Why conserve germfree-germplasm?

Dominique Dumet, d.dumet@cgiar.org and Lava Kumar, l.kumar@cgiar.org

Seeds of important grain legumes are conserved in IITA's Genetic Resources Center. Photo by IITA.
Seeds of important grain legumes are conserved in IITA's Genetic Resources Center. Photo by IITA.

Plant genetic resources (germplasm) are the foundation for sustainable agriculture and global food security. They possess genes that offer resistance to pests and diseases and resilience to abiotic stresses, such as drought tolerance, soil erosion, and other constraints.

However, genetic resources are eroding at unprecedented rates as a result of the loss of habitat, outbreaks of pests and diseases, and abiotic stresses. Therefore, it has become imperative to conserve genetic resources for agricultural sustainability and the preservation of global biodiversity.

In the mid-1970s, IITA has initiated an ex situ conservation of germplasm of important African food crops which are held in trust on behalf of humanity under the auspices of the United Nations. To date, IITA’s Genetic Resources Center (GRC) conserves over 27,000 accessions of six main collections of African staple crops, namely, cowpea and other Vigna, soybean, maize, cassava, banana, and yam. Germplasm is distributed worldwide for use in research for food and agriculture. Depending on the species’ reproductive biology and mode of dissemination, collections are stored in field, seed, or in vitro genebanks.

Conservation of virus-free germplasm. Source: L. Kumar, IITA.
Conservation of virus-free germplasm. Source: L. Kumar, IITA.

However, germplasm (seeds or vegetative propagules) infested with pathogens such as, viruses, fungi, bacteria, and nematodes, insects, mites and even weeds (hereafter all referred to as pests) can spread along with the planting materials. Because of this risk, planting materials are traditionally sourced from healthy-looking plants and as an additional safety measure they are treated with chemicals to eliminate bacteria, fungi, nematodes, insects and other pests. However, viral pathogens are difficult to detect and pose challenges to “clean” (pest-free) planting material production procedures. IITA’s collections were sourced over 35 years from several countries in Africa and other parts of the world.

Knowledge on viruses infecting crops conserved in the IITA genebank and the means for their detection and production of clean planting material have dramatically improved over the past two decades. To ensure that germplasm conserved is free of pests, particularly viruses, a systematic approach was taken to assess the health status of every accession in the genebank and produce clean planting materials for conservation.

For seed-propagated crops (maize and legumes), clean seed production requires planting accessions in contained screenhouses. Emerging plants are monitored for symptoms and each plant is tested using diagnostic tools for all known seed-transmitted viruses occurring in the territory where they were last grown. Plants that test positive for virus and/or showing virus-like symptoms are destroyed. Seeds are harvested from the virus-negative, healthy-looking plants. Clean seeds are then deposited in the germplasm collections. This work started in 2008, and so far over 4000 accessions of legumes have been evaluated and clean seed material produced have been conserved in the genebank.

Researcher in genebank. Photo by IITA.
Researcher in genebank. Photo by IITA.

For clonally propagated crops (cassava, yam and banana), production of clean planting material involves in vitro procedures using meristem culture. In cassava, source plants are subjected to thermotherapy (exposing plants to 27-30 °C) from 1 to 3 weeks prior to meristem excision and in vitro propagation. In vitro plants are indexed for viruses and plants that test positive are discarded while virus-negative plants are further propagated for conservation in the in vitro genebank. So far, over 2000 accessions of clonal crops have been subjected to this process to derive virus-free plants.

Production and conservation of “clean” planting material is expensive; however it improves the turn-around time for processing germplasm for exchange and dramatically improves its use. In addition, clean germplasm improves the viability of the material conserved in the genebank and prevents the risk of the accidental spread of pests from one region to another through the planting materials.