A cis-regulatory module activating transcription in the suspensor contains five cis-regulatory elements

The short and intricate life of the suspensor

The suspensor is a short-lived tissue critical for proper embryonic development in many higher plants. While the tissue was initially thought to simply suspend the embryo in the endosperm, it has been found through decades of research that it serves multiple important purposes. The suspensor has been found to be vital for proper embryo patterning and numerous studies have been undertaken into the complex transcriptional cross-talk between the suspensor and the embryo proper. Indeed, many suspensor mutants also display abnormalities in the embryo. The suspensor's role as a nutrient conduit has been shown using ultrastructural and histochemical techniques. Biochemical approaches have found that the suspensor is a centre of early embryonic hormone production in several species. The suspensor has also been frequently used as a model for programmed cell death as it shows signs of termination almost immediately upon developing. This review covers the essential functions of the suspensor throughout its short existence from multiple disciplines including structural, genetic and biochemical perspectives.

Using Giant Scarlet Runner Bean (Phaseolus coccineus) Embryos to Dissect the Early Events in Plant Embryogenesis

The giant embryo of the scarlet runner bean (Phaseolus coccineus) has been used historically to investigate the molecular and developmental processes that control the early events of plant embryo development. In more recent years, our laboratory has been using scarlet runner bean embryos to uncover the genes and regulatory events that control embryo proper and suspensor region differentiation shortly after fertilization. In this chapter we describe methods that we have developed to isolate scarlet runner bean embryos at the globular stage of development, and capture embryo proper and suspensor regions by either hand dissection or laser capture microdissection (LCM) for use in downstream genomic analysis. These methods are also applicable for use in investigating the early events of common bean (Phaseolus vulgaris) embryo development, a close relative of scarlet runner bean, which also has a giant embryo in addition to a sequenced genome.

Endoreplication and its consequences in the suspensor of Pisum sativum

KEY MESSAGE

DNA replication and continuous process of transcription during ongoing amitotic division accelerate the development of four-celled pea suspensor containing nuclei which create transient gradient of polyploidy necessary for correct embryo development. A suspensor, the link between embryo proper and surrounding tissues, differs significantly in size, morphology, and degree of polyploidy among the species. The suspensor of Pisum sativum consists of four polynuclear cells (two hemispherical and two elongated) formed in two layers. Their nuclei undergo endoreplication reaching, respectively, up to 256C and 128-256C DNA levels in its hemispherical and elongated parts. Our study shows that endoreplication first appears in the spherical part of the suspensor, and, subsequently, in the elongated one. At the next stages of suspensor development, the increase in DNA content takes place also in a similar order. Thus, despite simple construction of the suspensor, its development, supported by endoreplication, creates a certain gradient of polyploidy, which occurs in more extensive suspensors. Moreover, the rapid development of suspensor is supported both by the initiation of DNA replication prior to the completion of amitotic division of its polyploidal nuclei and by a continuous process of transcription, which is silenced by chromatin condensation throughout mitosis. Furthermore, the increase in DNA content correlates with the greater amount of transcripts; however, the multiplication of DNA copies does not entail an increase (but fluctuation) in the mean transcriptional activity of a particular nucleus during the next stages of suspensor development.

A shared cis-regulatory module activates transcription in the suspensor of plant embryos

The mechanisms controlling the transcription of gene sets in specific regions of a plant embryo shortly after fertilization remain unknown. Previously, we showed that G564 mRNA, encoding a protein of unknown function, accumulates to high levels in the giant suspensor of both Scarlet Runner Bean (SRB) and Common Bean embryos, and a cis-regulatory module containing three unique DNA sequences, designated as the 10-bp, Region 2, and Fifth motifs, is required for G564 suspensor-specific transcription [Henry KF, et al. (2015) Plant Mol Biol 88:207-217; Kawashima T, et al. (2009) Proc Natl Acad Sci USA 106:3627-3632]. We tested the hypothesis that these motifs are also required for transcription of the SRB GA 20-oxidase gene, which encodes a gibberellic acid hormone biosynthesis enzyme and is coexpressed with G564 at a high level in giant bean suspensors. We used deletion and gain-of-function experiments in transgenic tobacco embryos to show that two GA 20-oxidase DNA regions are required for suspensor-specific transcription, one in the 5' UTR (+119 to +205) and another in the 5' upstream region (-341 to -316). Mutagenesis of sequences in these two regions determined that the cis-regulatory motifs required for G564 suspensor transcription are also required for GA 20-oxidase transcription within the suspensor, although the motif arrangement differs. Our results demonstrate the flexibility of motif positioning within a cis-regulatory module that activates gene transcription within giant bean suspensors and suggest that G564 and GA 20-oxidase comprise part of a suspensor gene regulatory network.

Comprehensive Analysis and Expression Profiling of the OsLAX and OsABCB Auxin Transporter Gene Families in Rice (Oryza sativa) under Phytohormone Stimuli and Abiotic Stresses

The plant hormone auxin regulates many aspects of plant growth and developmental processes. Auxin gradient is formed in plant as a result of polar auxin transportation by three types of auxin transporters such as OsLAX, OsPIN, and OsABCB. We report here the analysis of two rice auxin transporter gene families, OsLAX and OsABCB, using bioinformatics tools, publicly accessible microarray data, and quantitative RT-PCR. There are 5 putative OsLAXs and 22 putative OsABCBs in rice genome, which were mapped on 8 chromosomes. The exon-intron structure of OsLAX genes and properties of deduced proteins were relatively conserved within grass family, while that of OsABCB genes varied greatly. Both constitutive and organ/tissue specific expression patterns were observed in OsLAXs and OsABCBs. Analysis of evolutionarily closely related "gene pairs" together with organ/tissue specific expression revealed possible "function gaining" and "function losing" events during rice evolution. Most OsLAX and OsABCB genes were regulated by drought and salt stress, as well as hormonal stimuli [auxin and Abscisic Acid (ABA)], which suggests extensive crosstalk between abiotic stresses and hormone signaling pathways. The existence of large number of auxin and stress related cis-regulatory elements in promoter regions might account for their massive responsiveness of these genes to these environmental stimuli, indicating complexity of regulatory networks involved in various developmental and physiological processes. The comprehensive analysis of OsLAX and OsABCB auxin transporter genes in this study would be helpful for understanding the biological significance of these gene families in hormone signaling and adaptation of rice plants to unfavorable environments.