I think the most promising routes use antibodies. You could either develop an ELISA or do western blot analysis of plant material - both need a good and specific antibody. To generate these, the protein of interest (or at least parts of it) are injected into animals (typically mice or rabbit) and then antibodies are pourified from the blood of these animals. These antibodies are polyclonal, but this approach is rather fast and can be done in a few weeks.
If you want to use a more sustainable source of antibodies, the antibody producing cells from these animals are isolated, immortalized and characterized as single clones to get monoclonal antibodies.
Since you are using an enzyme, you could also think about activity assays. So either a chromogenic or fluorogenic substrate is metabolized, or you could use coupled reactions where your enzyme uses a substrate which is refilled by another reaction - classical examples here are coupled reactions which use NADH or ATP.
It is also possible to measure the metabolic rate if the substrate or the product of your enzyme shows fluorescence. Then you can either measure the decrease of your substrate or the increase of your product.