Phototropism is growth of an organism which responds to a light stimulus. It is most often observed in plants, but can also occur in or organisms such as fungi. The cells on plant that are farst from light have a chemical called auxin that reacts when phototropism occurs. Gene rapy is an experimental technique that uses genes to treat or prevent disease. In future, this technique may allow doctors to treat a disorder by inserting a gene into a patient's cells instead of using drugs or surgery. 

So correct option is 'Reversible by gene rapy'.