Abstract
Vigorous shoots in chrysanthemums (Chrysanthemum morifolium) often develop from shoot tips (apical meristems with two leaf primordia) cultured in vitro following particle bombardment. The average fresh weight of regenerated shoots from a bombarded shoot tip of chrysanthemum ‘Jinba’ was 10 times more than that regenerated from unbombarded shoot tips. The average number of leaves per bombarded shoot tip was also more than that from an unbombarded shoot tip. The average number of leaves developing from a shoot tip increased with an increase in the amount of gold particles shot into the shoot tips. In addition, when the area destroyed in a shoot apical meristem (SAM) was varied by bombardment through nylon mesh with different pore sizes, the total number of leaves produced from each shoot tip increased with the size of the destroyed SAM area. Knowing the origin of these vigorous shoots, which may be from the bombarded meristem or from the lateral meristems, is important for the screening of transgenic plants. When the entire surface of a SAM was destroyed by bombardment, it was unable to rebuild itself; instead, lateral meristems were initiated at the base of the leaf primordia. Furthermore, the initiation of lateral meristems at the base of the leaf primordia was also observed in instances of restoration of the area of a partially destroyed SAM. This result indicates that vigorous lateral shoots initiate and develop from the bases of leaf primordia when SAMs are damaged to varying degrees. When leaf primordia-free shoot apical meristems (LP-free SAMs) were cultured after bombardment, vigorous shoots failed to develop from the wounded SAM; instead, the wounded LP-free SAMs regenerated a SAM by repairing the wounded areas and developed a non-vigorous single shoot. It was concluded the vigorous shoots do not participate in transgenic plant production because vigorous shoots arise from unbombarded lateral meristems. Finally, an effective and versatile method for transgenic plant production was established by combining micro-wound treatment on a SAM by bombardment and LP-free SAM culture to suppress the growth of vigorous lateral shoots after wounding.
