Characterization of the primary pollen signal in the postpollination syndrome of Phadaenopsis flowers

In many flowers, and especially in orchids, pollination regulates a syndrome of developmental events that collectively prepare the flower for fertilization while shedding of organs that have completed their function in pollen dispersal and reception. In this study, we performed a water extraction of the primary pollen signal(s) from the pollinia of Phalaenopsis flowers and characterized its biochemical nature. The primary pollen signal is readily soluble in water and is a relatively small molecular substance below 3000 MW. The pollen signal is probably not proteinaceous in nature, since biological activity was retained after digesting the pollen diffusate by Proteinase K or boiling for 30. By separating the pollen diffusate on an amino anion exchange column, we found that different fractions induced the postpollination syndrome suggesting that different pollen-borne substances may be involved in the pollination response. More than 90% of a radiolabeled free IAA standard coeluted with a specific fraction, however other collected fractions also induced the postpollination response, suggesting that IAA can not be the only primary pollen signal as previously described. High pressure liquid chromatography analysis revealed that the pollen diffusate contained two major peaks and five smaller peaks of detected substances. Fractions containing substances from two of these peaks completely mimicked the postpollination response of perianth senescence and ovary growth, while fractions of the other peaks only induced perianth senescence. By running additional standards, it was found that 1-aminocyclopropane-1-carboxylic acid peaked at the same retention time as one of the major pollen diffusate peaks, while the free IAA standard peak could not be correlated to any of the pollen diffusate peaks. In the future, further purification of these peaks, and analysis by gas chromatography coupled with mass spectrometry, will provide more information about the exact nature of the primary pollen signals.