Immunomodulation of bioactive gibberellin confers gibberellin-deficient phenotypes in plants

Immunomodulation is a means to modulate an organism's function by antibody production to capture either endogenous or exogenous antigens. This method was applied to plants to repress the function of gibberellins (GAs), a class of phytohormones responsible for plant elongation, by anti-bioactive GA antibodies. Two different antibodies were produced in Arabidopsis as single-chain variable fragment (scFv) fused to green fluorescent protein (GFP) with four different subcellular localizations: endoplasmic reticulum (ER), cytosol, apoplastic space or the outer surface of the plasma membrane. When targeting scFv-GFP to ER, plants showed the highest accumulation of scFv-GFP, with binding activity, strong GFP fluorescence in ER-derived compartments and mild but clear GA-deficient phenotypes, including a smaller leaf size, delayed bolting, shorter inflorescence length and decreased germination. Plants expressing scFv-GFP in ER responded to exogenous GA, and contained 15-40 times greater endogenous GA, than wild-type plants. They also showed increased gene expression for GA3ox1, GA20ox1 and GA20ox2, but decreased expression for GA2ox1, which are feedback and feedforward regulated by GA signalling, respectively. These results suggest that the level of free functional GA(4) decreased when trapped in the ER with scFv to the extent that mild GA-deficient phenotypes were created. A dramatic increase in the total sum of GA4 (free plus scFv-GFP bound) was detected as a result of the up-regulation of GA biosynthesis (feedback regulated), and a decrease in GA, catabolism as a result of protection by scFv-GFP binding. This study demonstrates that the use of immunomodulation to inhibit the action of bioactive GAs is an effective method of creating GA-deficient plants.