Ryegrass (Lolium spp.) is an outcrossing and genetically
heterogeneous species. Cultivars consist of many genotypes,
with each genotype responding differently in callus formation
and plant regeneration (Takahashi et al. 2004). Occasionally,
a responsive genotype derived from mature seed may lose its
regeneration ability, and the risk of somaclonal variation is
increased by prolonged callus culture. If responsive genotypes
can be maintained in vitro, researchers can produce transgenic
plants at any time with calli induced from shoot tips.
We induced vigorous and friable embryogenic calli from
shoot tips of the forage-type perennial ryegrass cv. Norlea
(Figure 2a) and co-cultivated them with A. tumefaciens har-
boring the binary vector pIG121Hm. Putative transgenic calli
and regenerated plants were screened using a histochemical
assay and PCR analysis. This approach is a minor modifica-
tion of the rice (Oryza sativa L.) transformation method
described by Hiei et al. (1994). In our approach, MS medium
was used instead of N6 medium, because MS medium may be
more successful for callus induction and growth in temperate
grasses (Yoshizawa et al. 1988). In addition, the hygromycin
concentration was increased to enhance the selection of
transformed calli.
Transient GUS assay was carried out with calli after co-
cultivation and distinct blue spots were observed (Figure 2b).
After 6 – 8 weeks, hygromycin-resistant calli of some genotypes
on the callus culture medium containing 100 mg L
–l
hygro-
mycin (Figure 2c) exhibited blue GUS staining ranging from
completely blue (Figure 2d) to partially blue; others showed
a complete absence of blue staining. PCR analysis of the
hygromycin-resistant calli was performed with two primer
sets to amplify a 644 bp gus fragment and a 408 bp hpt frag-
ment. With each primer set, a single fragment was amplified
from calli exhibiting totally blue GUS expression, partially
blue GUS expression, and no blue GUS expression. The sizes
of the amplification products were equivalent to the expected
sizes amplified from pIG121Hm. Most of the calli with no
blue GUS expression had no transgenes, suggesting that
non-transformed calli escaped the hygromycin selection.