Looking at just one gene would give an inaccurate phylogeny for several reasons. One is that the gene chosen might be the subject of horizontal gene transfer, which would make it seem closely related to whatever plant or other organism was the source of the gene not its actual relatives. Also, different genes mutate at different rates and are subject to different selection pressures in different plants, which means that different genes can give differerent phylogenies.
Work Step by Step
First, scan the section of the chapter near the figure with which this question deals, and see that it mentions horizontal gene transfer (review earlier chapters on this topic, if needed). Imagine how this could confuse a phylogeny. Then think about other evolutionary concepts which affect phylogenetics, such as mutation and selection and think of ways in which one gene is insufficient to properly do that work.