Understanding the basis for in vitro tissue proliferation of Rhododendron `Montego'

Date of Completion

January 1998


Agriculture, Plant Culture|Biology, Plant Physiology




In Rhododendron 'Montego', unlike other rhododendrons propagated by tissue culture, tissue proliferation (TP) is first observed in vitro as shoot clusters with small leaves and nodal tumors. In addition, unlike non-TP (TP($-$)) shoots, TP (TP(+)) shoots proliferate rapidly in vitro without cytokinin. Isolated TP tumors cultured with cytokinin had lower increases in fresh mass and a more tumorous phenotype than TP tumors cultured without cytokinin, indicating that endogenous cytokinin levels are sufficient and that exogenous levels are supraoptimal for growth. Adventitious or de novo production of TP(+) shoots from TP($-$) leaves was also shown to be influenced by exogenous cytokinin. In structural comparisons of TP(+) and TP($-$) shoot tips, TP(+) shoots had less developed vascular systems, altered internodal elongation, and axillary meristems displaced onto the stem above the leafaxil. Tumor formation began at a region of cell proliferation (RCP) at the base of the meristem.^ Further tumor development was characterized by organization of the displaced axillary meristem (tumor meristem) and extension of the RCP. Polar shoot tumors were smaller, had more organized meristems and were dominated by shoot growth from the tumor meristem. Nonpolar tumors were larger due to unorganized de novo meristem formation in association with an extensive RCP, leading to the formation of tumors with or without shoots.^ Uptake and metabolism was similar between TP($-$) and TP(+) shoots. Glucosylation was the major metabolic fate of IPA, as over 60% of extracted cytokinin was tentatively identified as IP9G. The slightly higher level of degradation of IPA to AMP and the disappearance of IPA in TP(+) shoots suggests that there is greater turnover of IPA in TP(+) shoots. However, no significant differences in the endogenous levels of Z, ZR, IPA and IPAR were found between TP($-$) and TP(+) shoots and TP(+) tumors in two experiments. These results suggest that continuous adventitious events, induced by exogenous cytokinin, maintains the TP(+) phenotype without significant changes in cytokinin uptake, metabolism, or endogenous cytokinin regulation. ^