Tomato plants!

Hi everyone! This is not a parenting question but I feel like one of you will know the answer to this!

I’m planning to garden with my daughter this year for the first time. I bought a few tomato seedlings and I’m getting ready to plant them in big 20 gallon containers. However, I am noticing that the bottom stems of each plant have turned yellow like the ones in this picture. Is this really bad news? Can I just cut them off? What should I do? Help!

Thank you!!