This paper evaluated the occurrence of hormetic dose responses in pollen reported over the past eight decades. Hormetic doses responses were induced by a wide range of chemical and physical agents in 34 plant species for pollen germination and pollen tube growth/elongation. Agents inducing such hormetic dose/concentration responses in pollen included nutrients, growth-promoting agents, plant and animal hormones, toxic substances, including heavy metals such as cadmium, gaseous pollutants such as ozone, as well as ionizing and non-ionizing radiation. This paper provides further evidence for the broad generality of the hormesis dose response, supporting substantial prior findings that the hormetic response is independent of biological model, inducing agent, and endpoints measured. Given the widespread potential of inducing hormetic dose responses in pollen, these findings indicate the need to explore their emerging biological, ecological, agricultural, economic and public health implications.
In flowering plants, the reproduction process is mediated by the pollen tube. This tubular structure provides the vehicle by which two immobile sperm cells are transported to the ovule, the location of two female reproduction cells (Sprunck, 2010; Dresselhaus and Franklin-Tong, 2013; Vogler et al., 2014). The pollen tube develops only after the pollen grain is transported, attaches, and then rehydrates on the receptive papillae that reside on the top of the stigma. After this process, the pollen grain becomes activated to form the so-called pollen tube, which may be characterized as a tubular protrusion that is chemically directed via complex physical and sensory processes through the pistil, eventually reaching the female gametophyte. At this point in the process, the male reproductive cells finally arrive at the target destination and initiate the process of fertilization (Palanivelu and Tsukamoto, 2012). Beyond their essential role in fertilization, pollen tubes are widely used to assess biological polarity, with broad applications to other cell types and biological processes. Of considerable theoretical and practical significance is that the pollen grain displays a self-organizing system that permits pollen to germinate, producing pollen tubes in vitro within an external signal mediated framework. The pollen germination rates, pollen tube growth rates, and final tube length are typically lower in in vitro studies than those observed in whole plants, most likely due to the absence of female factors that affect pollen germination, tube growth and growth directionality (Johnson and Presse, 2002; Chae and Lord, 2011). The selection of plants for pollen studies is affected by many species-specific processes as there are many factors that affect pollen tube growth, including the morphology of the reproductive tract (e.g. the stigma, style, the septum epidermis, and the funiculus). The nutrient containing extracellular matrix (ECM) contains a nutrient mixture that is important in enhancing pollen tube elongation Journal Pre-proof Journal Pre-proof 4 (Johnson and Presse, 2002; Chae and Lord, 2011). However, many studies employ bi-cellular pollen from species such as tobacco, which yields reliable findings (Volger et al., 2014).