Formal:

* Botanist: This is the most common and widely recognized term.

* Phytologist: This term specifically emphasizes the study of plants.

* Plant physiologist: This term focuses on the study of plant functions and processes.

* Plant geneticist: This term focuses on the study of plant genetics and heredity.

* Plant pathologist: This term focuses on the study of plant diseases.

* Horticulturist: This term focuses on the cultivation and management of plants, especially for food or ornamental purposes.

* Agronomist: This term focuses on the science and technology of crop production.

Informal:

* Plant expert: This term is a general descriptor for someone with knowledge about plants.

* Green thumb: This term is used colloquially for someone who is good at growing plants.

The best word to use depends on the specific context and the focus of the plant scientist's work.