PRE-CONDITIONING RHODODENDRON AND CHRYSANTHEMUM LEAF EXPLANTS AFFECTS TRANSIENT BETA-GLUCURONIDASE EXPRESSION AFTER BIOLISTIC PARTICLE BOMBARDMENT

Biolistic particle bombardment is used to transfer desirable genes into difficult to transform plants, but research is lacking on factors that can affect transformation success. This study was conducted to determine if manipulating culture conditions for Rhododendron 'PJM Hybrid' and 'Iridon' chrysanthemum (Dendranthema grandiflora Tzvelev.) leaf explants could alter transient gene expression of -glucuronidase (GUS) after bombardment. Both rhododendron and chrysanthemum leaf explants were bombarded with 1.0 μm gold particles coated with 2 μg of linear plasmid DNA, containing the GUS gene. Different explant treatments before bombardment were tested, including time in culture on regeneration medium, growing in the presence or absence of 0, 0.09, 0.2, 0.4, or 0.6 M sucrose, and growing in the dark for 0, 3, or 6 days. Transient GUS expression was histochemically quantified 2 days after bombardment as the percentages of GUS positive explants and the number of blue spots per GUS positive explant. Transient GUS expression was highest when 'PJM Hybrid' explants were incubated on regeneration medium 9 to 12 days before bombardment, whereas transient GUS expression by 'Iridon' explants was relatively high (>42%) for leaves incubated 3 to 18 days before bombardment. Exposure of 'PJM Hybrid' explants to media containing different concentrations of sucrose decreased transient GUS expression by 71% compared to those grown on sucrose-free medium. Alternatively, 27% more 'Iridon' explants transiently expressed GUS compared to those on sucrose-free medium, and the mean number of spots per GUS positive explant increased from 5 to 13. Transient GUS expression increased almost three fold when 'PJM Hybrid' explants were placed in the dark for six days compared those in the light. In contrast, the dark treatment had inconsistent effects on transient GUS expression by 'Iridon' explants. Manipulating explant culture conditions before biolistic bombardment influenced transient gene expression and illustrated that genotype-specific differences affect transformation potential.

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