Andrew Hudson, University of Edinburgh, Edinburgh, UK Christopher Jeffree, University of Edinburgh, Edinburgh, UK Leaves of different species show wide variation in morphology and anatomy, usually associated with specialized roles in photosynthesis. Formation of leaves, from naive meristematic cells at the growing shoot tip, differs subtly in monocotyledonous and dicotyledonous plants, although it appears to involve conserved gene functions. Introduction Leaves are the most variable of plant organs. They differ widely in shape, size and anatomy between different species, and even within individual plants. Most leaves are specialized photosynthetic organs, but others are adapted to different roles as, for example, spines or scales for protection, tendrils for support, or the traps of insectivorous plants. Although differing structurally, leaves share a number of characters that distinguish them from other organs of the plant. 1. They occur on the sides of stems and (together with leaf-like parts of the flower) are therefore termed lateral organs. 2. Unlike the shoot, they have a limited capacity for growth. 3. They are associated with secondary meristems (axillary meristems) that form at the junction between the upper (adaxial) part of the leaf and the stem and allow branching of the shoot. 4. Most leaves show dorsoventral asymmetry. They are usually flattened and may also have different tissues in their upper (adaxial) and lower (abaxial) parts. Leaves probably evolved in a common ancestor of euphyllous plants (e.g. flowering plants, conifers and ferns). Although the leaf-like organs of more primitive plants (e.g. mosses) have similar photosynthetic functions, they probably arose independently on more than one occasion. Parts of the Monocot and Dicot Leaf The leaf of a dicotyledonous plant (dicot) typically consists of a flattened leaf blade joined to the stem by a narrower petiole (Figure 1a). The petiole is usually continuous with the major central vein of the leaf (the midrib) and no distinct boundary may be apparent between petiole and blade, or between the lower (abaxial) petiole and the stem. However, specialized structures may form at the base of the petiole allowing leaf movement or loss under unfavourable conditions (as occurs in deciduous trees). The blade of a. Introduction Introductory article Article Contents. Parts of the Monocot and Dicot Leaf. Physiology and Function. Control of Leaf Initiation. Growth and Patterning of the Leaf. Cell Type Specification in the Leaf. Heteroblasty dicot leaf usually has a net-like vascular system, in which veins branch and rejoin. Major veins are usually thicker than the surrounding blade tissue. The blade is often similar in composition along its length and width, although its edge may form specialized structures, such as spines. In contrast, many leaves show an asymmetric distribution of tissues along the depth of the blade, with palisade mesophyll cells towards the upper (adaxial) surface and spongy mesophyll cells below. Further differences are often seen between epidermal cells of the adaxial and abaxial surfaces. Many dicots produce compound leaves with a number of individual leaflets on a common stalk (rachis) (Figure 1b). Each leaflet resembles a simple leaf in structure and development (although it has no axillary meristem associated with it). In addition, both simple and compound leaves can form blade-like outgrowths (stipules) from the base of the petiole (Figure 1b). Compound leaves have probably evolved from simple leaves on a number of occasions, and simple leaves may also have arisen by reduction of compound leaves. Some species are able to produce both simple and compound leaves during their lifetimes. The leaves of monocotyledonous plants (monocots) differ from dicots in several respects. 1. Many monocot leaves are sword shaped and lack a narrower petiole (Figure 1c). The basal part (the sheath) tightly encircles the stem and may overlap at its margins or form a tube, but the blade is usually free. Specialized structures are found at the sheath blade boundary: the ligule, a membrane or fringe of hairs that form a seal between the adaxial leaf and stem, and the auricle, which can act as a hinge between sheath and blade (Figure 1d). 2. All but the most minor veins run parallel to each other along the long axis of the leaf and rejoin only near its tip. Other tissues (e.g. epidermal hairs or stomata) may be arranged in similar longitudinal stripes. 3. Monocot leaves tend to show less differentiation between adaxial and abaxial tissues. ENCYCLOPEDIA OF LIFE SCIENCES 2001, John Wiley & Sons, Ltd. www.els.net 1
2 Leaf and Internode
Although these generalizations are valid for the leaves of grass-like monocots (e.g. maize), other monocots have dicot-like leaves with broader blades, veins that branch laterally from a central midrib, and more obvious differentiation of adaxial and abaxial tissues (Figure 1d). This has led to the suggestion that ancestral monocots had leaves similar to dicots, and that grass-like leaves arose by reduction of the dicot-like blade and increased growth of a region closer to the stem. Compound leaves in monocots are found only in palms. Unlike compound dicot leaves, these form from a single primordium, which becomes compound as cells between leaflets die late in development. Physiology and Function Most leaves are specialized photosynthetic organs and show adaptations to light harvesting and gas exchange (uptake of carbon dioxide, loss of oxygen). Their flattened shape presents a large area to incident light. Palisade mesophyll cells are responsible for most of the photosynthetic activity of the leaf. They are located adaxially (and therefore usually towards the light), contain numerous chloroplasts and have a large proportion of their surface area exposed for gas exchange (Figure 2a). Spongy mesophyll cells, although also photosynthetic, have fewer chloroplasts, but are separated by more extensive air spaces. The exposed surface area of mesophyll cells may therefore exceed the external surface area of the leaf by almost 20 times. Exchange of gases between the internal air spaces and the external atmosphere occurs through pores (stomata) in the epidermis (Figure 2b). In many plants, stomata are more frequent in the abaxial epidermis. Each pore is bounded by a pair of specialized epidermal cells (stomatal guard cells) that regulate its aperture. An increase in guard cell turgor pressure occurs in conditions favourable for photosynthesis (light or depletion of internal carbon dioxide by photosynthesis) causing stomata to open. Water stress or high internal carbon dioxide cause guard cells to lose turgor and stomata to close. Therefore the plant can balance the requirement for photosynthetic gas exchange with water loss by transpiration. Gas exchange and water loss through other epidermal cells is limited by a thickened external cell wall impregnated with the fatty polymer, cutin, and a hydrophobic surface layer (the cuticle) containing cutin and waxes (Figure 2b). The cuticle also reduces wetting of the leaf (e.g. by rain) and forms a barrier against attack by pathogens. Many leaves produce epidermal hairs (trichomes) consisting of one or more specialized cells. Many trichomes are branched, hooked or produce sticky or toxic compounds as a defence against pests (particularly insects) (Figure 2c). They may also protect against damage by UV light or reduce water loss by trapping a layer of still air around the leaf surface. The vascular tissues of the leaf resembles those in the rest of the plant, consisting of xylem, phloem and associated cells. Xylem is responsible for supplying the leaf with water and dissolved inorganic compounds. Phloem supplies the developing leaf with organic compounds, and exports excess products of photosynthesis from mature leaves (usually in the form of sucrose). The vascular cells of minor veins are surrounded by a single layer of photosynthetic bundle-sheath cells. In C 4 plants (e.g. maize) bundle-sheath cells are responsible for fixation of carbon dioxide that is produced from organic acids (usually malic) imported from neighbouring mesophyll cells. The bundle-sheath cells of C 4 plants are large, have large chloroplasts and are in intimate contact with neighbouring mesophyll cells a characteristic arrangement termed Krantz anatomy (Figure 2d). ControlofLeafInitiation The position at which leaves occur on a stem is termed a node and the stem tissue separating neighbouring nodes an internode. Leaves occur either singly or in groups at each node. Because the rate of leaf initiation and growth is affected by environmental conditions, leaf age is conveniently measured in plastochrons one plastochron being the time between initiation of leaves at successive nodes. Each leaf arises from a group of initial cells within the flank of the shoot apical meristem. The initials form a primordium growing in a new axis (Figure 3a), while surrounding cells form either stem tissues or axillary meristems. Leaf initials are present in at least four cell layers of the dicot meristem and may differ in number between species (e.g. 100 cells at primordium initiation in Arabidopsis thaliana, 150 in tobacco). Surgical experiments suggest that the identity of leaf initials is specified at least one plastochron before they form a primordium and that existing primordia may produce an inhibitory signal that prevents adjacent meristem cells assuming leaf fate (thus explaining the regular spacing of leaves on stems termed phyllotaxy). Repression of leaf fate in the meristem requires homeobox transcription factor genes of the knotted1 family, which are expressed in the meristem and stem initials, but excluded from leaf initials before primordium initiation. Conversely, MYB transcription factor genes of the phantastica family repress Figure 1 Parts of the monocot and dicot leaf. (a) A simple leaf of the dicot, Antirrhinum majus (snapdragon). (b) A compound leaf of the dicot Pisum sativum (garden pea). (c) Part of the grass-like monocot leaf of Zea mays (maize). (d) The broad monocot leaf of Spathiphyllum wallisii. 3
Figure 2 Leaf anatomy. (a) A section of the leaf blade of bean (Phaseolus vulgaris) showing adaxial and abaxial epidermal cell layers (e), a single layer of palisade mesophyll cells (pm) and several layers of spongy mesophyll (sm). This picture is of a mature leaf that was frozen and then broken before viewing in a scanning electron microscope. Bar, 50 mm. (b) The abaxial epidermis of a wheat leaf blade with a pair of stomatal guard cells (gc). The cuticle formed on the leaf surface includes numerous wax crystals. Key: e, epidermal cell; sa, stomatal aperture. Bar, 20 mm. (c) The abaxial epidermis of an immature bean leaf with both hooked and glandular trichomes. Labelled as in (a). Bar, 100 mm. (d) A section through a maize leaf showing typical Kranz anatomy associated with C 4 photosynthesis. This stained section was made perpendicular to the long axis of the leaf. Key: e, epidermis; m, mesophyll; bs, bundle sheath; v, vascular cells. Bar, 50 mm. Photograph provided by Jane Langdale, University of Oxford. knotted1-like genes in leaf initials and promote primordium initiation. Growth and Patterning of the Leaf The primordium of a dicot leaf flattens after initiation as the blade grows laterally. Fate mapping has shown that division of all cells in the tobacco leaf primordium contribute to growth in length and width and there are no specialized meristematic regions as found at shoot apices. Growth in width might be controlled by interaction between adaxial and abaxial domains of the primordium, because mutations that affect the identity of one of these domains tend also to prevent lateral growth. Although the primordia of monocot leaves differ from dicots in being flattened at emergence (Figure 3b), analysis of mutants has revealed that similar adaxial abaxial interactions might recruit meristem cells to form the margins of leaf primordia, giving them their characteristic flattened shape. Differences between the adaxial and abaxial parts of a dicot leaf are specified before primordium initiation, and a number of regulatory genes needed for either adaxial or abaxial cell fate in Arabidopsis thaliana show patterned expression in leaf initials at, or before, primordium formation. Analysis of their mutant phenotypes has 4
Figure 3 Leaf initiation. (a) The shoot apex of the dicot, Antirrhinum majus, showing the shoot apical meristem (SAM) and primordia of leaves formed from it (p). Leaves are produced in opposite pairs and numbered according to increasing age. (b) The apex of the monocot, barley. Leaf primordia (p) are more flattened than those of dicots, produced singly, and encircle the whole meristem. Bars, 100 mm. suggested (1) that adaxial and abaxial fates are mutually exclusive, and (2) that adaxial identity promotes formation of the axillary meristem at the junction between leaf and stem. The mechanisms specifying differences along the length of the leaf have been studied in the monocot maize, in which the ligule and auricle mark the boundary between sheath tissue and the blade further towards the tip (Figure 1c). The differences must be specified early because ligule formation is visible by the time the leaf is 2 3 plastochrons old (about 1 mm in length). Although knotted1-like homeobox genes are normally expressed only in the meristem, they might be responsible for patterning the long axis of the leaf because misexpression in leaves causes sheath tissue to develop in place of the blade. Specification of this axis directs expression of genes in particular regions, for example liguleless genes that are needed for ligule formation in cells around the sheath blade boundary. Cell divisions usually cease first at the tip of the leaf, and last in the basal region. However, the duration of division also varies for different cell types at similar positions (e.g. trichomes often stop dividing and differentiate before neighbouring epithelial cells). Most of the growth in area of the leaf is caused by expansion of cells after they have ceased division. Control of compound leaf architecture appears to involve different mechanisms in pea and tomato, consistent with the view that compound leaves arose independently in different plant groups. Tomato leaves are exceptional in expressing knotted1-like homeobox genes, and elevated homeobox gene expression can cause an increase in compounding. Unlike tomato, pea leaves do not express knotted1-like genes and require activity of a LEAFY-like transcription factor (which also specifies flower formation) to form a compound, rather than simple structure. Cell Type Specification in the Leaf Specification of two epidermal cell types stomatal guard cells and trichomes has been analysed in detail in Arabidopsis. Each pair of guard cells is produced from an epidermal precursor by several rounds of unequal cell division. The smallest daughter cell divides again to form a pair of guard cells while the surrounding daughter cells assume different fates. Although this pattern of division can explain spacing of stomata, a different mechanism has been found to operate in spacing of trichomes. Mutations in the TRYPTICHON (TRY) gene allow adjacent epidermal cells (which may be descended from different precursors) to differentiate as trichomes, suggesting that trichome initials normally repress trichome fate in their neighbours by TRY-dependent signalling. A number of 5
other transcription factor genes are expressed specifically in trichome initials, and are required for normal trichome initiation, growth and branching. Specification of vascular tissues in the leaf is poorly understood, although indirect evidence suggests involvement of the phytohormone auxin. High levels of auxin are proposed to promote both vascular cell fate and to increase uptake of auxin from surrounding tissues, therefore inhibiting vascular fate in neighbouring cells. Heteroblasty Because of similarities in their structure and development, leaves are considered equivalent to cotyledons (seed leaves) and floral organs. This has been supported by analysis of genes that specify the difference between these organs. For example, mutations that reduce activity of the LEAFY COTYLEDON gene in Arabidopsis thaliana allow cotyledons to develop with leaf-like characters. Similarly, loss of activity of floral homeotic genes, which together specify the identity of floral organs, leads to production of flowers consisting only of leaves. Most plant species also produce foliage leaves with different forms (a phenomenon termed heteroblasty). In most species the transition between forms occurs gradually as the plant matures. Genes that regulate maturation (also termed phase change), and therefore leaf form, have been identified in maize and A. thaliana. For example, reduction in activity of the Arabidopsis gene, TERMINAL FLOW- ER1, causes more rapid phase change whereas increased expression slows it. In some cases, the transition between forms may be more abrupt, or involve more dramatic differences. For example, seedlings of gorse, Ulex europaeus, produce flattened leaves whereas more mature plants produce only needle-like spines. In other species, the transition may be regulated by environmental signals. Deciduous trees may switch from production of leaves to protective bud scales in response to day length and aquatic plants may produce different leaf forms when submerged in water or exposed to air: for example Ranunculus aquaticus produces compound leaves with filamentous leaflets when submerged, but lobed leaves above the water surface. Further Reading Bell A (1991) Plant Form. An Illustrated Guide to Flowering Plant Morphology. Oxford: Oxford University Press. Esau K (1977) Anatomy of Seed Plants. New York: John Wiley. Freeling M (1992) A conceptual framework for maize leaf development. Developmental Biology 153: 44 58. Hetherington AM (ed.) (1994) The Tansley Review Collections. 1: Leaf development and function. New Phytologist 128: 19 507. Steeves TA and Sussex IM (1989) Patterns in Plant Development. Cambridge, UK: Cambridge University Press. 6