MPEP § 1604
The applicant for a plant patent must be the person who has invented or discovered and asexually reproduced the new and distinct variety of plant for which a patent is sought (or as provided in §§ 1.42, 1.43 and 1.47). The oath or declaration required of the applicant, in addition to the averments required by § 1.63, must state that he or she has asexually reproduced the plant. Where the plant is a newly found plant, the oath or declaration must also state that it was found in a cultivated area.
A Plant Patent Application (35 U.S.C. 161 ) Declaration, Form PTO/SB/03, may be used to submit a declaration. Form PTO/SB/81 may be used to appoint an attorney or agent. See MPEP § 402.
In an application for a plant patent, there can be joint inventors. See Ex parte Kluis, 70 USPQ 165 (Bd. App. 1945).
Form PTO/SB/03. Plant Patent Application 
