DMT and its indole family of compounds are indeed synthesized by plants. Some of their elucidated functions include stabilization and appropriate spatial conformation of proteins, particularly enzymes, assisting them in their capacity to modify their three dimensional structure(allosterism) as they bind and unbind from their substrates. Indoles can also function in a hormonal capacity. Additionally they are anti-oxidants that may scavenge unpaired reactive oxygen radicals that can be damaging to proteins and nucleic acids(RNA, DNA).

These fascinating compounds are most definitively not "just present". They are synthesized "intentionally" through complex metabolic pathways catalyzed by enzymes and co-enzymes driven by liberating energy stored in high energy phosphate bonds that are "created" directly through photo-phosphorylation or indirectly using stored substrate(carbohydrates).

Peace