According to Jared Diamond in Guns, Germs, and Steel, what are the eight prerequisites for turning a wild plant into a domesticate?
The answer to this question can be found in Chapter 7 of Guns, Germs, and Steel. There, Diamond describes a number of criteria that make it more likely that a plant will be domesticated.
The first criterion is size. If a plant has larger fruit, it is likelier to be domesticated because people will be able to get more food for a given amount of effort as they picked the food. The second criterion is taste. Plants that taste good are likely to be selected by people and domesticated. Then, on p.119, Diamond introduces a few more criteria. He says that plants are more likely to be domesticated if they have fruit that is very fleshy or has few seeds. He says they are more likely to be domesticated if they have oily seeds or if they have long fibers (for making into cloth).
After that, the criteria are less obvious. Diamond says that plants can only be domesticated if they do not have good mechanisms for dispersing their seeds. He says that some seeds have mechanisms that prevent them from all germinating at the same time. These kinds of plants would not be good for domesticating because their seeds would not all germinate at the same time and be mature at the same time. These are the criteria that are the prerequisites for domesticating wild plants.