Genetic transformation of yam

Leena Tripathi, l.tripathi@cgiar.org

Yam (Dioscorea spp.) is a multi-species, polyploid and vegetatively propagated tuber crop in the tropics and subtropics that provides food security and income to over 300 million people. There are 600 Dioscorea species; however, only a few of them are regularly cultivated for food. Dioscorea rotundata and D. cayenensis (both known as Guinea yam) are most popular and economically important in West and Central Africa where they are indigenous, while, D. alata (known as water yam) is the most widely distributed species globally. Yam is the second most important root and tuber crop in sub-Saharan Africa after cassava in terms of production with about 57 million metric tons. Over 95% of world yam production occurs in the yam belt of West and Central Africa with Nigeria alone accounting for about 66% of the world’s total. Some wild yam species are also known to produce secondary metabolites of pharmaceutical importance such as steroidal sapogenin, diterpenes, and alkaloids.

Despite the crop’s economic and sociocultural importance, its cultivation is generally limited by high costs of planting material and labor, decreasing soil fertility, low yield potential of varieties, and increasing levels of field and storage pests (nematodes) and diseases (anthracnose, tuber rots, and yam virus complex). In West Africa, about 11 million tons of yam are lost annually because of damage in storage initiated by nematodes. Plant-parasitic nematode damage is a critical factor in tuber quality reduction and yield loss in yam, both in the field and in storage, which is perpetuated over seasons through infected seed material. Nematodes also facilitate fungal and bacterial attacks.

Nematodes can be managed by nematicides, but are not commonly used due to their high cost. As yam is vegetatively propagated, nematode-affected tubers are transferred in infected seed yam material. Yam nematodes reproduce and build up large populations in stored tubers, causing severe damage and facilitating fungal and bacterial attacks that cause anthracnose disease, dry rot, soft rot, and wet rot. It is therefore necessary to control plant-parasitic nematodes to increase or at least maintain reasonable yields of yam and protect susceptible germplasm from total loss. Nematode-resistant varieties of yam can be very effective for nematode control.

Genetic transformation is an alternative tool for developing nematode-resistant varieties. This option is also important because host plant resistance has not yet been found in the major cultivated species (D. rotundata, D. cayenensis, D. alata) or close relatives with which they can be crossed by conventional breeding and selection for the trait. It is therefore necessary to control plant-parasitic nematodes to increase or at least maintain reasonable yields of yam and preserve susceptible germplasm.

Genes conferring nematode resistance are already available from the University of Leeds for plantain (Musa spp.) transformation at IITA, which can easily be made available and assessed against nematodes in yam. This can only be possible after the yam transformation system is established. To date, only a single report has been published on the stable transformation of D. alata by particle gun using reporter gene (Tor et al. 1993), and this still needs improvement. Tor et al. (1998) also reported transient gene expression in protoplast of Dioscorea spp. using polyethylene glycol (PEG)-mediated direct uptake method. The stable transformation of yam using PEG-mediated uptake still needs to be developed.

There is no report on Agrobacterium-mediated transformation of D. alata and D. rotundata, which is the preferred method for genetic engineering of plants, offering several advantages over direct gene transfer methodologies (particle bombardment, electroporation), such as the possibility of transferring only one or a few copies of DNA fragments carrying the genes of interest at higher efficiencies with lower cost and the transfer of very large DNA fragments with minimal rearrangement.

The development of stable transgenic plants requires an efficient regeneration system amenable to genetic transformation and stability of transgene under field conditions. Regeneration systems from meristems of D. rotundata and D. alata have recently been established at IITA (Adeniyi et al. 2008; Tripathi et al. unpublished). Recently, direct shoot organogenesis was also reported in petiole explants of D. rotundata, D. cayenensis, and D. alata (Anike et al. 2012). These regeneration systems are yet to be evaluated for their amenability to transformation. Regeneration through callus or somatic embryogenesis, which is ideal for transformation, remains to be established. There are only a few reports on plant regeneration from embryogenic cell cultures of Chinese yam (D. opposita), D. alata, and D. cayenensis (Belarmino and Gonzales 2008; Nagasawa and Finer 1988; Twyford and Mantell 1996; Viana and Mantell 1989). However, there is no report of regeneration from somatic embryogenesis of D. rotundata. We have obtained embryogenic callus but further research is needed to develop an efficient regeneration protocol using callus.

As a transformation system for yam is currently not available, therefore, IITA, with support from the Bill & Melinda Gates Foundation, is conducting research to develop a regeneration and transformation system for yam varieties most preferred by farmers. Once a transformation system for yam is established, the protocol will then be used for producing nematode-resistant varieties for effective control of this major pest in yam production systems.

References

Adegbite AA et al. 2006. Survey of plant-parasitic nematodes associated with yams in Edo, Ekiti and Oyo states of Nigeria. African J Agric Res 1: 125-130.

Adeniyi OJ et al. 2008. Shoot and plantlet regeneration from meristems of Dioscorea rotundata poir and Dioscorea alata L. African J Biotechnol 7: 1003-1008.

Anike FN et al. 2012. Efficient shoot organogenesis in petioles of yam (Dioscorea spp.). Plant Cell Tiss Organ Cult 111:303–313.

Belarmino MM et al. 2008. Somatic embryogenesis and plant regeneration in purple food yam (Dioscorea alata L.). Ann Trop Res 30:22-33.

Bridge J et al. 2005 Nematode parasites of tropical root and tuber crops. In: Luc, M., Sikora, R.A. and Bridge, J. (eds) Plant Parasitic Nematodes in Subtropical and Tropical Agriculture, 2nd edn. CABI Publishing, Wallingford, UK, pp. 221-258.

Nagasawa A, Finer JJ. 1988. Plant regeneration from embryogenic suspension cultures of Chinese yam (Dioscorea opposite thumb.). Plant Sci 60:263–271.

Tor M et al. 1993. Stable transformation of the food yam Dioscorea alata L. by particle bombardment. Plant Cell Rep 12: 468-473.

Tor M et al. 1998. Isolation and culture of protoplasts from immature leaves and embryogenic cell suspensions of Dioscorea yams: tools for transient gene expression studies. Plant Cell Tiss Organ Cult 53:113–125.

Twyford CT, Mantell SH. 1996. Production of somatic embryos and plantlets from root cells of greater yam. Plant Cell Tissue Organ Cult 46:17–26.

Viana AM, Mantell SH. 1989. Callus induction and plant regeneration from excised zygotic embryo of the seed propagated yams Dioscorea composite Hemsl. and D. cayenensis Lam. Plant Cell Tissue Organ Cult 16: 113–122.

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