Cell cycle regulation is of pivotal importance for plant growth and development. Although plant cell division shares basic mechanisms with all eukaryotes, plants have evolved novel molecules orchestrating the cell cycle. Some regulatory proteins, such as cyclins and inhibitors of cyclin-dependent kinases, are particularly numerous in plants, possibly reflecting the remarkable ability of plants to modulate their postembryonic development. Many plant cells also can continue DNA replication in the absence of mitosis, a process known as endoreduplication, causing polyploidy. Here, we review the molecular mechanisms that regulate cell division and endoreduplication and we discuss our understanding, albeit very limited, on how the cell cycle is integrated with plant development.

Cell cycle regulation in plant development.

In flowering plants, male reproductive development requires the formation of the stamen, including the differentiation of anther tissues. Within the anther, male meiosis produces microspores, which further develop into pollen grains, relying on both sporophytic and gametophytic gene functions. The mature pollen is released when the anther dehisces, allowing pollination to occur. Molecular studies have identified a large number of genes that are expressed during stamen and pollen development. Genetic analyses have demonstrated the function of some of these genes in specifying stamen identity, regulating anther cell division and differentiation, controlling male meiosis, supporting pollen development, and promoting anther dehiscence. These genes encode a variety of proteins, including transcriptional regulators, signal transduction proteins, regulators of protein degradation, and enzymes for the biosynthesis of hormones. Although much has been learned in recent decades, much more awaits to be discovered and understood; the future of the study of plant male reproduction remains bright and exciting with the ever-growing tool kits and rapidly expanding information and resources for gene function studies.

Molecular genetic analyses of microsporogenesis and microgametogenesis in flowering plants

In mammals, cadmium is widely considered as a non-genotoxic carcinogen acting through a methylation-dependent epigenetic mechanism. Here, the effects of Cd treatment on the DNA methylation patten are examined together with its effect on chromatin reconfiguration in Posidonia oceanica. DNA methylation level and pattern were analysed in actively growing organs, under short- (6 h) and long- (2 d or 4 d) term and low (10 mM) and high (50 mM) doses of Cd, through a Methylation-Sensitive Amplification Polymorphism technique and an immunocytological approach, respectively. The expression of one member of the CHROMOMETHYLASE (CMT) family, a DNA methyltransferase, was also assessed by qRT-PCR. Nuclear chromatin ultrastructure was investigated by transmission electron microscopy. Cd treatment induced a DNA hypermethylation, as well as an up-regulation of CMT, indicating that de novo methylation did indeed occur. Moreover, a high dose of Cd led to a progressive heterochromatinization of interphase nuclei and apoptotic figures were also observed after long-term treatment. The data demonstrate that Cd perturbs the DNA methylation status through the involvement of a specific methyltransferase. Such changes are linked to nuclear chromatin reconfiguration likely to establish a new balance of expressed/repressed chromatin. Overall, the data show an epigenetic basis to the mechanism underlying Cd toxicity in plants.

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In Posidonia oceanica cadmium induces changes in DNA methylation and chromatin patterning

The outer pollen wall, or exine, is more structurally complex than any other plant cell wall, comprising several distinct layers, each with its own organizational pattern. Since elucidation of the basic events of pollen wall ontogeny using electron microscopy in the 1970s, knowledge of their developmental genetics has increased enormously. However, self-assembly processes that are not under direct genetic control also play an important role in pollen wall patterning. This review integrates ultrastructural and developmental findings with recent models for self-assembly in an attempt to understand the origins of the morphological complexity and diversity that underpin the science of palynology.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-8137.2007.02060.x

Pollen wall development in flowering plants

This article provides detailed protocols for collecting pollen and outlines genetic crosses and phenotypic assays that are useful for characterizing mutants that affect pollen development.

/pdf/a-compendium-of-methods-useful-for-characterizing-4f3km8rxos.pdf

A compendium of methods useful for characterizing Arabidopsis pollen mutants and gametophytically‐ expressed genes

Male meiosis in higher organisms features synchronous cell divisions in a large number of cells. It is not clear how this synchrony is achieved, nor is it known whether the synchrony is linked to the regulation of cell cycle progression. Here, we describe an Arabidopsis mutant, named tardy asynchronous meiosis ( tam ), that exhibits a phenotype of delayed and asynchronous cell divisions during male meiosis. In Arabidopsis, two nuclear divisions occur before simultaneous cytokinesis yields a tetrad of haploid cells. In tam , cell divisions are delayed, resulting in the formation of abnormal intermediates, most frequently dyad meiotic products, or in rare cases, dyad pollen (two gametophytes within one exine wall). Temperature-shift experiments showed that the percentage of the abnormal intermediates increased at 27°C. Analysis of tam and the tam / quartet1 double mutant showed that most of these abnormal intermediates could continue through the normal rounds of cell divisions and form functional pollen, though at a slower than normal pace. The asynchrony of cell division started at the G2/M transition, with cells entering metaphase at different time points, during both meiosis I and II. In addition, chromosome condensation defects and mis-segregation were sometimes observed in tam . These observations suggest that the TAM protein positively regulates cell cycle progression, perhaps by promoting the G2/M transition. We speculate that there is a signal, perhaps TAM, that couples the normal pace of cell cycle progression with the synchrony of cell division during male meiosis.

/pdf/the-arabidopsis-gene-tardy-asynchronous-meiosis-is-required-563vd2d6bg.pdf

Yun-Chia Sophia Chen

Papers

The Arabidopsis gene tardy asynchronous meiosis is required for the normal pace and synchrony of cell division during male meiosis.