The carbon dioxide in Mars' atmosphere isn't the problem: it's the fact that the atmosphere is about 1% the thickness of the Earth's atmosphere. With such a thin atmosphere, the boiling point of water is below the freezing point, which means that you can't have anything living on the surface of Mars that requires liquid water like a plant (or anything else).

We can take plants to Mars that would be able to survive in the carbon dioxide, as long as the plants are somewhat isolated such that they can become self-sustaining by the oxygen they produce as well. However, Martian soil, while having all the nutrients that Earth plants need, are high in perchlorates that are toxic to many organisms from Earth. Without the perchlorates, the soil from Mars would be able to support some plants from Earth.

Provided you are not too restrictive with your definition of plants, yes. Lichen (a sort of composite plant/animal of algae and bacteria) was able to survive for over a month in a simulated Martian environment. These lichens were particularly hardy though, and there are many other issues which would complicate large-scale vegetation growth, such as the dearth of water. Although the atmosphere is predominantly CO₂, which plants might like, there isn't much of it; the pressure of the Mars atmosphere is only around 0.6% that of Earth's sea level pressure. In addition, Mars is cold. While the warmest days at the equator might reach 70°F, the nights there can still be -100°F. Part of this is due to the lack of an atmosphere to trap in heat energy. Low light levels (depending on latitude), various types of damaging radiation, and potentially unsuitable soil conditions would also impede progress.

Plants need really specific conditions to grow beyond atmospheric requirements. They need light, proper ratio of nutrients in the soil, and stable environmental conditions. Mars is -80F on average and even though it can get up to 70F during the day near Mars’ equator, it can plummet to -200F that night. A lot of plants would not be able to withstand those temperature fluctuations.

Some of the nutrients in Mars’ soil would be enough to support plant growth, but that really depends on the region you design to plant on Mars. Check this article out for more information:
here

The idea of transforming a planet to make it more earth-like and thus more habitable to humans is called “terraforming,” and was suggested by Carl Sagan as early as 1961 for Venus. Venus has an atmosphere that is rich in carbon dioxide and so Sagan speculated about introducing photosynthetic algae as a way to transform the atmosphere. Unfortunately, Mars does not have much carbon dioxide in its atmosphere. Mars is very cold (no greenhouse gases), very dry (no liquid water), and is constantly being bombarded by solar UV radiation. Currently, no organism, not even plants or algae or bacteria could survive the harsh conditions on Mars. Mars today is very different than the early earth, which had lots of carbon dioxide, nitrogen, and water, and was warm because of the greenhouse effect. All those conditions are required for plants. Mars does have a lot of frozen carbon dioxide and water in its polar icecaps, so there is speculation that if this could be melted and released, then this could create an atmosphere around Mars that could support plant life. One idea is to use giant mirrors to reflect the sun’s rays into the Martian icecaps to release the frozen carbon dioxide.

Once the temperature on Mars is warm enough and there is enough carbon dioxide and water in the atmosphere, then plants could do photosynthesis and produce oxygen eventually transforming the atmosphere to be similar to earth. It is estimated that the overall process to convert Mars to be habitable to humans would take more than 100,000 years, and so most people are not willing to wait that long.