Banana is a succulent tropical fruit. Edible bananas do not produce seeds and are propagated vegetatively through suckers. A banana or plantain stand usually have about 7-10 suckers in a year; natural regeneration is slow. Most of banana and plantain production is in backyards or small scale farms for consumption. Suckers obtained are often infected with several diseases or infested by pests; such sucker propagates disease or harbors the pest, thus, reducing the lifespan of a new establishment, that is why a major constraint to the expansion of production is the scarcity of planting material. One of the ways to raise healthy suckers is through tissue culture.
Tissue culture is a means to prepare disease-free planting materials via the use of a liquid, semi-solid, or solid growth medium, such as broth or agar, and in vitro under sterile growing conditions to provide healthy and high yielding planting material for the banana establishment
Components Of Plant Tissue Culture
4 components of plant tissue culture media must be mentioned; they are:
- Explants: meristem, node, flowers, leaf, embryo, shoot, etc.
- Culture medium: micro and Macronutrients (MS basal medium), carbon source (sucrose), vitamins (thiamine), PGRs (auxin, cytokinin), others (antioxidants, additives, support).
- Sterile conditions: explant disinfection, media sterilization, filtered air.
- Controlled environment: light duration & intensity, temperature.
Banana Tissue culture Facility
To ensure a successful generation of banana seedling through the tissue culture technique, the facilities needed are:
- Media preparation room
- Washing and disinfection room
- Transfer or inoculation room
- Culture room
- Chemical storage room
- Screen house or Greenhouse with 50% shade.
Processes of Banana Micropropagation
The micropropagation of banana is guided by some principles to ensure the development of derived quality suckers. The processes involved include;
- The collection of sucker with the desired trait
- Cleaning and disinfection of suckers.
- Maintenance of shoot cultures
- The proliferation of shoots (subculture)
- Preparation of Tissue Cultured Banana suckers for field planting.
#1. Collection of Suckers
There are different types of banana suckers namely, peepers, sword, and maiden suckers. Any of these types of suckers can be selected and collected for tissue culture. After the selection of desired suckers, the desired suckers should be detached from the main mother plant without cracking the corm. The detached banana suckers are excised to obtain approximately 4 inches (10 cm) of inner pseudostem tissue containing the banana meristem.
#2. Cleaning and Disinfection of Suckers
Washing and disinfecting of suckers prior usage is important to ensure the production of health tissue-cultured suckers. Wash the pseudostem of the suckers collected from the field with running water to remove adhering soil particles. Disinfect the suckers from micro pathogens by Immersing the pseudostem in a container of undiluted household bleach (5.25% NaOCl) for 30–45 minutes. Gently strain the bleach solution and keep the surface-sterilized pseudostem in the container.
Here is the beginning of the tissue culture of banana. Start by trimming and peeling off the outer leaf sheath of the sucker that comes in contact with the bleach. Transfer the sucker to a clean cutting dish and continue cutting until the shoot measures 1×1 cm, with the corm tissue as thin as possible. Transfer the shoot tip of the sucker to a fresh cutting dish and cut the shoot into quarters longitudinally, through the center. Then, transfer each quarter onto a solid culture medium.
#3. Maintenance of Shoots Cultures
Keeping the shoots in good condition to ensure the culture process is not contaminated. The shoot cultures must be kept in an air-conditioned room under a 16-hour photoperiod 40 µE/m2S-1 (provided by two 40-watt fluorescent tubes). Observe the cultures closely to detect faults and contamination of the system. Discard contaminated cultures as soon as they are noticed. Similarly, observe for some physical qualities such as browning and bulging of corm tissue, the greening of leaf tissues, and the growth of new shoots during the first month of culture. The shoot tips are ready for subculture when shoots coming out from the apex of the leaf axis are almost 2 cm tall.
#4. Subculture of Shoots
The shoots are ready for subculture when shoots coming out from the apex of the leaf axis are about 2 cm tall. The subculture begins by transferring the shoot or sections of the shoot to the fresh culture medium in vitro whenever the propagules are about 2 cm tall. Overgrown shoots are less proliferative. If shoots are beyond 2 cm, make a longitudinal cut through the apex of the growing shoot. Subculture onto half-strength MS medium supplemented with 5 mg/l BAP and 100 ml/l coconut water. This medium, without auxins, is used to avoid the early forming of nubbins at high frequencies. All subculturing needs to be conducted in sterile conditions.
The subculture of the shoots lasts for about 3–4 weeks until the desired number of shoots is obtained. You can repeat the subculture for at most than 5 cycles. An increase in the number of subculture cycles will lead to off-type banana mutations such as dwarfism, elongation, or other abnormalities. As soon as sufficient shoots then, you can proceed to the next stage of the culture, rooting.
Rooting of the subcultured shoots starts when there are sufficient stocks to meet production targets. Rooting is initiated by the separation and trimming of well-formed shoots with 3 or more leaves. The rooting of the shoots is initiated in the rooting medium either auxin NAA or IBA, activated charcoal. After 4-6 weeks in the rooting culture, the rooted plants are ready for planting out. A new planting material with a well-developed root system and at least 4 leaves is developed for transplanting.
Preparing Cultured Banana plantlets for field planting
Before planting tissue-cultured banana plantlets into the soil, the seedlings need to be acclimatized to the external environment. Acclimatization of the plantlets can be done in several ways such as; transferring them to a liquid medium without agar, exposing the plantlets to partial sunlight in the tissue-culture vessel under greenhouse conditions for 21 days. Any observed agar medium adhering to the tissue-cultured plantlets should be gently washed off before planting into potting media in a nursery.
Acclimatize in a potting mix with good moisture-holding and drainage characteristics, like 2 parts Sunshine Pro mix, 1 part perlite, and 3 parts medium to coarse-grade vermiculite. Ensure the media is moist to maintain the health of the tissue-cultured seedlings. Boost the fertility of the media by applying liquid fertilizer. Acclimatize by placing the banana seedlings in a partially shaded area (50% shade) for 3 weeks before exposure to full sunlight.
Banana pests like aphids, whiteflies, and spider mites are commonly found on banana plants during acclimatization in greenhouses or clustered nurseries; these should be managed by applying insecticide. However, after the plants are transplanted into the field, these pests are typically not problematic. The full acclimatization process should take about 8 weeks, or when seedlings reach about 8 inches or taller, depending on variety, before field planting.
Difference Between Tissue-Cultured Banana Suckers And Conventional Suckers
Conventional banana suckers
- Has a low multiplication ratio, 5-10 suckers in 12 months.
- Suckers may be infected or infested
- Non-uniform growth of emerging banana stand
- Often associated with poor yield.
- Limited availability of planting material.
Tissue Culture banana suckers:
- Possesses a high multiplication ratio, producing about 1000 plants in 12 months.
- Suckers are usually healthy and disease-free.
- Suckers possess uniform and even germination rate
- Rapid germination and increased yield
- Availability of suckers all year round.
Tissue culture banana plants are usually ready for harvesting at an average of 9–10 months after transplanting into the field provided all management operations such as weeding, fertilization, propping, etc. are carried out when due.