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LITERATURE REVIEW

Papaya General Description ( papaya L.) is a member of Family. This family comprises of 31 species in the four genera: three genera from America (Carica, Jacaritia and ) and one from equatorial Africa (Cylicomorpha). Papaya is an economically important fruit crop in Hawaii, Australia, India, Srilanka, Phiilipines and South-east Asia including Thailand. It is also known as papaw, pawpaw, papayer (French), melonenbaum (German), lechosa (Spanish), mamao, mamaociro (Portuguese), mugua (Chinese) and malakol (Thailand) (Nakasone & Paull 1998).

Origin and Distribution The origin of Carica papaya is in Tropical America. Its seeds were distributed from the Caribbean to Malacca and India by travellers and botanists in the eighteenth century. The distribution was continued throughout Asia and Pacific. Carica papaya is grown in all tropical countries and many subtropical countries between 32 °North and South latitudes but the high commercial production is found between 23 °N and S latitudes (Nakasone & Paull 1998).

Characteristic

Stem Carica papaya is a fast-growing tree herbaceous like 5-7 meters in height. Papaya normally has a monaxial stem without branching but it has multi-stems when damaged. When the stem is wounded white milky latex oozed from the wound. Although papaya can be up to 9 meters height, it is easily damaged and makes harvesting of fruit difficult (Nakasone & Paull 1998).

Leaves The cluster of leaves at the apex and along the upper of the stem makes up the foliage on tree. New leaves emerge from the apex and old leaves senescence and fall. Leaves are palmately lobed with prominent venation; the blade is deeply divided into 7-11 segments and can measured 3

40-50 cm in diameter with 15 mature leaves per plant. The leaves contain white milk latex (Nakasone & Paull 1998).

Flower Papaya flowers are born in florescences which appear in the axils of leaves. It can be female, male or hermaphrodite flowers. Female flowers are held close against the stem as single flowers or in cluster of 2-3 flowers. Male flowers are smaller and more numerous. Hermaphrodite (perfect) flowers are intermediate between the female and male (Nakasone & Paull 1998).

Fruit The fruit superficially resembles a melon puriform, oval and elongated in shape. The fruits range in size from 7-30 cm. The fruit is normally composed of 5 carpels. Fruits from female trees are spherical whereas the shape of fruits from hermaphrodite trees is affected by environmental factors that modify floral morphology during early development of the inflorescence. Green fruits contain an abundance of milky latex. Ripe fruits have yellow-orange coloured skin. Mature fruits contain numerous grey-black spherical seeds 5 mm in diameter (Nakasone & Paull, 1998).

Importance Papaya is mainly cultivated for its edible fruits as a fresh fruit and for use of drinks, jams, candies and dried fruit. Ripe fruits are usually eaten fresh and green fruits are also used as a cooked vegetable. Papaya also has several industrial uses. Biochemically, its leaves and fruits produce several proteins and alkaloids with important medical and industrial application. The latex of green fruits contain a proteolytic enzyme, papain, used in the beverage, food and pharmaceutical industries for production of chewing gum, chill-proofing beer, tenderising meat, treat digestive disorders, degum natural silk, extracted fish oil. It is also used in the cosmetic industry, in soap, shampoo and face lifting preparations (Nuñez, 1982). Evolutionary, papain may be associated with protection from frugivorous predators and herbivores (Australian Government, Department of Health and Ageing, 2003) 5

Table1 Nutrient composition of papaya per 100 grams.

Nutrient Composition per 100 grams

Energy 59.0 Kcal Moisture 84.4 % Protein 1.0 g Fat 0.1 g Carbohydrate 13.5 g Fibre 0.5 g Ash 0.5 g Calcium 31.0 mg Magnesium 0.8 mg Phosphorus 17.0 mg Iron 1.0 mg Sodium 2.0 mg Potassium 337.0 mg Vitamin B1 0.08 mg Vitamin B2 0.15 mg Niacin 0.1 mg Ascorbic Acid (Vitamin C) 69.3 mg Carotene 2431.0 µg

Source: Agri-Food Business Development Centre

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Production World papaya production for 2003 was estimated at 48,000 thousand tonnes, according to the Food and Agriculture Organization (Table 2). Brazil was the largest producer and the production was estimated at 1,600 thousand tonnes. Mexico came second with the production of 700 thousand tonnes followed by Nigeria with 755 thousand tonnes.

Table 2 World papaya production by country

Country 2001 2002 2003 (MT1/) Brazil 1,489,324 1,597,700 1,600,000 Mexico 873,457 876,150 955,694 Nigeria 748,000 755,000 755,000 India 700,000 700,000 700,000 Indonesia 500,571 605,174 626,745 Thailand 120,000 120,000 125,000

1/ Metric tonnes Source: Food and Agriculture Organization of the United Nations (2004)

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In 2002, Mexico was a major exporter with the value of 30,080 million dollars (Table 3) followed by Malaysia, Brazil and USA with the value of 26,247, 21,624 and 13,604 million dollars respectively.

Table 3 Export of papaya by country

2000 2001 2002 Country Quantity Value Quantity Value Quantity Value (MT) (x 000 US) (MT) (x 000 US) (MT) (x 000 US) Mexico 59,819 23,691 74,033 30,323 68,553 30,080 Malaysia 44,134 18,201 53,961 24,603 60,892 26,247 Brazil 21,513 17,696 22,804 18,503 28,541 21,624 USA 6,191 14,422 8,324 17,243 7,106 13,604

Source: Food and Agriculture Organization of the United Nations (2004)

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Nutrition Papaya is a wholesome fruit. Papaya has more carotene compare to other fruits such as apples, guavas, sitaphal and plantains (Mumtaz, 2005). The fruit in 100 grams contains protein (1.0 g), carbohydrate (13.5 g) and fibre (0.5 g). It is a good source of minor such as Calcium (31.0 mg), Potassium (337.0 mg) and Magnesium (0.8 mg).

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The major importer of papaya was USA followed by Japan, Canada and Germany. The volume of papaya import was increased every year from 2000 - 2002 to 68,559 metric tonnes (Table 4).

Table 4 Import of papaya by country

2000 2001 2002 Country Quantity Value Quantity Value Quantity Value (MT) (x 000 US) (MT) (x 000 US) (MT) (x 000 US) USA 69,887 53,140 84,401 62,365 88,559 58,337 Japan 5,796 16,503 6,569 16,389 6,606 15,156 Canada 4,885 8,236 5,484 8,903 5,624 8,236 Germany 3,502 7,315 5,032 9,716 5,965 10,476

Source : Food and Agriculture Organization of the United Nations (2004)

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Papaya cultivated areas and its production was stable since 2000. The growing areas are approximately 10,000 Ha and the production is approximately 100,000 MT (Table 5).

Table 5 Production of papaya in Thailand

Year Area (Ha1/ ) Production (MT) 2004 10,500 125,000 2003 10,500 125,000 2002 10,000 120,000 2001 10,000 120,000 2000 9,800 119,000

1/ Hectares Source: Food and Agriculture Organization of the United Nations (2002)

In 2000 – 2002, papaya export value was increased from 197,000 to 843,000 $. The export value, however, was decreased in 2003 due to the decrease in papaya quantity (Table 6).

Table 6 Export of papaya in Thailand

Year Quantity (MT) Value (x 1000 US) 2003 2,123 843 2002 2,681 932 2001 495 373 2000 182 197

Source: Food and Agriculture Organization of the United Nations (2002)

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Plant pathogen interaction

Plant pathogen interaction In nature, encounter a diverse range of enemies such as fungi, bacteria, nematodes and viruses. Initiation of defence responses recognises either plant or pathogen derived signals (elicitors) such as degraded cell wall components or exogenous signals produced by pathogen (Smith, 1991). The inducing response including production of chitinase and phytoalexin results in failure of the pathogen to cause pathogenicity in plants. It has been shown in many studies that resistant plants are able to recognise and induce defence response more rapidly (Baker, 1997)

Signal Transduction Signal transduction at the cellular level refers to movement of signals from outside of the cell to inside that carrying specific information to the defined. The movement of signals response can be simple, the receptor that constitute channels with, upon ligand interaction, allow signals to be passed in the form of small ion movement, either into or out of the cell. The ion movement results in changes in the electrical potential of the cell. This event includes phosphorylation and dephosphorylation. Protein phosphorylation is a key mechanism for intracellular signal transduction in both eukaryotic and prokaryotic cells. The addition (or removal) of phosphate molecules from protein has profound effects of their function. As protein phosphorylation can be rapidly increased or decreased even in the absence of new protein synthesis, change in protein phosphorylation is a very rapid and energy efficient mechanism for responding. This process is catalysed by protein kinase and this classified into three major groups based on their substrate specificities, serine/theronine kinase, tyrosine kinase and histidine kinase. Serine/threonine kinase is found in both plants and animals. Tyrosine kinase family is found in bacteria, yeast and plant. Histidine kinase is exclusively found in plants (Urao, 2000).

Histidine Kinase The most abundant signalling pathway is represented by two component systems as Histidine Kinase. Histidine Kinase are composed of two conserved proteins: a histidine protein 11

kinase (HK) and a response regulator protein (RR) (West, 2001). The signal processing is initiated by the autophosphorylation of the sensor kinase at a conserved histidine residue within its histidine domain, using ATP as a donor. The phosphoryl group is then transferred to an aspartic acid residue in the receiver domain of the response regulator, where it results in conformational changes that modulate the activity of the effect domain (Rogov, 2004). Using Arabidopsis as a model, histidine kinase in plant can be divided into 3 receptor groups; ethylene, cytokinin and phytochromes.

Source: (West, 2001) Figure 1 Two-component phosphotransfer system. (a) A single component phosphorelay system, the phosphoryl group (P) from specific conserved His (H) residue transferred to a specific Asp residue (D) and the effector domain responses. (b) A multi-component phosphorelay system often begins with a hybrid histidine kinase that has an addition regulatory domain at the C-terminus. More than one His-Asp phosphoryl transfer reaction takes place.

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Ethylene Receptor Using Arabidopsis as a model, ethylene is perceived by a family of five receptors, ethylene receptor1 (ETR1), ETR2, ethylene response sensor1 (ERS1), ERS2 and ethylene insensitive4 (EIN4) that share similarity with bacterial two-component regulators. The receptor family can be divided into two subfamilies based on their gene and protein structure. Members of the type-I subfamily, which include ETR1 and ERS1 contain an amino-terminal ethylene-binding domain (also called the sensor domain) and a well-conserved carboxy-terminal histidine kinase domain. The type-II subfamily receptors, which include ETR2, ERS2 and EIN4 contain an amino-terminal ethylene-binding domain and a degenerate histidine kinase domain that lacks one or more elements that are necessary for catalytic activity (Guo, 2004).

Ethylene Ethylene is a gaseous plant hormone as a very simple molecule. Ethylene is produced from L-methionine. Methionine is activated by ATP to form S-adenosylmethionine through the catalytic activity of S-adenosylmethionine synthetase. Starting from S-adenosylmethionine two specific steps result in the formation of ethylene. The first step produces the non-protein amino acid 1-aminocyclopropane-1-carboxylic acid (ACC). It is catalysed by ACC synthase with pyridoxal phosphate acting as a co-factor. Formation of ACC is the rate-limiting step in ethylene biosynthesis. Production of ethylene from ACC is catalysed by ACC oxidase. This reaction is oxygen-dependent. At anaerobic conditions ethylene formation is completely suppressed. Fe2+ is a co-factor and ascorbate is a cosubstrate. CO2 was shown to activate ACC oxidase. Distribution of the ethylene gas within the plant occurs through intercellular spaces and when dissolved in the symplast from cell to cell. Long-distance distribution is achieved by releasing ACC into the vascular tissue where it is moved to the site of action. Ethylene promotes maturation and abscission of fruits. Many climacteric fruits such as apple, banana and tomato show a strong increase in ethylene levels at the late green or breaker stage. In addition, ethylene regulates senescence and fading of flower and abscission of petals and leaves. It has evolved as the central of cell death programs in plants. Biotic and abiotic stresses 13 frequently result in formation of ethylene. Increase amounts of ethylene are evolved from plant tissue infected by pathogen as well as by injured tissues (Sauter, 2003).

Papaya Ringspot Virus

Taxonomy and morphology Papaya Ringspot Virus is a member of family Potyviridae, genus Potyvirus. It is a filamentous particle approximately 780x12 nm with positive single stranded RNA and coat protein subunits of 36 kDa. There are two biotypes of Papaya Ringspot Virus; Papaya Ringspot Virus-Papaya biotype (PRSV-P) and Papaya Ringspot Virus-biotype of cucurbits or watermelon (PRSV-W). PRSV-P infects both papaya cucurbits and Chenopodiaceae family while PRSV-W infects only cucurbits (Naqvi, 2004).

Symptoms The symptoms caused by PRSV variable depending on the virus isolates, stage of infection, temperature and plant size. Early symptoms appear first on young leaves in the crown of plants and main symptoms of Papaya Ringspot Virus are leaf mottle and distortion, ring and spots on the surface of misshapen fruits and streaks on stem and petioles. Quality and yield of infected plants are markedly reduced (Naqvi, 2004; Šuti , 1999).

Geographic Distribution Papaya Ringspot Virus was first found in Carica papaya in Hawaii (Brunt,1990). PRSV is widespread in the Middle East, the South and Central American regions, China, France, Germany, India, Italy, Mexico, Taiwan and the USA (Brunt et al., 1996).

Transmission Papaya Ringspot Virus can be transmitted easily with infective plant sap which may be as important for field spread as it is for experimental purpose (Šutić, 1999). PRSV is naturally transmitted by aphid non-persistently and is not seed transmitted. PRSV maybe transmitted by 21 species in 11 genera of aphids including Myzus pericae (green peach aphid), M. euphorbiae , Aphid gossypii(cotton aphid), A. spiraecola (green citrus aphid), A. craccivora (cowpea aphid), A.nerii (oleander aphid), Rhopalosiphum maidis (corn leaf aphid) and Toxoptera aurantii (black citrus aphid) (Šutić, 1999).