Response of Rhododendron 'Montego' with 'tissue proliferation' to cytokinin and auxin in vitro

Abstract

To characterize the in vitro behavior of Rhododendron 'Montego' with tissue proliferation (TP) to cytokinin and auxin, comparisons were made of normal [TP(-)], dwarf TP [TP(+) dwarf], and long TP [TP(+) long] shoot cultures. On basal medium TP(-) and TP(+), long shoots failed to multiply and had a low relative growth rate (RGR) of 0.1, whereas TP(+) dwarf shoots produced 31.8 shoots per tip, with most shoots being <5 mm long, and RGR was 0.3. Addition of 15 μM 2iP to basal medium induced the production of more than six shoots per TP(-) tip and doubled their RGR; TP(+) long shoots produced 16.8 shoots, most <5 mm long, and had an RGR of 0.3; TP(+) dwarf shoots produced only 16% as many shoots as on basal medium, but still exhibited an increase in RGR. Leaves from TP(-) and TP(+) sources failed to produce shoots on basal medium, but 74% of TP(-) leaves formed shoots when cultured on 1 μM IBA and 30 μM 2iP. TP(+) leaves were able to form shoot meristems on media containing only 5 μM 2iP (26% of explants), but these meristems failed to elongate into shoots. Calli from TP(-) leaves, TP(+) leaves, and TP(+) tumors grown on medium containing 10 μM NAA and 15 μM 2iP had higher RGRs than the same calli on basal medium during the first 8 weeks of culture. Over time, RGR decreased in both TP(-) and TP(+) leaf calli, but increased in TP(+) tumor callus. The increased RGR resulted from differentiation of shoot meristems on 85% of the calli between week 4 and week 8. Our results suggest that TP(+) tissues have altered hormone metabolism or sensitivity that leads to dramatic differences in in vitro behavior and probably contributes to tissue proliferation observed in whole plants. Chemical names used: 6-(γ,γ-dimethylallylamino) purine (2iP); indole-3-butyric acid (IBA); α-naphthaleneacetic acid (NAA).