Viewpoint: The Poisonous Pollens, why do they exist? (New Phytol)

Besides being the carrier of male gametes in plants, pollen cells also unexpectedly contained some toxic properties that are collectively called “Pollen Defensive Compounds”. This raises the question: what is the evolutionary significance of these toxic compounds? In this Viewpoint by Rivest and Forrest, three hypotheses are presented, particularly as they pertain to the role of bees in driving the evolution of pollen defenses. The first, Pleiotropy hypothesis, claims that these defensive compounds just happen to be defensive, it reflects nothing but physiological spillover of the adjacent plant tissue. Alternatively, the Protection-against-pollen-collection hypothesis indicates that low-efficiency pollinators and pollen thieves drive the evolution of pollen defense in plants. The third is that the pollen toxic metabolites are protective from microorganism colonization. The review summarizes numerous studies the shed light on these hypotheses, but as yet the primary driver of pollen defenses is uncertain. It seems may be that they protect pollen from both herbivores and pathogens. Additional studies to better understand poison pollen are proposed. (Summarized by Nanxun Qin) New Phytol. 10.1111/NPH.16230