Perhaps. It all depends on how you look at it. Personally I don't think of plants as moral agents, because they don't have a capacity to consciously suffer. At least that is according to my definitions.

But perhaps for some definition of consciousness and some definition of suffering even a plant can suffer. Perhaps according to those definitions even a traffic light can suffer. Sounds crazy, but it all depends on the definition. If something responding to stimuli to serve some goal is consciousness then a traffic light has consciousness. There is no universally accepted defintions.

Yet then the difference between a pig and a traffic light is so extreme that a cut off point seems reasonable. I certainly also don't consider mosquitoes as important as pigs. Then we can assign certain moral weight to anything within some range of intelligence and capacity to suffer. At some point we might even have to consider sufficiently powerful AI moral agents. Perhaps the neuron count would be part of that scoring equation.

The least amount of harm that we can inflict on other living creatures weighted by this set of scores while maximizing our own happiness, yet not over estimating our own worth, should be the goal.

Under those conditions eating plants is still better than eating animals, because animals eat plants and thus you indirectly cause more plant death by eating animals.

We would then have some suffering index and could calculate that by going vegan you lower your suffering index by a factor 10, similar to how we know it reduces your carbon footprint by a factor 2 or so.