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An Introduction to Flowering Plants: Monocots and Eudicots
There is no doubt about it that plants are main producers of ecosystem and important in every aspect of our daily lives. Many products which are used in food, nutraceutical, pharmaceutical, textile, cosmetics, perfumery, coffee, tea and beverage industries are in fact derived from plants. They are biosynthesized in different parts of plants and are known as natural products or secondary metabolites. Many of these compounds are defensive in nature which are produced during primary metabolic activities in plants. Many pigments in flowering plants are also secondary metabolites which are crucial for their pollination. Secondary metabolites include alkaloids, flavonoids, betalains, glycosides, tannins, terpenoids and saponins. They will be introduced in the next chapter.
This book deals with flowering plants, that is, angiosperms as they make one of the abundant group of plants of economic importance. However, before discussing major products of angiosperms, their biosynthesis and applications, it is important to discuss what are angiosperms? How did they evolve? What is their body organization and what kind of cells they have? So in the next section, a brief introduction of angiosperms and their classification is discussed.
1.1 An Introduction to Major Group of Angiosperms: Monocots, Eudicots and Basal Angiosperms
All plants are considered to be a group of related organisms which are capable to synthesize their own sugars during photosynthesis, possess the cell wall, and generally with the differentiation of their bodies in roots, stems, leaves, flowers or flower- like structures. But recent trends in molecular phylogenetics have shown that they are not as much closely related as thought before. In fact, plants can be best described as 'a group of different organisms which evolved independently during course of evolution and share similar characteristics like ability to synthesize their own food within their chloroplasts, have chlorophyll a as a necessary photosynthetic pigment and possess the cell wall which largely comprises of cellulose'. Their body is differentiated in vegetative and reproductive organs (spore or seed-producing structures) and are therefore classified in one kingdom plantae. Division within kingdom plantae is based either on the presence or absence of vascular tissues (xylem and phloem) or spore-producing structures. Bryophytes like liverworts, hornworts and mosses are non-vascular spore producing plants while pteridophytes are vascular plants which produce spore, for example, ferns, horsetails and clubmossses. Other two major groups are seed-producing plants, that is, gymnosperms which produce seeds which are not enclosed within their ovaries, and angiosperms or flowering plants, in which seeds develop within carpels and are covered by ovary wall.
Angiosperms also known as flowering plants are the largest monophyletic group of seed producing plants which have evolved many efficient ways of survival over the period of time. They are unique from other group of plants due to the development of endosperm (nutritive tissue around embryo within seeds), flowers with carpels and stamens having two pairs of pollen sacs and phloem for transportation of sugars. Their fossils are over 135 million years old. Angiosperms are considered to be close relatives of living gymnosperms but some recent evidence suggested that seed ferns represent sister group to angiosperms. They are relatively evolved group of plants as compared with gymnosperms as they possess several mechanisms which ensure successful asexual and sexual reproduction, one of the main reason which makes them one of the abundant group of seed plants.
Although monocots (angiosperms with one cotyledons) and dicots (angiosperms with two cotyledons) are referred as two main groups of angiosperms but modern classification which is based on molecular evidences have characterized angiosperms as core and basal angiosperms according to their monophyletic origin (descendants of common ancestors) and facts provided by molecular data including studies from DNA sequences from chloroplasts gene rbcL. Therefore, modern system of plant taxonomy, that is, Angiosperms Phylogeny Group (APG) system is a molecular-based systematics which retains order and families of Linnean systems and includes groups which are monophyletic. APG I was published in 1998 which was followed by APG II in 2003 (Chase et al., 2003) and APG III in 2009 (Bremer et al., 2009) and then APG IV in 2016. However, further development in molecular techniques, advancement in techniques related to metabolomics and proteomics is exploring the molecular phylogenetics which will form foundation of evidence-based classification of flowering plants.
Evolutionary evidences suggest that basal angiosperms which are characterized by absence of xylem vessels are primitive, however, some recent phylogenetic analysis reported that Amborella trichopoda is sister to all extant angiosperms and is at the base of angiosperms phylogenetic tree. They are composed of only few species which include many aquatic plants like water lilies (Figure 1.1), Amborella and star anise. Core angiosperms are represented by monocots and core eudicots. They include three major groups including monocots, eudicots and magnoliids, and the latter group was once considered to be dicots but now it is placed in a separate group. Important magnoliids include plants like avocado, black pepper, magnolia, nut-meg, bay leaf, tuliptree or yellow poplar.
Figure 1.1 (a-b) Basal angiosperms, (a) Nymphaea alba from family Nymphaeaceae, (b) Magnolia sp. is another basal angiosperm which belongs to family Magnoliaceae.
Eudicots also known as true dicots, composed of more than 75% of angiosperms and are characterized by their monophyletic origin and presence of tricolpate pollens (having three apertures). This group of angiosperms represents abundant clade of angiosperms. Figure 1.2 shows a cladogram of flowering plants based on information from APG I, II and III. A cladogram represents an evolutionary diagram which is used to explain evolutionary relationships within a group of related organisms which share common ancestors. Orders of basal angiosperms (Amborellales, Nymphaeales and Austrobaileyales) represent primitive groups whereas core eudicots are represented as advanced or modern group of flowering plants. Magnoliids like Laureales, Magnoliales, Canellales and Piperales are evolved with monocots. Eudicots represent abundant group of flowering plants, among which core eudicots include two highly evolved and diverse clades which evolved separately are asterids (lamiids and campanulids) and rosids (fabids and malvids) (based on APG III) which are classified on the basis of their tendency to produce fused or free petals (Figures 1.3 and 1.4). Evolutionary traits, apomorphies, which are important in classification are represented where the origin of a clade takes place. Eudicots represent group of many economically important plants like members of family Apiaceae, Asteraceae, Brassicaceae, Cucurbitaceae, Fabaceae, Malvaceae, Rosaceae and Solanaceae.
Figure 1.2 A cladogram of angiosperms based on information from the Angiosperms Phylogeny Group (APG III, 2009) (Bremer et al., 2009).
Figure 1.3 (a-e) Rosids (fabids and malvids) are characterized by the presence of free petals (a) Quisqualis indica, (b) Chamelaucium uncinatum, (c) Millettia peguensis is an economically important plant with insecticidal properties and antiviral activities, (d) Tropaleum majus is an ornamental member of family Tropaeolaceae, and (e) Rosa sp. which belongs to Rosaceae is one of the popular ornamental and medicinal shrub.
Figure 1.4 (a-d) Asterids (lamiids and campanulids) are core eudicots which are differentiated from other eudicots due to the presence of fused petals (a) Petunia hybrid, (b) Daisy, (c) Lycopersicon esculentum, (d) Duranta erecta.
Other main group of flowering plants, that is, monocots represent one of the highly evolved clade with monophyletic origin (Figure 1.5). They are characterized by presence of only one cotyledon, non-woody stem, fibrous roots, long and slender leaves with parallel venation and scattered vascular bundles. They produce inconspicuous, mostly non-fragrant flowers with floral parts in multiple of three often which are arranged to form a spikelet in case of grasses. Table 1.1 shows comparison of monocots and eudicots. Commelinid clade represents most derived group of angiosperms which includes many plants from Arecales, Commelinales, Poales and Zingiberales. Monocots include palms, orchids and grasses which evolved about 60 millions years ago and are composed of almost 10,000 species. Fossils of palms and members from arum family are the oldest known monocots which are reported to found in rocks almost 100 millions years old. Monocots include many economically important plants which make our staple food like all cereals and grasses are monocots. They are important source of biofuel and...
