-ist

(Greek > Latin: a suffix; one who believes in; one who is engaged in; someone who does something)

phytophilist
A collector of plants (flowers, etc.).
phytosociologist
Someone who specializes in that aspect of ecology dealing with the origin, composition, structure, and classification of plant communities.
pietist
pirinophilist
A collector of cotton reels.
piscicapturist (s) (noun), piscicapturists (pl)
A fisherman or a catcher of fish: "There are many men in the world who are piscicapturists as a profession or just for the sport of doing it."
pisciculturist
Someone who breeds and rears fish for scientific or commercial purposes.
plagiarist (PLAY juhr ist) (s) (noun), plagiarists (pl)
Someone who purloins or utilizes the words, writings, or ideas of another person, and passes them off to others as his or her own creations: A reporter was accused of being a plagiarist, or a literary thief, when he failed to give credit to the person who wrote the original report.

Some musicians are plagiarists who take something composed by an old musical master and decompose it.

—Based on a quotation by Evan Esar in Esar's Comic Dictionary
planetologist
1. Someone who studies the planets including their origins and compositions.
2. An expert or specialist in planetology.
platformist
platitudinist
plebicolist (s), plebicolists (pl) (noun forms)
1. Someone who flatters, or courts the favor of, the common people; a demagogue.
2. Giving the appearance of being a friend of the people; an orator who appeals to the passions and prejudices of his audience.
plebiocologist
Someone who who flatters the common people in order to get them to follow his or her viewpoints and to get the people to take action in accordance with what is being promoted.
pluralist (s), pluralists (pl) (nouns)
Someone who holds multiple offices; especially, a clergyman who holds more than one ecclesiastical benefice.
plutologist (s) (noun), plutologists (pl)
Someone who is a specialist in the study of the wealthy and their powerful influences: "For her advanced umiversity degree, Karin intends to interview several plutocrats so she can become a qualified plutologist."
plutonomist (s) (noun), plutonomists (pl)
Someone who specializes in the political economy of the wealthy: "Mr. Mark was recently appointed the plutonomist for a prestigious business magazine."