Plant Latex- A Concise Review on the Exudate

: In this review article an effort has been made to provide basic knowledge about plant latex. In this paper the origin and evolutionary aspects of latex has been mentioned. The geographical distribution and classification along with the physical and anatomical traits have also been brought into limelight. Moreover, the presence of phytochemicals, proteins and variation of the content of rubber in latex has been referred in the paper respectively. This paper would be of great help in gaining deeper knowledge about the plant latex for future researches.


I. INTRODUCTION
In nature latex is found as a thick milky emulsion. Sometimes, it may be thin and clear and the colours may vary. It is not necessary that only white colour is found to be present in the latex. This emulsion coagulates on exposure of the air. The exudation occurs after an injury takes place. Latex is different from other plant saps; it is produced separately (Mishra and Parida, 2020). The exudate is produced by specialized cells known as the laticifers. The laticifers are further classified into two sub-classes, articulate and non-articulate laticifers. Latex has been reported to occur in around 900 genera and 12000 species. Like the other plant parts, latex is also rich in many phytochemicals. These phytochemicals play vital roles in the lives of plants and human beings (Chavan et al., 2015).

II. ORIGINS OF LATICIFER AND LATEX
The laticifers exist irregularly throughout the plant kingdom, it has been proposed that the laticifers and latex have independently evolved many times (Farrell et al., 1991). The way of development and the differences in the structures observed among the several types of laticifers (Dussourd and Denno, 1991) reinforce the idea that the laticifer and latex have evolved independently and convergently. However, at present there is no evidence how and from what the laticifer and latex have evolved (Konno, 2011).

III. COMMON AND CONVERGENT CHARACTERISTICS OF LATICIFERS AND LATEX
Diversity of the latex constituents has been observed among the closely related species. In contrast to this latex of plant species that are distant in phylogeny often reveal similar or the same constituents in common that seem to have evolved convergently. A very evident example is the frequent presence of rubber molecules (cis-1,4-isoprenes polymer) in the latex of the plant families Apocynaceae, Asteraceae, Moraceae, etc. (Mooibroek and Cornish, 2000). Rubber commonly occurs in the latex of distant plant groups, as stickiness is one of the basic and common features necessary for the function of latex. The existence of rubber in latex among the different groups with completely different laticifer structures, including both articulated laticifers and the nonarticulated laticifers (Moraceae, Apocynaceae) shows the probability that rubber in latex have evolved convergently in numerous plant groups (Konno, 2011).
IV. DIVERSITY OF LATEX COMPONENTS WITHIN FAMILIES, GENERA, AND SPECIES In spite of its similar appearance, latex components are occasionally diverse even between closely related species in the same families and genera. An evident example is of family Moraceae, comprising around 1000 species of latex-exuding plants. The latex components vary among the species. Cysteine protease is the major defense substance in the latex of Ficus virgata and several other Ficus species, (Konno et al., 2004) but, in some species in place of cysteine protease, phenanthroindolizidine alkaloids exist as defense substances (Konno, 2011). Meanwhile, the latex of mulberry trees (Morus spp.) that are toxic to insects are sugar-mimic alkaloids like DNJ (Deoxynojirimycin), D-AB1 (1,4-dideoxy-1,4-imino-D-arabinitol) (Konno et al. 2007) (Shunze Jia et al., 2020), and a unique chitin-binding protein, MLX56 (Wasanoet al., 2009). These defense substances are completely different from the Ficus species.
Further, it has been observed that the compositions (molecular species and concentration) of sugar-mimic alkaloids vary significantly amongst the mulberry populations in the same or in sibling species from different places in East Asia (Konno et al., 2006). Therefore, it is clear that components in the latex of plants belonging to Moraceae family are distinct within a family, a genus, and even within a species. Similarly, in the genus Asclepias of Apocynaceae family (Seiberet al., 1982;Rasmannet al., 2009), and genus Euphorbia of Euphorbiaceae family (Lynn and Clevette-Radford, 1987), diversity of the latex components has been observed. In the latex of A. speciosa and A. californica cardenolides were completely absent whereas, high concentrations of cardenolides were found in the latex of A. curassavica and C. procera (Seiberet al., 1982). It was also observed that among the species containing cardenolides, the composition of cardenolides in latex varied greatly (Seiberet al., 1982;Rasmannet al., 2009). The reason for diversity in the latex constituents would have resulted from plantherbivore interactions that the species have experienced (Konno, 2011).

V. GEOLOGICAL DISTRIBUTION OF LATEX
It has been observed that latex frequently occurs in plant families and species in tropical regions (12.2% for families and 14.0% for species). While in temperate areas less laticiferous plants (4.9% for families and 5.9% for species) has been observed (Lewinsohn, 1991). In comparison to 8.9% of worldwide latex bearing plants (Farrell et al. 1991) it has been discovered that 20-35% plants of Tropical America (Amazon) exude latex (Lewinsohn, 1991). Meanwhile, only 15-30% plants exude latex in the tropical Africa region (Reitsma, 1988). The reason for this kind of difference in the percentage is the interaction between the plants and the herbivorous insects. In the tropical regions this interaction is more intense than the temperate areas as the frequent occurrence of latex bearing plants is consistent with the defending roles of latex and laticifer against herbivorous insects.

Stickiness and Clots
In some plant species like Asclepias syriaca, Hevea brasiliensis, and Lactuca sativa latex is highly viscous (Dussourd and Eisner 1987; Dussourd 1995), whereas the latex of Morus spp., Nerium oleander etc. latex exudation is non-sticky.In most cases, after the exudation latex becomes stickier, and in some case finally develop clots. Solidification of fluid is called as clotting while stickiness is corelated with elasticity, high viscosity and adherence to a surface. The stickiness of latex acts as a defense function against the herbivores by gluing the mouth parts (Dussourd, 1993) and trapping the body of the insects (Dussourd, 1993(Dussourd, , 1995. Rubber particles play important role in providing stickiness to the latex (Gidrolet al., 1994). It is obligatory for the plant to maintain high pressure inside the laticifers so that the latex has easy flow inside the laticifers and exudes large amount of latex at the site when any damage occurs. Without clotting of the latex at the damaged sites, pressure of the latex flow would decrease speedily and not flow from the wounds on subsequent damages caused by insect bites. Thus, clotting and stickiness both play an essential role against herbivores insects and other organisms as well.

Amount of Secretion of Latex
Within the same family, genus and among the closely related species, the amount of latex exudation from a point of damage vary greatly. For example, the latex exudation from the young leaves of Asclepias barjoniifoliais four times more, per equal size of wound in comparison to the leaves of Asclepias angustifolia (Rasmannet al., 2009). Similarly, Ficus variegata and Ficus virgata exude white non-transparent latex in large amount compared to the very small amount of orangecoloured non-transparent latex oozed by Ficus benguetensis (Konno, 2011).

VIII. ANATOMY OF LATICIFERS
Latex is an exudate that is reserved in the extremely elongated cells that extend along the plant and this canal structure that secretes latex is called laticifer. As a large part of laticifer cells are occupied by big vacuoles that develop in the course of laticifer formation (Cai et al., 2009), the majority of latex secreted is likely to originate in vacuoles. Sometimes it is possible that the exuded latex may also comprise of fluid from the cytoplasm of laticifer cells. This is due to the degradation and mixing of the cytoplasm of laticifer cells with vacuoles in the finishing stage of laticifer development (Zhou and Liu, 2010). Laticifers are spread in the roots, stems, petioles, leaves and leaf veins of the plants.

IX. COMPOSITION OF PLANT LATEX 9.1. Existence of Various Chemicals and Proteins
Latex contains a great variety of secondary metabolites and proteins, especially for the defense purpose in a speciesspecific manner. A variety of secondary metabolites such as alkaloids, cardenolides, phenolics, terpenoids, etc. have been found in the plant latex. Apart from the secondary metabolites wide range of proteins such as chitin-binding proteins, chitinases, glucosidase, lectins, oxidases, phosphatase, proteases etc. exist in latex. Recent studies have suggested that many of these proteins and secondary metabolites in laticifers and latex play defensive roles against herbivore insects.

Content of Rubber in Latex
Rubber (cis-1,4-isoprene polymer) is a terpenoid widely found present in the latex of various plant species. Rubber is known to be present in the latex of around 8 plant families and 300 genera (Bushman et al. 2006;Metcalfe, 1967;Mooibroek and Cornish, 2000). The two important traits observed in latex i.e., stickiness and white colour is due to the presence of rubber as particles dispersed in fluid. Rubber in latex may play important roles in sealing wounds and protecting the plants from infection and further oozing of latex (Konno, 2011). The frequent existence of latex in high concentration in many unrelated families like Euphorbiaceae, Moraceae, Apocynaceae etc. indicates that rubber plays a very common and important role in latex.

Alkaloids
Alkaloids are alkaline compounds containing nitrogen. These are often found to be present in the plant latex of many plants. Some of these affects particularly neurotransmission, these are considered toxic to many animals. Plant families like Apocynaceae, Campanulaceae, Moraceae, Papaveraceae etc. are rich in latex containing alkaloids. Morphine in Papaver somniferum (Papaveraceae), (Itenovet al. 1999;Hartmann 1991) isoquinoline alkaloids like chelidonine, copticine and sanguinarine in Chelidonium majus (Papaveraceae) (Tomè and Columbo, 1995) and piperidine alkaloid like Lobeline in Lobelia cardinalis (Campanulaceae) (Oppelet al., 2009) have been found in the latex. Morphine affects the central nervous system of humans (Waldhoeret al. 2004) and sanguinarine affects neurotransmission as it inhibits various neuroreceptors and this makes sanguinarine toxic to the vertebrates and insects (Schmelleret al., 1997). Lobeline effects on the nicotinic acetylcholine receptors (Felpin and Lebreton, 2004). Apart from these the sugar-mimic alkaloids are present in the latex of mulberry trees, Morus spp. (Moraceae) (Konno et al.,2004). These alkaloids are toxic to several insects as they inhibit glucosidases (Hirayama et al., 2007).

Phenolics
Phenolics are known to function as plant defenses. These phytochemicals are sometimes found in the plant latex in large amounts. For example, the latex of Ipomoea batatas (sweet potato) (Convolvulaceae) contain high concentration of phenolics like eicosyl, hexadecyl and octadecyl ester of p-coumaric acids. These altogether make up to 3% of fresh vine latex and 10% of root latex of the ''Jewel'' variety of sweet potato (Snook et al., 1994).

Terpenoids
Numerous sesquiterpene lactones including lactucin have been found in L. sativa (cultivated lettuce) of Asteraceae family (Sessa et al. 2000). The sesquiterpene lactones, lactucopicrin and 8-deoxylactucin prevent feeding by locusts (Rees and Harborne, 1985). These sesquiterpenes also have antifungal properties. Lettucenin A, in latex inhibits the growth of pathogenic fungus Cladosporium herbarum (Sessa et al., 2000). The latex of Euphorbia biglandulosaand other species of the Euphorbia genus contain phorbol, its derivatives (Noack et al., 1980) and diterpenoids in the plant latex. These phytochemicals are toxic for insects and are responsible for causing tumor and skin inflammation (Gershenzon and Croteau, 1991).

Cardenolides
Cardenolides are a group of cardiac-active steroids. These steroids are present in latex of many plants belonging to Apocynaceae family. Cardenolides inhibit the Na+ /K+ -ATPase. In tropical Southeast Asia, the latex of the plant Antiaristoxicaria (Moraceae) is used in preparations of dart poisons as, the latex is rich in cardenolides (Carter et al., 1997). Cardenolides are known for their defensive roles against the herbivores. An evident example is of Asclepias curassavica, the latex of the plant is rich in cardenolides and after the ingestion of the latex the generalist caterpillars, Trichoplusiani (Noctuidae), showed toxic symptoms like convulsions with spasms, immobilization, regurgitation and unresponsiveness lasting over a day (Dussourd and Hoyle, 2000).

X. CONCLUSION
The evolutionary aspects, diversity, physical traits and anatomical features makes latex an interesting topic for research. The content of latex and the amount of rubber in it, varies greatly from family, genus and species, further studies are required