related with mor- systems, increased emphasis in plant development may phology. It is true that molecular data provide good infor- open original perspectives. In particular, plant development mation in relation with morphology, but it is also true that presents features that may help to understand basic con- the role played by genes and proteins in the maintenance or ceptual aspects. In parallel to an increased attention in plant stability of shapes will be better understood once a suffici- systems, new approaches need to correct a past drawback ent description of plant forms has been done. This needs a that has limited perspectives inhibiting original points of strong theoretical basis as well as mathematic support but view and compromised future results: Development con- does not always need sophisticated experimentation. cerns primarily changes in shape, but paradoxically its The limitations of the traditional functional approach to protocols and strategies are predominantly based on the the analysis of plant development need to be considered. analysis of the nature and dynamic relationships of mole- The current functional perspective has reached a limit and cules. A functional view has been predominant in the analy- now needs original inputs from descriptive morphology. In sis and description of all morphological processes. Under the following paragraphs the historical antecedents to our this working schema the current views are incomplete and claim will be commented as well as some recent descriptive may thus, be unsatisfactory. The main answers offered in issues. In addition, theoretical and conceptual aspects that response to the developmental questions consist in the des- may be useful for understanding development will be em- cription of a series of biochemical elements and their inter- phasized. The reader will see that plant development may actions. Primary questions are avoided and substituted by be interpreted as the integration of different independent others either because they fall out of current ways of thin- processes and thus, it may serve as a conceptual framework king, either because they are too elemental. Some of these model for more general approaches. The identification and primary questions concern the description of the shapes of appreciation of each particular developmental process as an plants and their changes, for example: why do the flowers independent entity may contribute to new and productive of crucifers have the shape of a cross? Others may concern views of genome organization, development and evolution. the relationships between different plant structures in evolu- In this sense, work in plants may open ways to understand tion: Does the degree of conservation in flower shape in a the general principles of development in living beings. The given family correspond with a parallel degree of conserva- accurate morphological description of plant structures based tion in seed shape? In the absence of precise descriptions of on a mathematical approach may be a fundamental aspect in plant morphology, development is often seen from a pers- the identification of such processes and their role for the pective where the main question (morphology) is replaced integration of plant development. by a secondary one (the description of the biochemical ele- ments involved). HISTORICAL VIEW: THE DESCRIPTION OF PLANT Tables of contents of journals devoted to development SHAPES IS A FUNDAMENTAL ASPECT IN show to what extent research in the field is dominated by BOTANY the description of genes and proteins: their structure and function, their interactions and regulation are today the The description of plant shapes is an aspect of traditional basic aspects in the description of development. This importance in the history of botany. From early times, em- Received: 31 March, 2010. Accepted: 1 June, 2010. Opinion Paper International Journal of Plant Developmental Biology 4 (1), 68-71 ©2010 Global Science Books phasis was made in the geometrical appearance of plant CONCEPTUAL ASPECTS IN DEVELOPMENT: organs and subsequently, many genera and species of plants DIFFERENT PROCESSES VERSUS CHANGES IN were named according to their morphological characteris- THE SAME PROCESS tics (parviflora, magnipetala, rotundifolia, etc.). Similarly, many plant families receive their names from characteristics In a recent article entitled “Caterpillars evolved from ony- in the shape of their flowers (Papilionaceae, Magnoliaceae, chophorans by hybridogenesis”, Donald Williamson (2009) Asteraceae, Campanulaceae, Crucipherae, etc.). Searches in argued in favour of the hypothesis that proposes that insects the IPNI database (International Plant Names Index; http:// with a metamorphosis may have evolved modularly. The ar- www.ipni.org/index.html) with adjectives indicating mor- ticle is of interest to plant biologists because the hypothesis phological characters may yield many examples of this his- there proposed may be successfully applied to plant systems. torical trend (Cervantes and Tocino 2009). According to this point of view, insect caterpillars would be Plant ontogeny may be interpreted as periods of conti- the result of the integration of onicophoran genomes with nuous growth that stop at reference points or are interrupted pre-metamorphic insect genomes. Independently of the fea- by switches to alternative growth patterns. In general, struc- sibility of the proposed “hybridization” mechanisms, there tures representing reference points in development (flowers, may be several examples candidates for modular develop- seeds, leaves, pollen grains, cones, etc.) approach geometri- ment. For example, different animals sharing a common cal forms toward which the growth of a set of cells is ac- type of larvae may result from the integration of diverse curately regulated. Plant development may thus be seen as genomes (an earlier genome for the larva and another for an integrated and continuous process that goes through cha- the adult precursor). In theory, this hypothesis may be tested racteristic stages or reference points often characterized by by the genomic analysis but this still remains as an open the approximation to a geometric figure. possibility whose strategy is not yet clear and was not pro- Fig. 1 illustrates this view for Arabidopsis. We may vided by the article. On the other hand, and even if the main consider the ontogenic development of this species as the hypothesis is not verified by a genomic analysis, the article continuous transitions between reference points represented contributes to an aspect of conceptual importance in the by stable geometrical forms (seed, rosette, flower pollen analysis of development that concerns the identification of grain). The possibility of a precise morphological descrip- particular and independent developmental processes. A tion for each of these structures indicates such a reference clear conclusion of this work that particularly affects plant point. The concept of ontogenetic trajectory (Rice 1997) biology (Williamson 2009) is that at least two different pro- may be applied either to the transition between two cones- cesses of development are at play for each of the examples cutive reference points or to the coordinated developmental given. For animals with larvae, larval development is dif- program that leads to the establishment of one of such ferent from adult development. For insects with metamor- points. Overall, the process of plant ontogeny resembles the phosis, caterpillar development is different from adult deve- current views of evolution with periods of gradual change lopment. The identification of particular developmental
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