When a soul attains the lowest forms of birth such as an animal, plant, etc. as a result of sinful deeds in the past, how can that soul uplift itself through virtuous deeds? If we consider the act of a plant giving itself to be eaten as a virtuous deed, what virtuous deeds can be attributed to animals?

It is difficult for a soul to do virtuous deeds if it is born as an animal, plant, etc., because of its sinful deeds.
Notes:
Only in a human form of life, can one do virtuous or sinful deeds. It is not possible to do so when born as a plant or animal. (It is a different matter that sin or virtuous deed befall them).
Only sins are destroyed through taking birth as animals or birds. Once the results of sins and virtues are exhausted through such births, they area gain born as human beings. Through a human birth, one has the opportunity to do virtuous deeds and attain an elevated state. If one end up committing sins once again in the human birth, they might end up taking the birth of insects, etc. All of these births are for us to exhaust effects all the sins and virtues and to accumulate them.