If all the plants perish from the earth then there will be no life.
Plants are the main producers of oxygen. During the process of photosynthesis plants take in carbondioxide and give out oxygen which helps the humans in reducing the level of CO₂ in the atmosphere. If plants are perished then the monsoons will not be cyclic, that is there will be disturbances in the season cycle. And also the temperature on earth will increase.
So it is our duty to save our environment.