Highlights

Abstract

Graphical abstract

Keywords

1. Introduction

2. Literature search strategy & statistics

3. Historical framework: early hormetic findings

4. Radiation – UV and ionizing

5. Pollution and pollen

6. Polyamines

7. Discussion

8. Conclusion

CRediT authorship contribution statement

Declaration of competing interest

Acknowledgement

Appendix A. Supplementary data

References

Abstract

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.

1. INTRODUCTION

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).

Key Features of Pollen Germination in Cereals

Genetic and Proteomic Screening for Key Factors in Pollen Germination

Key Pathways Identified for Pollen Germination in Cereals

Concluding Remarks

References

Germination of Pollen

In flowering plants, the distribution of pollen (see Glossary) across land by wind and/or animals drives the spread of genetic variation within a species. Agriculturally, successful fertilization of flowers in an increasingly harsh and unpredictable climate is the key determinant of grain production and yield in cereal crops [1]. Mature pollen usually contains only two cell types – sperm cells and the much larger vegetative (or tube) cell – and desiccates before release from the anther. On adhering to a compatible stigma, pollen grains respond to signals from the mother plant and become active, undergoing cytoplasmic reorganization and activation of stored RNA and protein to produce a pollen tube originating from the vegetative cell. The pollen tube elongates within the pistil until it reaches the egg cell, where the sperm cell divides to enable the double-fertilization event typical of plants (Figure 1, Key Figure).
The factors that control successful pollen germination have been the focus of plant reproduction, evolution, and breeding research to increase crop yield and to overcome hybridization barriers [1,2]. Barriers to fertilization include interspecific incompatibility as well as self-incompatibility, which maintains genetic diversity within a local population. However, overcoming some of these constraints in a breeding environment can contribute to the production of novel elite lines for specific end-uses. Since 2003, pollen transcriptomic studies, together with identified mutants, have rapidly expanded our knowledge of the regulatory mechanisms that govern the fertilization process, including signaling pathways and cytoskeleton proteins [3–11]. However, most of this research has occurred in model plant species such as arabidopsis; despite massive agronomic and economic interest, relatively little is known about these molecular processes in cereal crops (the Poaceae). This review focuses on the unique characteristics of pollen grain adhesion, hydration, germination, and pollen tube growth in monocot cereals, and provides an overview of our current understanding of the regulators governed by the male gametophyte in the best-studied species, rice (Oryza sativa) and maize (Zea mays) (Figure 1C).

Viable seeds of numerous plant species are not capable of germinating immediately after harvest under conditions suitable for the germination process. Such seeds are termed primarily dormant. Primary dormancy is established during seed development and maturation on the mother plant. This type of dormancy is particularly common in wild plants. Dormancy is a very important phenomenon which prevents germination on mother plants, facilitates seed dispersal, ensures plant survival of natural catastrophes, and reduces intra-specifc competition (Bewley et al. 2013). The phenomenon has turned out to be non-obligatory, its expression depending on environmental factors, e.g., temperature. Seeds can be fully dormant; such seeds are not able to germinate at any temperature (Hilhorst 2007). There are also seeds which are not capable of germination only within a certain temperature range, whereas they germinate at temperatures outside that range. Thus, the expression of dormancy in such seeds depends on temperature, and dormancy release is associated with widening the range of germination temperature. Under natural conditions, primarily dormant seeds are exposed to fuctuating environmental conditions, e.g., light, temperature, moisture, and the presence of gases, which leads to dormancy state cyclicity (Finkelstein et al. 2008). Primary dormancy can be removed also by cold stratifcation, dry storage, light, or chemicals (Bewley et al. 2013). It is commonly accepted that the balance between abscisic acid (ABA) and gibberellins (GAs) and/or sensitivity to these hormones are responsible for regulation of the dormancy state and germination of seeds in response to environmental signals (Finkelstein et al. 2008; Rodríguez-Gacio et al. 2009). ABA is considered as the most important hormone responsible for the establishment of dormancy during seed development and for maintenance of dormancy during seed imbibition. In turn, GAs have been cited as factors involved in dormancy release and/or germination. Dormancy release has been shown as involving a decline in the ABA content and an increase of the GAs level (Bewley et al. 2013).

مقاله سال 2016

زیرگونه‌های خربزه‌های کوچک وحشی اگرستیس، یک نوع گیاه مهاجم در بسیاری از نواحی آسیا و شمال آمریکا هستند. تولیدمثل این‌ گونه‌ها از طریق دانه انجام می‌شود. بنابراین ، آزمایشات آزمایشگاهی به منظور تعیین دمای کاردینال و تأثیر نوسانات دما، گرما ، استرس غوطه‌وری در آب، PH، طول عمر دانه، و عمق کاشت دانه خربزه برای جوانه‌زنی و پیدایش جوانه انجام شدند. از بین سه مدل نمونه آزمایش شده، مدل خطوط تقاطعی بهترین برآورد را در مورد دمای کاردینال داشت. دمای پایه ، بهینه و ماکسیمم برای جوانه زدن دانه خربزه به ترتیب 20 ، 35، و 45 درجه سانتی‌گراد تخمین زده شد. بالاترین میزان جوانه‌زنی خربزه وحشی در °C 20 /30 و °C 35 /25 (روز / شب) مشاهده شد. فقط قرارگیری طولانی‌مدت آن‌ها ( تا 10 روز) در دمای °C 90 و یا بیش از 5 دقیقه در °C 120 در کاهش جوانه‌زنی دانه خربزه وحشی اثرگذار بود. غوطه‌وری در آب تا زمانی که 3 ماه از انجام آن گذشته باشد، تأثیر ناچیزی در کاهش جوانه‌زنی دانه خربزه دارد. دانه‌های این علف هرز در طیف گسترده‌ای از مقادیر PH جوانه‌زده ، اما میزان جوانه‌زنی تحت شرایط اسیدی (PH 5) بیشتر از شرایط پایه (PH 8 , 9) بود. جوانه‌زنی دانه‌های خربزه وحشی با گذشت 23 ماه از کاشتن آن‌ها در خاک در عمق 15 سانتی‌متری تا 50% کاهش پیدا کرد. پیدایش جوانه‌های خربزه وحشی در عمق‌های کمتر بیشتر از عمق‌های بیشتر بود. این نتایج نشان می‌دهند که گسترش موفق خربزه وحشی در منطقه تا حدودی می‌تواند با تحمل آن در برابر شرایط محیطی توضیح داده شود. ترکیبی از طول عمر دانه ، عدم رکود دانه و جوانه‌زنی سریع دانه تحت شرایط نامطلوب آب و هوایی ممکن است توضیحی برای رشد سریع دانه‌های هرز در محصولات تابستانی باشد. در بین استراتژی‌های مدیریت که می‌توانند برای محدود کردن هجوم خربزه‌های وحشی در زمین‌ها بکار برده شوند، به نظر می‌رسد که کاشت عمیق دانه در طی آماده‌سازی خاک ، امیدوارکننده‌تر باشد.

1. مقدمه
خربزه‌های کوچک وحشی و خوش بوی اگرستیس ، مختص منطقه اوراسیا می‌باشند و از گونه‌های مهاجم در بسیاری از قسمت‌های جهان هستند. آن به‌عنوان یکی از دانه‌های هرز مهم خانواده کدو محسوب می‌شود و محصولاتی مانند کتان ، ( پروستکو و چاندلر ،1998، تینگل و همکاران ،2003)، بادام زمینی ( گریچار ، 20007a) و دانه سویا ( پریچار، 2007b، سهرابی و همکاران ،2013) را موردحمله قرار می‌دهد. این خربزه‌های وحشی ، تاک‌های بالارونده‌ای هستند که دارای گل‌های نر و ماده در یک گیاه می‌باشند و از طریق دانه تولیدمثل می‌کنند. ویژگی‌های اصلی که این گونه‌های خودرو را از خربزه‌های کاشته شده متمایز می‌کند، اندازه گیاه و میوه آن است. گیاهان و میوه‌های گونه‌های کاشته شده آن، بزرگ‌تر از گونه‌های خودرو هستند. میوه‌های این گیاهان خودرو به شکل بیضوی و کروی هستند و دارای قطر 1.9 تا 2.5 سانتی‌متر و طول 2 تا 4.5 سانتی‌متر می‌باشند که هر یک تقریباً حاوی 180 دانه است( کونون و همکاران، 2009؛ سهرابی و همکاران ،2014).