In plants, male reproductive development is extremely sensitive to adverse climatic environments and (a)biotic stress. Upon exposure to stress, male gametophytic organs often show morphological, structural and metabolic alterations that typically lead to meiotic defects or premature spore abortion and male reproductive sterility. Depending on the type of stress involved (e.g. heat, cold, drought) and the duration of stress exposure, the underlying cellular defect is highly variable and either involves cytoskeletal alterations, tapetal irregularities, altered sugar utilization, aberrations in auxin metabolism, accumulation of reactive oxygen species (ROS; oxidative stress) or the ectopic induction of programmed cell death (PCD). In this review, we present the critically stress-sensitive stages of male sporogenesis (meiosis) and male gametogenesis (microspore development), and discuss the corresponding biological processes involved and the resulting alterations in male reproduction. In addition, this review also provides insights into the molecular and/or hormonal regulation of the environmental stress sensitivity of male reproduction and outlines putative interaction(s) between the different processes involved.

BackgroundThe in vivo determination of the cell-specific chromosome number provides a valuable tool in several aspects of plant research. However, current techniques to determine the endosystemic ploidy level do not allow non-destructive, cell-specific chromosome quantification. Particularly in the gametophytic cell lineages, which are physically encapsulated in the reproductive organ structures, direct in vivo ploidy determination has been proven very challenging. Using Arabidopsis thaliana as a model, we here assess the applicability of recombinant CENH3-GFP reporters for the labeling of the cell’s chromocenters and for the monitoring of the gametophytic and somatic chromosome number in vivo.ResultsBy modulating expression of a CENH3-GFP reporter cassette using different promoters, we isolated two reporter lines that allow for a clear and highly specific labeling of centromeric chromosome regions in somatic and gametophytic cells respectively. Using polyploid plant series and reproductive mutants, we demonstrate that the pWOX2-CENH3-GFP recombinant fusion protein allows for the determination of the gametophytic chromosome number in both male and female gametophytic cells, and additionally labels centromeric regions in early embryo development. Somatic centromere labeling through p35S-CENH3-GFP shows a maximum of ten centromeric dots in young dividing tissues, reflecting the diploid chromosome number (2x = 10), and reveals a progressive decrease in GFP foci frequency throughout plant development. Moreover, using chemical and genetic induction of endomitosis, we demonstrate that CENH3-mediated chromosome labeling provides an easy and valuable tool for the detection and characterization of endomitotic polyploidization events.ConclusionsThis study demonstrates that the introgression of the pWOX2-CENH3-GFP reporter construct in Arabidopsis thaliana provides an easy and reliable methodology for determining the chromosome number in developing male and female gametes, and during early embryo development. Somatically expressed CENH3-GFP reporters, on the other hand, constitute a valuable tool to quickly determine the basic somatic ploidy level in young seedlings at the individual cell level and to detect and to quantify endomitotic polyploidization events in a non-destructive, microscopy-based manner.Electronic supplementary materialThe online version of this article (doi:10.1186/s12870-015-0700-5) contains supplementary material, which is available to authorized users.

In the plant kingdom, events of whole genome duplication or polyploidization are generally believed to occur via alterations of the sexual reproduction process. Thereby, diploid pollen and eggs are formed that contain the somatic number of chromosomes rather than the gametophytic number. By participating in fertilization, these so-called 2n gametes generate polyploid offspring and therefore constitute the basis for the establishment of polyploidy in plants. In addition, diplogamete formation, through meiotic restitution, is an essential component of apomixis and also serves as an important mechanism for the restoration of F1 hybrid fertility. Characterization of the cytological mechanisms and molecular factors underlying 2n gamete formation is therefore not only relevant for basic plant biology and evolution, but may also provide valuable cues for agricultural and biotechnological applications (e.g. reverse breeding, clonal seeds). Recent data have provided novel insights into the process of 2n pollen and egg formation and have revealed multiple means to the same end. Here, we summarize the cytological mechanisms and molecular regulatory networks underlying 2n gamete formation, and outline important mitotic and meiotic processes involved in the ectopic induction of sexual polyploidization.

No abstract

Whole-genome duplication through the formation of diploid gametes is a major route for polyploidization, speciation, and diversification in plants. The prevalence of polyploids in adverse climates led us to hypothesize that abiotic stress conditions can induce or stimulate diploid gamete production. In this study, we show that short periods of cold stress induce the production of diploid and polyploid pollen in Arabidopsis (Arabidopsis thaliana). Using a combination of cytological and genetic analyses, we demonstrate that cold stress alters the formation of radial microtubule arrays at telophase II and consequently leads to defects in postmeiotic cytokinesis and cell wall formation. As a result, cold-stressed male meiosis generates triads, dyads, and monads that contain binuclear and polynuclear microspores. Fusion of nuclei in binuclear and polynuclear microspores occurs spontaneously before pollen mitosis I and eventually leads to the formation of diploid and polyploid pollen grains. Using segregation analyses, we also found that the majority of cold-induced dyads and triads are genetically equivalent to a second division restitution and produce diploid gametes that are highly homozygous. In a broader perspective, these findings offer insights into the fundamental mechanisms that regulate male gametogenesis in plants and demonstrate that their sensitivity to environmental stress has evolutionary significance and agronomic relevance in terms of polyploidization.