A plant leaf is an organ. in what way is a human lung similar to a plant leaf?

Other Groups

Living plant leaves are colonized by a large and diverse group of fungi. Those fungi are distributed widely, occurring on a wide range of plants in different climates, although Dix and Webster (1995) distinguished between fungi that are found on angiosperm leaves and those that occur on the leaves of gymnosperms. Some of the early colonizers are saprobic or weakly parasitic fungi that are restricted to the leaf surface until the leaf becomes senescent, at which time they colonize other leaf tissues as well. Among the earliest colonizers of angiosperm leaves are yeasts; red yeasts in the genera Sporobolomyces and Rhodotorula, and white yeasts, including Cryptococcus species, are common (Dix and Webster 1995). Aureobasidium pullulans is a ubiquitous and cosmopolitan filamentous fungus that occurs on angiosperm leaf surfaces in early summer and autumn in temperate regions. In culture, this fungus produces copious quantities of slimy conidial masses ranging in color from creamy white or pink to dark brown or black. Sclerophoma pythiophila, the anamorph of Sydowia polyspora, resembles Aureobasidium and is common on conifers (Dix and Webster 1995). Other common fungi on angiosperm leaves include species of Alternaria, Cladosporium, Botrytis, Epicoccum, and Stemphylium. The hyphae and reproductive structures of many leaf-colonizing fungi such as these are pigmented. Leaf-colonizing fungi in temperate regions tend to be more numerous on lower leaves and on more peripheral (further from the trunk) leaves in the canopy (Dix and Webster 1995). Fungal growth often is better on the abaxial (lower) than on the axial (upper) leaf surface because the former surface is more sheltered and has a higher number of stomata than the latter.

Common leaf-spotting microfungi in tropical, subtropical, and temperate regions include species of Alternaria, Ascochyta, Cercospora, Cladosporium, Corynespora, Phyllosticta, Pestalotia, and Pestalotiopsis (Cook 1975, 1978; Holliday 1980). Many produce lesions or other symptoms of infection on stems, flowers, and fruits as well as leaves. In temperate forests, common leaf-spotting fungi on broad-leaf trees and shrubs belong to the genera Coccomyces, Discula, Gnomonia, Mycosphaerella, Rhytisma, and Venturia (Funk 1985). The life cycle of a typical foliage parasite of temperate trees involves the production of ascomata on infected leaves either during the fall or early spring. Ascospores are discharged about the time that the new foliage appears in the spring. The fungus overwinters as a saprobic mycelium or as ascomata in infected leaves or twigs to complete the cycle (Manion 1981).

The presence of foliar pathogens is recognized macroscopically by water-soaked areas; distortion; or more commonly, chlorotic and/or necrotic lesions on the leaves. Occasionally, the fungi will cause total necrosis or shriveling of the leaves. To identify a fungus, it is usually necessary to examine its reproductive structures under a compound light microscope. Fungi causing foliar diseases on plants commonly produce reproductive structures such as acervuli, pycnidia, perithecia, or apothecia in association with the necrosis. If fungal reproductive structures are not present on the leaves or the reproductive structures are not mature, it may be necessary to incubate the leaves in a moist chamber (see “Standard Ground-Based Techniques,” later in this chapter). Keys and descriptions for genera and species of fungi occurring on leaves of temperate trees, shrubs, and woody climbers can be found in Ellis and Ellis (1985), and the same for foliar fungi of trees of western North America can be found in Funk (1985).

Parasitic fungi occurring on grasses in temperate regions include the ascomycetes Gaeumannomyces, Gibberella, Monographella, Phyllachora, and Pleospora (Stemphylium anamorphs) as well as the specialized groups of pathogens previously noted. The common hyphomycetous fungi on grasses include Alternaria, Cladosporium, Drechslera, Fusarium, Pseudocercosporella, Rhynchosporium, and Ulocladium species. Common coelomycetes on grasses include Ascochyta and Colletotrichum. Few parasitic fungi occurring on living rushes and sedges in temperate areas have been reported other than rusts, smuts, and members of the Clavicipitaceae. Included among those fungi are the ascomycete Didymella, the hyphomycete Pseudocercosporella, and the coelomycetes Ascochyta and Colletotrichum. For keys, additional literature references, and descriptions of taxa see Ellis and Ellis (1985).

Common foliar parasites of grasses in tropical regions include species of Cercospora, Drechslera, Magnaporthe (anamorph Pyricularia), Rhynchosporium, and Sphaerulina. Pyricularia species are particularly important in the tropics as parasites of rice, other cereals, and grasses. Pyricularia grisea (synonym, P. oryzae; teleomorph, Magnaporthe grisea), the causal agent of rice blast, is an important pathogen in many rice-growing countries. The fungus also parasitizes more than 50 other species of grasses and sedges (Ou 1985). Symptoms consist of lesions on leaves, nodes, panicles, and grain. The leaf spots are elliptical with more or less pointed ends and grayish centers with dark margins. They begin as small, water-soaked areas and develop into lesions of up to 1.5 × 0.5 cm. Heavy spotting or infection at the tillering stage can kill the host (Holliday 1980).

Needle-cast fungi occur in temperate regions where conifers (gymnosperms) are common. Many needle-cast diseases are caused by facultative parasites in the Rhytismataceae (Ascomycota). This family comprises 43 genera and 344 species, some of which fruit on foliage and stems of angiosperms (Cannon and Minter 1986). Species commonly causing needle blights and casts of conifers are members of genera such as Lophodermium, Lophodermella, Elytroderma, Hypodermella, and Lirula (Hunt and Ziller 1978; Sinclair et al. 1987 and references therein); they often produce symptoms and initiate fruiting bodies in the first growing season. However, those fruiting bodies often do not sporulate until the foliage is a year old or older. Hence, symptoms such as chlorotic, discolored foliage and repeated defoliation (only current-year needles remain on the tree) may be readily observed, but fruiting bodies collected on current-year needles could be immature. It may be necessary to collect older foliage remaining on the tree or to augment the collection with the previous year's cast foliage to make species determinations.

17.3.1 Control of Leaf Development by the GRFs

In dicotyledonous plants, leaf primordia are initiated at the peripheral zone of the vegetative SAM (Figure 17.3A). Initially, they have a rod-like structure that soon takes on dorsoventral polarity to constitute a flat lamina consisting of two anatomically distinct surfaces: the adaxial and abaxial sides (reviewed in Rodriguez et al., 2014). In the next step, the leaf lamina expands to acquire its final size and shape. First, cell proliferation occurs throughout the small-leaf primordium, which can be visualized using reporters of mitotic cyclins such as CYCLINB1;1-GUS (Donnelly et al., 1999; Figure 17.3B). As the organ grows, the region containing proliferative cells becomes restricted to the base of the organ, and the cells located in the distal part of the leaf begin their expansion (Figure 17.3B). Finally, cell proliferation ceases and the leaf lamina continues to grow only by cell expansion. A similar spatial organization of the processes contributing to organ growth occurs in monocotyledonous plants. In a growing maize leaf, cells divide only in the division zone at the base of the leaf, while they expand at the more distally located expansion zone. In both dicots and monocots, the size and persistence of the division zone is one of the main factors determining final leaf size (Nelissen et al., 2012; Rodriguez et al., 2010; Ferjani et al., 2007; Vercruyssen et al., 2014).

Arabidopsis mutants in AtGRFs have small leaves with fewer cells (Kim et al., 2003, 2012; Kim and Kende, 2004; Figure 17.1B). Additionally, similar phenotypes are found in plants overexpressing miR396 (Rodriguez et al., 2010; Liu et al., 2009; Figure 17.3C) or in gif1/an3 mutants. The leaf size seen in gif1 is significantly reduced as a result of combination with gif2 or gif3 mutants (Lee et al., 2009). Detailed analysis of these mutants indicated that cell proliferation is prematurely terminated (Rodriguez et al., 2010; Ferjani et al., 2007).

On the other hand, increasing GRF levels by overexpression of GRF1, GRF2, or GRF5 under strong promoters (Horiguchi et al., 2005; Kim et al., 2003; Gonzalez et al., 2010), or by introducing miR396-resistant alleles of GRF2 or GRF3 (Rodriguez et al., 2010; Debernardi et al., 2014), produced bigger leaves with more cells (Figure 17.3D). Overexpression of GIF1 also increased leaf size (Kim and Kende, 2004) and, in agreement with its function as a coactivator, synergistically boosted the effect of rGRF3 and GRF5 on leaf size (Debernardi et al., 2014).

In young developing leaves, miR396 and GRFs have opposing gradients of expression. MiR396 is expressed in a gradient along the longitudinal axis of the organ, with higher expression at the distal part (Figure 17.3B). In turn, miR396 represses GRFs, generating an opposing gradient of expression with higher levels in the proximal part of the organ (Figure 17.3B). Posttranscriptional regulation of GRFs by miR396 results in the expression of GRFs in cells undergoing mitosis (Debernardi et al., 2012; Rodriguez et al., 2010). Expression of AtGIFs is also restricted to the proximal part of leaf primordia, as expected from their function as GRFs coactivators (Ha Lee and Hoe Kim, 2014).

Similar defects in leaves and comparable GRF, GIF, and miR396 expression patterns in relation to cell proliferation were obtained in monocotyledonous plants (Liu et al., 2014; Candaele et al., 2014), indicating a conserved function for GRFs in leaf development across plant species.

Altogether these results indicate that the miR396–GRF–GIF network controls leaf organ growth after primordia initiation. In developing leaves, GRF–GIF protein complexes promote cell proliferation in the proximal part of the leaf, while miR396 restricts the activity of the complexes by posttranscriptionally repressing GRF expression in the distal part, where cell expansion and differentiation occur.

Overexpression of miR396 in sensitized backgrounds such as rdr6 or as1/as2 brought about plants with lotus-like or needle-like leaves with loss of their adaxial side (Mecchia et al., 2013; Wang et al., 2011a). These phenotypes were partially rescued by a miR396-insensitive allele of AtGRF9 (Wang et al., 2011a). Moreover, AtGRF2 expression was preferentially detected in the adaxial side of the leaves (Mecchia et al., 2013). The results suggested that the miR396–GRF–GIF network might also play a role in dorsoventral polarity of the organ after primordia initiation.

4.15.4.2 Aroma Products from Leaves and Allied Plant Materials

Essential oils from plant leaves are main ingredients in many products.100 Geranium oil serves as a source of (S)-citronellol (9), obtained by saponification and subsequent fractional distillation. Eucalyptus is classified into two groups – the globulus type and citriodora type – which differ considerably in terms of their composition. Peppermint is cultivated mainly in the United States and also in China and India. Another species usually referred to as Japanese mint is grown in China, Brazil, India, and Japan and is used for l-menthol (7) isolation. Spearmint oil is a good modifier of peppermint and two species (Native and Scotch) are recognized as important. The main producer is the United States while India and China have also emerged in recent years. Citronella also has two separate species in cultivation (Sri Lanka and Java type), of which Java type is particularly important as an industrial source of (R)-citronellal (10). Likewise, lemongrass continues to be important as a citral (11) source, but to a less degree due to the advent of synthetic citral. Patchouli is cultivated in Sumatra, Seychelles, Madagascar, etc., and it is used in many men’s fragrances. Lavender is grown in southern France while Lavandin, a hybrid developed by crossing Lavandula officinalis with spike or aspic lavender (Lavandula latifolia), is cultivated in France and northern Africa. Lavandin was once regarded as a less expensive substitute of lavender oil but now has attained a firm position in its own right. It is inexpensive and reliable with consistent quality. Basil is one of the representative cooking herbs (especially in Italy and Thailand) and also an important F&F product source. Thyme is well known for its antibacterial and antifungal properties. Three species (two from Europe and one from Mexico) are known as oregano even though each of them belongs to a different genus, naturally exhibiting discrete odor profiles. Rosemary is cultivated in Spain, France, and Croatia, and besides being a cooking herb it also has a long history as a deodorant and pesticide, and nowadays its extract is used as an antioxidant agent in processed food (Table 5).

Table 5. Aroma extract from leaves

OilSource plantAroma-active constituentsReference(s)GeraniumPelargonium graveolens(S)-Citronellol, geraniol, (4S)-rose oxide95,96EucalyptusEucalyptus globulus1,8-Cineole95Eucalyptus citriodora(±)-Citronellal, (±)-citronellol95PeppermintMentha piperita(−)-Menthol, (−)-menthone (35), (−)-menthyl acetate (36)95,100Japanese mintMentha arvensis(−)-Menthol95SpearmintMentha spicata(R)-Carvone95,100CitronellaCymbopogon winterianus(R)-Citronellal, geraniol, (R)-citronellol95LemongrassCymbopogon flexuosisCitral95Cymbopogon citratusPatchouliPogostemon patchouli(−)-Patchoulol (37)95,96LavenderLavandula officinalis(R)-Linalool, (R)-linalyl acetate, (R)-lavandulyl acetate (38)95,96BasilOcimum basilicum(R)-Linalool, estragole (39)95,100ThymeThymus vulgarisThymol (40), carvacrol (41)95,100OreganoOriganum vulgareCarvacrol, thymol95,100Coridothymus capitatusCarvacrol, thymol95,100Lippa graveolensThymol, carvacrol, 1,8-cineole95,100RosemaryRosmarinus officinalis1,8-Cineole, (+)-camphor (42), (−)-borneol (43)95,100

IX. Chlorophyll Degradation and Interconversion of Chlorophylls a and b

In senescing higher plant leaves, chlorophylls are degraded by a specific pathway involving enzymes that are elevated in senescing leaves (Hörtensteiner, 2006). Obviously, unicellular algae like Chlamydomonas do not undergo organ senescence. Nevertheless, there are circumstances where chlorophyll degradation would be advantageous by preventing the accumulation of photosensitizing free chlorophyll. These circumstances include periods of nutritional deficiency, when nonessential parts of the photosynthetic apparatus (e.g., LHCs) are selectively degraded so that the protein amino acids can be redeployed for more pressing needs. Also, during normal photosynthesis, labile chlorophyll protein complexes, such as the D1 subunit of PS II, are turned over.

The first step of chlorophyll a is catabolism in plants is hydrolysis of the phytyl ester bond by chlorophyllase (Table 20.1) to form chlorophyllide a and phytol. Although it had been thought that chlorophyll degradation is irreversible once it had been dephytylated, later evidence indicated that in Synechocystis sp. PCC 6803 a portion of the hydrolysis products can be recycled to form new chlorophyll a molecules (Vavilin and Vermaas, 2007).

In the second step of chlorophyll degradation, chlorophyllide a is demetalized to form pheophorbide a. An enzyme that catalyzes the demetalation has not been identified, and the reaction might occur nonenzymatically. Next, the macrocycle ring of pheophorbide a is opened at the 5 position by oxygen-dependent pheophorbide a monooxygenase (Hörtensteiner et al., 1998). The open-chain product (a bilin) is then modified in several ways and ultimately transported to the vacuole (Hinder et al., 1996). Chlorophyll b catabolites are not detected in senescing leaves (Kräutler and Matile, 1999), and pheophorbide a monooxygenase does not accept pheophorbide b as a substrate (Hörtensteiner et al., 1995). These observations suggest that catabolism of chlorophyll b must begin with its conversion to chlorophyll a.

In vitro conversion of chlorophyll b to chlorophyll a was detected in preparations from a mutant barley strain that is unable to form chlorophyll b, indicating that conversion of chlorophyll b to chlorophyll a must occur by a different process than reversal of the reaction that synthesizes chlorophyllide b from chlorophyllide a (Rudoi and Shcherbakov, 1998). Conversion of chlorophyll b to chlorophyll a in extracts of cucumber cotyledon etioplasts required both the membrane and stromal fractions as well as ATP (Ito et al., 1994). The ATP was probably required to form reduced ferredoxin (see below). A likely intermediate in the reduction of chlorophyll b is 7-hydroxymethyl chlorophyll a. NADPH, but not ATP, was required for reduction of chlorophyll b to 7-hydroxymethyl chlorophyll a by intact barley etioplasts and etioplast membranes (Ito et al., 1996; Scheumann et al., 1996). In lysed etioplasts, reduced ferredoxin was required for the reduction of 7-hydroxymethyl chlorophyll a to chlorophyll a (Scheumann et al., 1999). The reverse reaction, conversion of chlorophyll a to chlorophyll b, was not detected in these systems. Chlorophyllide b was also a substrate in the lysed barley etioplast system, being converted to chlorophyllide a (Scheumann et al., 1999).

In the green alga Chlorella protothecoides, degradation products of both chlorophyll a and chlorophyll b are excreted into the medium, which indicates that in contrast to senescing plant leaves, this alga can degrade chlorophyll b without its having to be converted to chlorophyll a (Gossauer and Engel, 1996). Chlorophyll degradation products that resemble bilins were excreted into the medium from a chlorophyll b-less Chlamydomonas mutant (Doi et al., 1997). The structures of these products have not been completely identified. When held under anaerobic conditions, Chlamydomonas cells accumulate the 132-demethoxycarbonylation product of pheophorbide a, pyropheophorbide a (Doi et al., 2001). The pyropheophorbide a accumulates presumably because its conversion to bilins requires oxygen, as does the formation of bilins in the catabolism of chlorophylls in plants (Hörtensteiner, 2006).

A Chlamydomonas enzyme has been characterized that removes the 132-methoxycarbonyl group of pheophorbide a to form pyropheophorbide a (Suzuki et al., 2002). This enzyme, named pheophorbide demethoxycarbonylase, has a somewhat different reaction mechanism than the pheophorbidase enzymes that have been described in a few plants (Suzuki et al., 2002), and it appears to be restricted to green algae. The role of pheophorbidases in plants is not known, but the pheophorbide demethoxycarbonylase in Chlamydomonas is probably involved in chlorophyll catabolism. Pheophorbide demethoxycarbonylase is present in only trace amounts in light-grown cells, but it is induced when cells are kept in the dark for 3 days (Doi et al. 1997). The enzyme has been extracted from cells and characterized as a 170-kD homodimer. Its ability to use pheophorbide b, in addition to pheophorbide a, as a substrate has not been reported. A Chlamydomonas gene for this enzyme has not been identified, nor has a gene for a ring-opening pheophorbide oxygenase. However, an expressed putative chlorophyllase gene has been identified (Table 20.1).

The ability of plants to interconvert chlorophylls b and a suggests the possible existence of a “chlorophyll cycle,” which might function to adjust the chlorophyll a/b ratio in response to changing external conditions (Ito et al., 1996; Ohtsuka et al., 1997). At this time, there is no evidence to suggest that such a cycle operates in Chlamydomonas.

Evidence for Tapeworm Expulsion and Leaf Swallowing in Other Animals

Observations of undigested plant leaf material and large masses of tapeworms in the dung of Alaskan brown bears by Barrie Gilbert (Department of Fisheries and Wildlife, Utah State University) suggest some striking similarities with African apes in both mechanism and function. Gilbert observed what first appeared to him as anomalous plant ingestion in the fall months just prior to hibernation by Alaskan brown bears during a 6-year behavioral study at Brooks River in Katmai National Park, Alaska.

During the spring in coastal regions, a brown bear’s diet consists 100% of estuarine sedge. Carex sp. (Cyperaceae) was the species ingested before salmon arrived. At this time of year, sedge consumption is highly correlated with an increased phase of protein content and low structural cellulose. In the fall, however, foraging is consistently on high fat-content items, definitely eliminating sedge, which is rather inefficiently digested, even in its least woody phase of growth in spring. In the fall, Carex sp. is extremely high in fiber tissue, rather sharp edged and coarse. At this same time, large dung masses, almost completely composed of long tapeworms, are often observed. The tapeworms appeared to be expelled almost in entirety.

These observations lead Gilbert to question what an animal would suffer in energy loss, etc., if it went into hibernation for 6–7 months with a gut lined with tapeworms. He presumed that, as the temperature of bears is within a few degrees of normal during hibernation, gut parasites would not become particularly inactive. Bears eat prodigious quantities of decomposed salmon, making their liquid feces rich in digestible nutrients. With the hyperphagia (excessive eating) of bears in fall and the volume of liquid salmon in their guts, tapeworms would flourish and thus be a distinct liability during hibernation. However, parasite levels in hibernating bears are reportedly quite low.

What Gilbert proposed is that the coarse leaf material functions as a rasping plug or abrasive pad to scrape off the scolices (head of tapeworm). In effect, if a bear were to ingest coarse leaf material prior to going into hibernation, this might help to decrease its tapeworm load. This mechanical action would account for the observation of dung composed almost completely of tapeworms. Previously, the loss of tapeworms each year before hibernating and reinfection in the spring has been ascribed to a change in diet. This suggests that the manner in which items of no nutritional value are ingested is of critical importance.

John Holmes (Professor Emeritus, Department of Biological Sciences, University of Alberta) has noted a similar phenomenon in Canadian snow geese. In the summer prior to their migration south, juvenile birds in particular, carry significant tapeworm burdens. Also at this time of year, Holmes observed large boluses of undigested grass and tapeworms in goose dung. When the parasite loads of these flocks were measured after migrating south, their mid-lower guts were found to be completely clean, with no tapeworms attached. Geese may be ridding themselves of competitors for limited energy stores needed to complete the long migration to their wintering grounds.

18.1 Introduction

Whole plant or parts of plant (leaves, fruit, roots, bark, flower, etc.) which provide health-promoting or curative properties are generally known as herbal medicine or phytomedicine. Herbalism implies the alternative health care outside conventional medicine. Advances in clinical research and improvement in quality control and analysis have contributed to recent resurgence of the use of alternative and traditional medicinal system and further establishing its importance in treating and preventing diseases (Ekor, 2014). Herbal products are available in both commercial and crude preparations, the latter being more often used in the developing countries and are formulated as mixture (Alissa, 2014). Plant-based medicines have been used long before recorded history; Chinese, Egyptian, and Indian systems of alternative and complimentary describe uses of plants since ages (Pan et al., 2014; Jaiswal et al., 2016). In India only, more than 40% of the total 17,000–18,000 flowering plants find their usage documented in traditional medicine systems. The global market for Indian traditional medicine is estimated to 120 billion USD, reflecting the high demand for Indian natural products (Jaiswal et al., 2016). It is estimated that about 4 billion people (about 80% world population), at least up to some parts of their primary health care, rely on herbal medicinal products (Ekor, 2014). Herbal products are also now becoming mainstream in the developed countries like European countries, North America, and Australia (Braun et al., 2010; Anquez-Traxler, 2011; Blendon et al., 2013). The use of herbal products in health care continues to grow globally along with continuous introduction of new products; issues related with safety are also increasingly recognized (Ekor, 2014).

Medicinal herbs consist of diverse pharmacologically active compounds, which are responsible for its multi-target effects, in contrast to conventional drugs, that have simple composition and definite mechanism of action. While the drug–drug interaction has been well established in terms of resulting metabolic impact, very little is known about the herb–drug interaction and the available evidence is rather weak (Wang, 2015). The combination of herb and drug in most of medical practices has been proved to be beneficial; nevertheless, reports from some studies have suggested adverse reactions (Tsai et al., 2012). Clinical research reported many cases of adverse effect of herb–drug interaction, although majority of them are devoid of any severe consequences. Most of the reports on adverse effect of herb–drug interaction come from case reports, but with little information and of poor quality. The highest level of evidence regarding the herb–drug interactions comes from case reports coupled with pharmacokinetic trials (Tsai et al., 2012; Izzo et al., 2016).

Coadministration of herbal product with drug may result in cross reactivity of their components with the drug or result into alteration in pharmacokinetics of the drug (Alissa, 2014). Pharmacokinetic studies, necessary to evaluate the overall effect of drug, are very difficult to carry out for medicinal plants because of their complex phytochemical profile (Mazzari and Prieto, 2014). Moreover, the phytopharmacological profile of plants also vary with climatic conditions, postharvest methodologies, part used, etc., making it more difficult to determine the clinical pharmacokinetic and pharmacodynamic effects. Even, standardization for active compound does not rule out the possible variation in other constituent, which possibly results into altered bioavailability and pharmacological activity in human (Alissa, 2014).

Despite the high risk, there is scarcity of literature regarding the herb–drug interaction. This chapter aims to briefly discuss the impact and mechanism of interaction with drug and adulterant-mediated toxicity of some commonly used herbal medicine. In the later sections of this chapter, an attempt has been made to address the clinical implications of herb–drug interaction in patient with specific diseases.

Plant Tissue Biosensors

As discussed earlier, several fruits, plant leaves, and vegetables have been successfully used to develop tissue-based biosensors for a wide range of substances. Table 1 shows that a very diverse range of plant tissues have been used to develop various tissue-based biosensors with excellent detection limit and linear concentration range, commonly in the micromolar range. As indicated previously, the reproducibility of most of these biosensors is within 0.3–4.0% RSD and reported tissue-based biosensors for dopamine, glycolic acid, glutathione, and peroxide have lifetimes greater than 1 month and, in some cases, up to 4 months (Table 2). These various plant-tissue-based biosensors have been applied to the determination of various substances in a diverse range of sample materials, such as alcoholic beverages, river water, wastewater, urine, serum, whole blood, pharmaceutical preparations, cosmetic creams, vegetables, and fruits.

The information in Table 1 shows that plants that contain PPO such as apple, avocado, banana, coconut, mushroom and potato have attracted wide use for the construction of tissue-based biosensors for determination of atrazine, catechol, dopamine, and paracetamol. Also, plant materials that contain ascorbic acid oxidase (AAO), such as cabbage, cucumber, green zucchini squash, yellow crook neck squash, have attracted considerable interest in the development of tissue-based biosensors for the determination of ascorbic acid, glutathione, and organophosphorus pesticide. It is important to note here that while the determination of ascorbic acid was based on the direct biocatalytic effect of AAO, the measurement of glutathione and organophosphorus pesticide (ethyl paraoxon) was based on the inhibitory effect of these substances of the biocatalytic activity of AAO. Another common enzyme that is often used (Table 1) is tyrosinase, which was obtained from plant materials, such as mushroom and sugar beet, and used for the construction of tissue-based biosensor for the determination of mycotoxins, tyrosine, and phenolic compounds. It is also important to note that the use of some of the tissue-based biosensors for monitoring organic-phase biocatalytic reactions has been reported. Also, an increasing use of these biosensors for flow injection analysis (FIA) and as detectors for high-performance liquid chromatography has been reported. Figure 3 shows the successful use of a yeast-based carbon paste bioelectrode for the FIA measurement of different alcohols and the determination of alcohol in alcoholic beverages. Also, a rapidly responding tissue- and microbe-based carbon paste electrode has been successfully used as an electrochemical detector for liquid chromatographic determination of p-cresol, dopamine, and ethanol.

The successful exploitation of the multienzyme composition of plant tissues in eliminating interferences and, hence, improving the selectivity and stability of tissue-based biosensors have also been reported. The presence of AAO in zucchini has been successfully used to eliminate ascorbic acid interference from the determination of dopamine or norepinephrine. Also, the incorporation of papaya tissue into a carbon paste matrix has been effectively used to destroy surface active proteins.

Another area of interest is the use of tissue-based biosensors in bioreactors. Figure 4 shows the successful use of a coconut-tissue-based bioreactor for the determination of catechol in river water and wastewater. Evidently, catechol concentrations can be determined reliably in these samples by this approach down to the micromolar levels.

6 Conclusion and future perspectives

FA is a dietary compound present in various plant seeds and leaves, both in free form and covalently conjugated to polysaccharides, glycoproteins, polyamines, lignin and hydroxy fatty acids of plant cell walls. FA exhibits a wide variety of pharmacological activities, such as antioxidant, anti-inflammatory, antimicrobial, antiallergic, hepatoprotective, anticancer, antithrombotic effects, increased sperm viability, antiviral, vasodilatory actions, metal chelation, modulation of enzyme activity, activation of transcriptional factors, gene expression and signal transduction. Furthermore, recent in vitro and in vivo studies revealed that it has potential for prevention and treatment of numerous cancers, such as breast, lung, cervical, prostrate, hepatic, colon and pancreatic cancer. FA exhibits potential anticancer effect by inhibiting the growth of cancer cells through suppression of proliferation, initiation of apoptosis and cell cycle arrest as well as modulating various signaling pathways. Most of the cancer related studies have been performed in vitro with limited in vivo results, therefore the observations on the cancer prevention and treatment needs further investigation. It is essential to explore the pharmacological attributes and the mechanism of actions of metabolites for a better interpretation of FA as an anticancer agent. Still, the mechanisms underlying the antitumor potential of FA are not completely understood and more in vivo analysis is required. Therefore, further clinical studies on anticancer efficacy of FA are also warranted to investigate the systemic bioavailability of FA from dietary sources or specific drug formulations, optimum dose, and duration. In addition, the limited bioavailability of FA and its metabolites pose a challenge for their clinical development. To resolve these issues, there is a need to explore advanced drug delivery systems as viable options for delivering remedial concentrations of FA into the systemic circulation as well as target tissues.

1 Symptoms, host range, and transmission

CMV causes typical mosaic symptoms on melon and cucumber leaves, plant stunting and fruit yield reduction. Mottle or mosaic is also often observed on fruits. In some cucumber cultivars, a rapid and complete wilt is observed on adult plants, a few days after CMV infection. In zucchini squash, CMV symptoms are very severe, including mosaic, yellow spots and leaf distortions (Fig. 1C). Infected plants remain stunted and generally fruit setting is drastically reduced and may even be stopped. Fruits are deformed with pinpoint depressions. CMV infection is rarely observed in watermelon, which reacts with dark necrotic lesions.

CMV infects a wide range of plants (over 1200 different species) and is transmitted by more than 60 aphid species in the nonpersistent mode. There are a few conflicting reports of CMV seed transmission in cucurbits. If confirmed, this could be significant for long distance dissemination of strains, but due to the abundance of CMV reservoirs, this should have probably no or only limited impact on local epidemics in crops.

How is a plant leaf similar to a human lung?

Which human organ is similar to leaf?

Why leaves are called lungs of plants?

What is the breathing organ of a leaf?