In addition to the above, you should also consider the laboratory facilities of your institution or university. The topic of your thesis could be based on studying one of the related problems in the field of agriculture in your country using biotechnology techniques, such as studying genetic variation or resistance to biotic and abiotic stresses.
1) you will not be doing PhD on your own resources, so your topic is of less concern than the facility available at the institute you will be doing it.
2) read and research in literature and make your own proposal.
3) If you cant do the above and depends on others to help with readymade food, better not go for a PhD. Apparently, PhD means doing something new by critical thinking and innovating. You you feel that is not your cup of tea, better stop here.
I am agree with suggestion of Mr Abhijeet Singh. To choose a PhD topic you read many literature then you will be able to make your own proposal. you will get lots of idea by reading current research article related to plant biotechnology and crop improvement. for crop improvement.
Multi stress resistance crop plant is a good field in plant biotechnology and it have future prospect also but it depends on your resource and supervisor along with institutional choice.
In addition to the above, you should also consider the laboratory facilities of your institution or university. The topic of your thesis could be based on studying one of the related problems in the field of agriculture in your country using biotechnology techniques, such as studying genetic variation or resistance to biotic and abiotic stresses.