Syrian rue's seeds carry harmine and harmaline, beta carboline alkaloids of the same class as those in the ayahuasca vine, which makes it a monoamine oxidase inhibitor and therefore capable of the same enzyme-blocking role in a combination.
On that basis David Flattery and Martin Schwartz argued at length in 1989 that the Avestan haoma, and by extension the Vedic soma, was Peganum harmala. Their case is philological and pharmacological together: they read the Zoroastrian textual descriptions of haoma's effects as matching harmaline intoxication, they trace the plant's names and its persistence in Iranian folk practice, and they argue that the modern Parsi use of ephedra represents a substitution made after the original was lost.
The objections are equally serious. Harmaline alone is unpleasant, producing nausea and a heavy, leaden state rather than the exhilaration and strength the hymns describe. The Vedic descriptions of a plant pressed for juice on mountains do not obviously fit a seed-bearing desert shrub. And the argument depends on reading poetic texts as clinical description, which is the same methodological move that weakens the fly agaric case.
There are at least three live candidates for soma and haoma: ephedra, on grounds of ritual continuity; fly agaric, on Wasson's reading; and Syrian rue, on Flattery and Schwartz's. None is established. The atlas holds this as an open question and records all three plants as candidates rather than picking one.