Environmental Engineering Reference
In-Depth Information
possibility of genetic shifts, hybrid seed production fields can be used for many years, perhaps even
the lifetime of a hybrid cultivar.
22.7.5 t
ranSgEnic
S
witchgraSS
Transgenic switchgrass plants have been created using bombardment with tungsten-coated particles
or Agrobacterium-mediated procedures (Richards et al. 2001; Somleva et al. 2002). Transgenes for
herbicide resistance and reporter genes have both been expressed in transgenic switchgrass plants.
Controlled crosses between transgenic and nontransgenic plants resulted in the expected expression
of both genes in T
1
plants. So far, the greatest limitation to transformation of switchgrass comes from
the limited number of genotypes that are capable of plantlet regeneration from tissue culture. Because
of this limitation, Alamo is the only switchgrass cultivar that has been successfully transformed and
regenerated from culture (Denchev and Conger 1994; Alexandrova et al. 1996a, 1996b; Dutta Gupta
and Conger 1999; Richards et al. 2001; Somleva et al. 2002; Mazarei et al. 2008). Because of the
climatic limitations of Alamo, deployment of transgenic switchgrass into upland genotypes and to
geographic regions to which Alamo is not adapted becomes complicated by the need to transfer
transgenes from Alamo to other genotypes. Transfer of transgenes from Alamo to other genotypes
will necessarily include large chromosome segments that may be deleterious to northern strains of
switchgrass and reduce genetic complementation between upland × lowland hybrids.
Deployment of transgenes in commercial switchgrass cultivars for use in North America will
likely require the use of a hybrid system that prevents the introduction of transgenes into wild
or natural switchgrass populations. Such a system could be based on the hybrid seed production
scheme of Martinez-Reyna and Vogel (2008) in which two parental clones are increased by somatic
embryogenesis and transplanted into alternating rows. Using this system, the transgenic parent must
be utilized as the female and the nontransgenic parent utilized as the male. In addition, the female
transgenic parent must have a sterility system, such as cytoplasmic male sterility, such that the
parental clone itself and all of its progeny are male sterile, incapable of releasing transgenic pollen
into the wild. Such a system could borrow from the maize hybrid model in which there are fewer
rows of male parents relative to female parents, to avoid large increases in seed production costs
associated with lack of hybrid seed production on the male parents.
22.7.6 dna m
arkEr
S
ElEction
Development of efficient DNA marker systems and identification of associations between DNA
markers and quantitative trait loci (QTL) has been hampered in switchgrass because of the
complexity of its genome organization. Existence of diploid (2
n
= 2
x
= 18) plants would greatly
simplify the discovery of efficient and inexpensive markers and the association of those markers
with QTL. Only one diploid accession has ever been reported, originating near Chippewa Falls,
Wisconsin (Nielsen 1944). In 1996, the site at which this accession was originally collected had
been developed into an urban landscape, destroying all native switchgrass, illustrating just how
important habitat loss and fragmentation may be to preservation of switchgrass germplasm. More
recently, diploid switchgrass plants have been discovered and confirmed, resulting from haploidy in
tetraploid seed stocks (Casler and Price 2010, unpublished data; Young et al. 2010).
Although RAPD markers were used for some of the earlier germplasm diversity research, most
current efforts are focused on development of expressed sequence tag (EST) markers or EST-
SSR (simple sequence repeat) markers (Tobias et al. 2005, 2006; Narasimhamoorthy et al. 2008).
Expressed sequence tag markers hold great promise as tools for selection because many EST
markers can be traced to functional genes that can be associated with target plant traits and they are
amenable to development of single nucleotide polymorphism (SNP) markers.
Strategies for use of DNA markers as selection tools rely on development of predictable and
reliable associations between markers and traits that may take on one of several forms. First and
