Black Sigatoka Responsive Banana Genes Biology Essay

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Banana is a tropical perennial herb that belongs to the genus Musa. It is often confused with a tree due to its height, the leaves are group together forming a pseudostem which has trunk-like structure, this false trunk has a rosette of 10 to 20 leaves at the top and can reach 6 meters height. The leaves have a size of 3.50 m long and 65 cm wide and can reach 9 meters height.

The possible place of origin is Malaysia or India but Alexander the Great was the responsible of its dissemination among western nations. Afterwards, Antonius Musa, the personal doctor of the Roman Emperor Octavius Augustus, promoted the cultivation of banana on his territory.

Banana fruits have a great source of vitamin B6, vitamin C and minerals as phosphorus, potassium, calcium, and the large amounts of energy (90 cal per 100 gr), but its importance relies on being a staple commodity and in some cases, the only source of nutrients for several millions of people living in developing countries where rice, wheat and corn also plays a major role.

1.2. Worldwide context

According to FAO (2007) the production of bananas worldwide was 81 million tonnes, being the staple food for nearly 400 million people. Its high yield is one of the reasons for its economic importance, one plant produces between 50 to 100 fruits at a time, and a head of nine hands can weigh between 22 to 65 Kg. In addition, from its plantation 10 to 15 months later bears fruit, blooms all year and lasts several years. Even without rainy climates, banana can grow by irrigation.

Nevertheless, only one fifth of total banana production worldwide is exported or imported at international level.

The resulting industry has become a crucial source of income, employment and also a help to the economical status of developing countries especially in America, Africa and Asia that export this crop.

1.3. Situation in Peru

Banana and plantain have a great social and economical importance because of being one of the key products in the regular diet, mostly in the residents of the eastern tropical parts of the country (Figure 1).

According to III National Agricultural Census (1994), 17% of total agricultural area is cultivated with bananas and plantain and 70% of that is focused on the jungle region, almost 148 thousand farmers are engaged in this activity, thus, banana has become an important food source and allows the generation of income due to its local and regional trade.

During the last decade, organic bananas produced in Tumbes and Piura (northern coast) have been exported as non-traditional products, where markets from USA and Europe are the main target.

Figure 1. Peruvian regions where Musa grows and its national production in percentage (Krauss et. al., 1999)

Nowadays; planting, operation and marketing of banana and plantain are facing technical, social and economical problems that are a challenge for researchers in finding and implementing solutions, through generation and transfer of new technologies that enhance productivity and profitability of the crop, making it sustainable, internationally competitive and equitable for both producers and consumers.

2. Banana and its interaction with pathogens

Banana and plantains are one of the most important crops worldwide, they are very important in terms of total production in comparison with other crops and one of the main reasons is that banana can be harvested any day of the year and due to this lack of seasonality, it is exposed continuously to environmental hazards as diseases or pests.

Among the biotic threats that cause diseases in banana plantations are viruses, bacteria and fungi, nematodes can exert great damage as well by parasitic association, while insect pests may also produce a serious harm.

Bacteria might cause systemic vascular diseases in banana. According to Jones (2009), the most serious ones correspond to Moko bacterial wilt, blood bacterial wilt and Xanthomonas wilt. Ralstonia solanacearum biovar 1 race 2, an aerobic gram negative bacterium, causes Moko disease which still attacks susceptible cultivars from Costa Rica to Peru producing great losses (Thwaites, 2000). The symptoms will be determined by the route of infection and the type of strain of R, solanacearum.

Blood bacterial wilt is present in Indonesia and the taxonomy of the causal agent has not been determined yet, but the symptoms observed are very alike those produced by R. solanacearum (Moko disease) such as wilting, necrosis and collapse of leaves, fruits usually are rotten or dry.

Bacterial wilt of Enset attacks cultivars in Ethiopia in the first place, but now has spread to Uganda. This disease is caused by Xanthomonas campestris pv.musacearum (Thwaites, 2000).

Regarding to viral diseases, a wide range of virus attacks banana plantations but probably the major threat among them is Banana bunchy top virus (BBTV) which will disrupt the formation of fruit leading to great losses in production. BBTV has been detected in Asia, Pacific and Africa countries and the main responsible of its local spread is the black banana aphid, Pentalonia nigronervosa, while transfer of infected material lead to its long-distance spread (Jones, 2009).

Not only the aerial part of the plant may be affected by pathogens; for instance, the destruction of root and rhizome tissue in Cavendish cultivars is caused by Radopholus similis, the burrowing nematode that is found is some regions of Africa. Rhizomes can also be affected by the banana weevil, Cosmopolites sordidus, which is considered as a pest, this insect burrows into this underground part of the plant weakening it (Jones, 2009).

2.1. Fungal pathogens

Bananas from the cultivar Gros Michel were devastated by the fungi Fusarium oxysporum f. sp. cubense, which is the causal agent of the Fusarium wilt. It was first described in Australia in 1876 and led to the replacement of the susceptible Gros Michel cultivar by the resistant Cavendish; nowadays, in certain regions of Asia, the latter cultivar has been threaded by another strain of the fungus named F. oxysporum f. sp. cubense race 4 also known as "tropical race 4" (Ploetz and Pegg, 2000).

2.1.1. Mycosphaerella fijiensis Morelet

But certainly, fungi that belong to the genus Mycosphaerella is the most feared worldwide due to its great potential for devastation. Mycosphaerella fijiensis Morelet and Mycosphaerella musicola Leach ex Mulder cause Sigatoka leaf spot diseases, but the first one is the main cause of Black leaf Streak or Black Sigatoka being the most devastating disease that affects banana at present.

M. fijiensis penetrates the stomata of banana leaves and colonizes the intracellular spaces between mesophyll cells (Harelimana et. al., 1997), establishing a biotrophic relationship for 21 or 28 days before any necrosis reaction occur (Marín et. al., 2003). The first symptom of the disease is the appearance of dark streaks on the lower leaf surface with subsequent necrosis lesions of the tissue, and depending whether the variety is susceptible or resistant, it may lead to total loss of the plant.

The reason why this disease is the most devastating and costly among banana production is due to a decrease in the total yield which is a consequence of a reduction in the photosynthetic surface and also because of the premature ripening of the fruit.

Ascospores and conidia are the responsible agents in the spread of the disease. Conidia under high humidity conditions and ascospores are dispersed by wind for a few hundred kilometers, nevertheless, the main cause of long distance dispersions is the transfer of infected plantations into new areas (Marín et. al., 2003).

M. fijiensis was first identified in Fiji in 1963 and has spread to Africa, Asia and Pacific countries (Figure 2). The disease was developed in Latin America for the first time in Honduras (1972) and it spread either to the north as to the south to Guatemala, Belize, southern Mexico, El Salvador, Nicaragua, Costa Rica, Panama, Colombia, Ecuador, Bolivia and Peru (Mourichon et. al., 1997).

Figure 2. Global distribution of Black Sigatoka disease (Jones, 2000)

M. fijiensis is mainly controlled with systemic fungicides as benomyl, propiconazole, tridemorph and strobilurins that have a post-infection action. Even though, good results have been observed in field, studies showed a decrease in sensitivity by the pathogen to the fungicide and even resistance patterns, for instance, a decrease in sensitivity was observed using propiconazole (Romero and Sutton, 1997) and trifloxystrobin in Costa Rica (Chin et. al., 2001), resistance and sensitivity was also detected to benomyl in the same country. Therefore, new strategies have to be developed to control fungicide resistance risk.

In order to improve banana cultivars through genetic engineering techniques, the distribution of variability of the pathogen was studied and with help of molecular tools, the highest level of allelic diversity among M. fijiensis populations from different geographical regions was found in Southeast Asia, and over 88% of the alleles identified in Africa, South America and Pacific Islands were also detected in Southeast Asia, suggesting that probably M. fijiensis was originated in Southeast Asia and then spread to other regions of the world (Mourichon et. al., 1997). With the aim to overcome the tragic scenarios caused by this fungus, detection of resistance genes has to be carried out in such a way that those genes can be introduced in susceptible cultivars.

3. Plant disease responsive genes

Evolution of the interaction between hosts and their pathogens has taken place for millions of years, where the host sets the barriers and synthesizes specific proteins against the invader whereas the pathogen has to circumvent them in order to reproduce.

3.1. Resistance mechanisms

Race-specific resistance, also called the "gene-for-gene" plant defense mechanism is the specific interaction of a product of the host resistance (R) gene and a product of the pathogen avirulence (Avr) gene, this can lead to a compatible interaction where the disease is fully developed with subsequent sporulation or incompatible interaction where no disease is produced. On the other hand, race non-specific resistance is due to the action of several genes (Miller, 2009).

According to Miller (2009), different types of defense responses are observed when a plant is infected by a pathogen, some of these responses are common, including:

Oxidative burst, also called respiratory burst, is the rapid release of reactive oxygen species (ROS).

Hypersensitive response (HR), characterized by the rapid death of cells in the infection area.

Production of reactive oxygen intermediates (ROI) for signaling.

Production of nitric oxide.

Tissue and cell wall reinforcement, enhancing lignin formation.

Synthesis of pathogenesis-related (PR) proteins.

Accumulation of substance as acid (JA) and salicylic acid (SA).

Production of antimicrobial metabolites as phytoalexins.

Synthesis of enzymes as chitinases and glucanases against the invader.

Even though, there are many defense pathways that may be specific for a pathogen to avoid the development of a disease, a well-organized cross-talk among them should be present.

It is also important to remark that defense mechanisms can be either constitutive or inducible. The former one is a passive type of defense and is the first obstacle to be overcome whereas the latter one is an active type of system that requires energy and is due to a specific recognition of a pathogen (Keen, 1999).

When the passive defense mechanism is overcome by the pathogen, the inducible response is switched on leading to a localized necrosis either from the action of the invader (compatible interaction) or from a programmed cell death by the plant itself (incompatible interaction) (Maleck and Dietrich, 1999). After that, pathogen-related (PR) proteins are expressed locally and systemically giving rise to a systemic acquired resistance (SAR) that confer more protection against a wider range of pathogens. Pathogen-related (PR) and antifungal proteins are powerful weapons, the former are synthesized by plants upon infection and has seventeen classes, from PR-1 to PR-17; PR-1 and PR-2 create transmembrane pores, PR-2 is a β-1,3-glucanase and PR-3, 4, 8 and 11 are chitinases (Ferreira, 2007). In contrast, antifungal proteins are present in a vast range of organisms and can be part of the constitutive and induced resistance to fungi, two main groups stand out from this category: PR-1 proteins that belong to a conserved family and are induced after infection, and chitinases that cleave the β-1,4-glycosidic bond between the N-acetyl-D-glucosamine units of chitin, they can be divided into exochitinases (acting on non-reducing ends) and endochitinases (cleavage at internal points). Kasprzewska (2003) suggested a double action of chitinases in the process of fungal infection: apoplastic chitinases partially degrade chitin releasing oligosaccharides acting as elicitors for the plant, and vacuolar chitinases hydrolyze newly synthesized chitin chain, thus stopping fungal growth.

3.2. Resistance (R) genes

After fungal penetration, the fungal haustorium produces numerous compounds as enzymes needed for food acquisition or molecules called elicitors that will elicit or repress the host response against infection. In order to exert a strong resistance response a receptor coded by a single constitutive resistance (R) gene should specifically bind to a specific pathogen molecule referred to as elicitor or effector coded by an avirulence (Avr) gene (Bent, 1999; Miller et. al., 2009), this binding will trigger a cascade of events leading to a hypersensitive response (HR).

Even though, there are many R genes present in a wide range of plant species with pathogen specificity, the proteins which they encode are similar structurally (Bent, 1996).

According to Miller (2009), there are five classes of R genes based on structural domains of their products:

Cytoplasmic nucleotide-binding site - leucine-rich repeat proteins (NBS-LRRs)

With N-terminus TIR motif

With N-terminus CC motif

With N-terminus without obvious CC motif

With N-terminus TIR motif and C-terminus WRKY domain and nuclear localization signal

Extracytoplasmic eLRRs anchored to a short transmembrane (TM) domain

Cytoplasmic serine-threonine (Ser/Thr) receptor-like kinases (RLKs) with extracellular LRRs

Cytoplasmic Ser/Thr kinases without LRRs

Membrane anchor fused to a coiled coil domain

Miller also mention a sixth class of R genes that do not fit in the prior classification, corresponding to toxin reductase Hm1, membrane protein Mlo, cytoplasmic Ser/Thr kinase Rpg1, protein without interaction domain HS1, putative cell surface glycoproteins for receptor mediated endocytoses (RME), jacalin-like protein RTM1 and heat shock-like protein RTM2.

This study is focused on two classes of resistance genes, one of them belongs to LRR class and more attention will be paid in this