I will suppose you mean gene injection through genetic therapy, possibly CRISPR-Cas, not merely dropping naked DNA on the blood stream.
Well, we already have plant genes.
Plants and animals have a common ancestor. They branched out in the tree of life about 850 million years ago, give or take. Probably on a Tuesday.
In evolutionary science, genes shared by different species may be orthologous:
Homologous sequences are orthologous if they are inferred to be descended from the same ancestral sequence separated by a speciation event: when a species diverges into two separate species, the copies of a single gene in the two resulting species are said to be orthologous. Orthologs, or orthologous genes, are genes in different species that originated by vertical descent from a single gene of the last common ancestor.
And here is an article by Dr. Natasha Glover, whose PhD is in plant genomics and biotechnology:
What genes do I have in common with a plant?
In short: we humans share 12,792 orthologous pairs of genes with the thale cress.
Our current estimate for the number of human genes gives us an amount of 46,831. So if you inject a thale cress gene in a human at random, there is approximately a chance of one in four that you will be just duplicating a gene. Depending on which one the person may get sick or may even become more resistant to cancer (if the thale cress has the p53 gene and you happen to copy that). Most likely, though, nothing will happen.
If you do happen to give that person a plant exclusive gene, though... Same results. You won't get a person able to do photosynthesis, because on top of that depending on a very complex set of gene expression, we don't have chloroplasts. You might get genes for resistance against the TMV virus, which is anticlimactic because it cannot infect humans anyway. But you may also create a human that is able to generate their own linoleic and α-linolenic acids, two kinds of unsaturated fatty acids, which would have zero impact on their general quality of life too.
Stan Lee said in his autobiography that if you are aiming to write a story set in a fantastic world with mutant superheroes, you should tone the science down. In this kind of fiction, the more you science things, the more boring things get.