Cadang-Cadang

Characteristics

Viroids are small, single-stranded RNA molecules, ranging from 246 to 375 nucleotides long. Other viruses , they do not code for protein coats but contain genes for autonomous replication. They are more commonly found in a latent stage, and their mode of infection is mainly mechanical, but it is documented that they are of vertical transmission through pollen and seed. ^[4] The sequence and structure of CCCVd has been documented. There are four low-molecular-weight RNA species (ccRNAs) found with cadang-cadang disease. ^[5]Of the oven, two are related to the early stage of the disease: ccRNA 1 fast and ccRNA 2 fast. After several years, ccRNA 1 slow and ccRNA 2 slow. Moreover, they share sequence homology with other viroids. ^[2]

Conditions for a viroid to infect its host. ^[6] Not all of the conditions pertain to cadang-cadang.

Tinangaja disease is caused by coconut trinangaja viroid (CTiVd), which has 64% sequence homology with CCCVd. This disease has been found in Guam . Coconuts from Asia and South Pacific have been found with similar nucleic acid sequences of CCCVd. The pathogenicity of CTiVd is uncertain. ^[7] Coconut cadang-cadang viroid , also known as CCCVd, is responsible for the disease of coconut plant first reported in the early 1930’s in the Philippines . CCCVd is the smallest known pathogen and it is biologically distinct from other viroids; it consists of circular or lineal single-strandedRNA with a basic size of 246 or 247, it is thought to be transmitted by seed or pollen(with low transmission rates) and occur in almost all plant parts. Once infected, coconut palm shows yellow leaf spots and nut production ceases; from appearance of first symptoms to tree death, time ranges from around 8 to 16 years, the goal pest is Generally Greater in older plants. It is estimated that over 30 million coconut palms have been killed by cadang-cadang since it was first recognized and the loss of production is about $ 80 – $ 100 for each planting site occupied by an infected tree.

Molecular structure

CCCVd are small, circular, and single-stranded. They can not replicate themselves and are completely dependent on a host.

CCCVd have a sequence of 246 nucleotides, 44 of which are common with most viroids. ^[8] CCCVd can add a cytosine residue in the 197 position and increase the size to 247 nucleotides. It is known that CCCVd forms with 247 nucleotides cause severe symptoms. Their minimum size is 246, but they can produce forms from 287 to 301 nucleotides. ^[9]

In CCCVd, changes in molecular forms are related to the steps of the disease. There are four different RNAs found in CCCVd, two of them fast and the other two slow (the difference between fast and slow RNA is their mobilities in electrophoresis gel ). In the early stages of the disease, RNA fast 1 and RNA fast 2 appear, while in the late stages RNA slow 1 and RNA slow 2 are detected.

Geographical distribution

The CCCVd is widely spread in Philippines and it is mostly found in Bicol Region , Masbate , Catanduanes , Samar and other smaller islands in the area. It is known that the present northernmost boundary is at the latitude of Manila and the southernmost latitude of Homonhon Island . This fact is important to the proximity of the disease to the major coconut and oil palm growing area of Mindanao . ^[10] An isolated focus of infection has been found on Solomon Islands , Oceania .

The present data suggests that the epidemiology of cadang-cadang viroid may not be spreading into any specific route.

Variations

In Guam , a similar viroid (CTiVd) has been found that causes a similar disease named tinangaja disease. This viroid has 64% sequence homology with the cadang-cadang viroid. ^[11] There are other related viroids with the CCCVd, which are found in Asia and the South Pacific . They have a high degree of homology but pathogenicity is uncertain. ^[12]

Transmission

The mode of natural inoculation is viroid is unknown. There has been evidence that transmission through pollen and seed has been reported to be very low in the proportion of healthy pollen, with 6 months after germination . ^[13]

CCCVd can spread through mechanical inoculation primarily through contaminated farm tools such as harvesting scythes or machetes , due to the improper sanitary conditions. ^[14] The efficiency of the mechanical inoculation has been influenced by factors such as the age of the test plant and the mode of inoculation.

Experimental transmission of the CCCVd using insects as vector has-been unsuccessful ALTHOUGH there are evidences FEW que la transmission of the viroid might be related to the wounds of coelopteran insects, this goal Has not Properly Investigated beens. ^[14]

The natural hosts of cadang-cadang identified are Cocos nucifera , Corypha utan , Elaeis guineensis and Roystonea regia . The experimental hosts are Adonidia merrillii , Areca catechu , Caryota cumingii , Dypsis lutescens , Saribus rotundifolius , Phoenix dactylifera , Ptychosperma macarthurii and Roystonea regia . ^[15]

Detection

The diagnosis by symptoms is not reliable enough so it’s better to do a molecular diagnosis based in test samples. Some of these methods (like Dot-blot hybridization ) allow the scientists to detect the viroid even six months before noticing the first symptoms.

The first step is the purification to obtain the nucleic acids of the plant cells . The leaves of the plant are cut. Afterwards, they are blended ( homogenize ) with sodium sulfite . Then the extract is filtered and clarified by centrifugation (10,000 g during 10 minutes). The next step is to add polyethylene glycol (PEG). Finally, after nearly two hours of incubation at 4 ° C, and after another centrifugation (at low speed) the nucleic acids can be extracted by chloroform procedures, for example.

When approximately 1 g of coconut tissue has been purified, the electrophoresis method can be started, which will help to identify the viroid by its relative mobility. ^[16] The CCCVd is one of two dimensional polyacrylamide gels with a silver stain .

The viroid can be detected by a more sensitive method called dot blot molecular hybridization. ^[17] In this method, CCCVd is amplified by the polymerase chain reaction (PCR ) and the clones of CCCV are used to synthesize a complementary DNA or RNA chain. These sequences are radioactively labeled so when they are put to the test with the intention to analyze (on a supporting membrane) and exposed to x-ray film, then if CCCVd is present it will appear as a dark color. This dark tonality only appears when nucleic acid hybridization occurs.

Pharmacology and treatment

Coconut cadang-cadang disease has no treatment yet. However, chemotherapy with antibiotics has been tried with tetracycline solutions; antibiotics failed trying to alter progress of the disease since they had no significant effect on any of the parameters. When the Treated plants Were at the early stage, tetracycline failed to prevent prevention injections the progression of the palms to more advanced courses, nor Did They Significantly affect the mean number of husks or nuts. Penicillin treatment had no apparent improvement.

Control strategies are elimination of reservoir species, vector control, mild strain protection and breeding for host resistance. ^[18] Eradication of diseased plants is usually Performed to minimize spread aim is of dubious efficacy due to the Difficulties of early diagnosis as the virus etiology remains unknown and the one hand Discovered Are The three courses in the disease development.

Easily confused pests

Coconut Cadang Viroid (CCCVd) can be confused with another viroid called Coconut Tinangaja Viroid (CTiVd). That is because they have 64% of their sequence of nucleotides in common. They both affect coconut palm, and infected plants have similar symptoms: yellow spots and even death. ^[19]

There are few differences between the two causes of small fruits, while CCCVd causes round and reduced nuts. Moreover, CTiVd has 254 nucleotides of length, while CCCVd has 246.

Symptoms

Symptoms of cadang-cadang develop slowly over 8 to 15 years, making it difficult to diagnose at an early time. ^[6] There are three main stages of defined series of characteristics: early, medium, and late stages. ^[20] The first symptoms in the early stages of infection. ^[20] These symptoms include scarification of the coconuts which also become rounded. The leaves (fronds) display bright yellow spots. About two years later, the medium During internship, ^[21] the inflorescences Become Eventually stunted and killed, so no more coconuts are Produced. Yellow spots are larger and in greater abundance to give the appearance of chlorosis. During the final stage, roughly 6 years after the first symptoms are recorded, the yellow / bronze fronds start to decrease in size and number. Finally, all the leaves coalesce, leaving the trunk of the palm “standing like a telephone pole”. ^[6]

Palms under 10 years of age are rarely affected by cadang-cadang; the incidence of disease increases until about 40 years of age and then ^[22] “No recovery has ever been observed, and the disease is always fatal”. ^[4] African oil palm has similar symptoms as coconut but also has orange spotting on palms. ^[23]

Environment

Geographical location

Philippines: Southern Luzon , Samar , Masbate , and other small islands. Tinangaja was found in Guam on Mariana Islands . ^[22] Evidence shows that CCCVd is negatively correlated with altitude . ^[3]

Dispersal

The CCCVd viroid can be transmitted by pollen , seed, and mechanical transmission; it infects all parts of the host after flowering.

Insect vectors

No insect vector is known at this time, and the rate of dispersal is not related to the proximity of clusters.

Control

No definite control measures exist at the present.

• Genetic resistance and vector control are not possible because they are not viable.
• Eradication is ineffective because of the long period between infection and appearance of symptoms, which is approximately 1 to 2 years.
• Cross-protection (see also Influenza vaccine: Cross-protection ) is a possibility for the future. Cross infection means inoculating the coconut with a mild strain of the viroid to give the coconut tree some degree of protection from the killer form. This is similar to Edward Jenner’s pioneering smallpox vaccine . He used cowpox to confer induced immunity on humans. ^[24] But in the case of Cadang-cadang, this is still under research.

References