Plant species have been improved ever since people stayed in one place and began planting seeds. Slowly, they selected the best varieties from wild plants which were barely edible:- carrots, parsnip, grass seeds of barley, wheat, oats types, fruits. Slowly, they saved and re-planted the seeds from the plants with the best taste, or biggest seeds, or that grew best in a dry year. Gradually, our species improved.
Most of the last century has also seen moderately slow improvement - selection of better pest resistance, breeding plants which respond better to chemical fertilisers, breeding temperate species for sub-tropical conditions (e.g. apples can now be grown in most countries). This has all been done by pollinating one plant with another, or by grafting a bud onto a better root stock, in ways that are by now traditional.
However, in the last 10 to 15 years, there has been a great step forward with the possibility of direct genetic manipulation. The genetic code has been dismantled, and it is possible to take a chunk of genetic material from one species and transplant it into another, so that one plant acquires characteristics from another, for example, total resistance to a disease, or total resistance to a particular chemical, or drought resistance.
Current Situation. Genetic manipulation is claimed to give miraculous results, however, there are powerful lobbies against it - mostly campaigning on the basis that nature forms a fragile network - if, for example, the pollen from maize is altered, then the butterflies and insects that feed on it may either be altered or not be able to digest it, and starve, and this will cause further effects that are entirely unpredictable. One English newspaper called them 'Frankenstein foods', because nobody knows what will happen. Field trials are being done, but there is some evidence that modified genetic material is escaping into 'the wild' and affecting other species. The 'network of nature' is so large and complex that it is almost impossible to predict the consequences of such an 'escape' of altered materials (and perhaps it always will be)