Axis formation occurs in plants, as in animals, during early embryogenesis. However, the underlying mechanism is not known. Here we show that the first manifestation of the apical-basal axis in plants, the asymmetric division of the zygote, produces a basal cell that transports and an apical cell that responds to the signalling molecule auxin. This apical-basal auxin activity gradient triggers the specification of apical embryo structures and is actively maintained by a novel component of auxin efflux, PIN7, which is located apically in the basal cell. Later, the developmentally regulated reversal of PIN7 and onset of PIN1 polar localization reorganize the auxin gradient for specification of the basal root pole. An analysis of pin quadruple mutants identifies PIN-dependent transport as an essential part of the mechanism for embryo axis formation. Our results indicate how the establishment of cell polarity, polar auxin efflux and local auxin response result in apical-basal axis formation of the embryo, and thus determine the axiality of the adult plant.

Efflux-dependent auxin gradients establish the apical–basal axis of Arabidopsis

MHC class I molecules function to present peptides eight to ten residues long to the immune system. These peptides originate primarily from a cytosolic pool of proteins through the actions of proteasomes, and are transported into the endoplasmic reticulum, where they assemble with nascent class I molecules. Most peptides are generated from proteins that are apparently metabolically stable. To explain this, we previously proposed that peptides arise from proteasomal degradation of defective ribosomal products (DRiPs). DRiPs are polypeptides that never attain native structure owing to errors in translation or post-translational processes necessary for proper protein folding. Here we show, first, that DRiPs constitute upwards of 30% of newly synthesized proteins as determined in a variety of cell types; second, that at least some DRiPs represent ubiquitinated proteins; and last, that ubiquitinated DRiPs are formed from human immunodeficiency virus Gag polyprotein, a long-lived viral protein that serves as a source of antigenic peptides.

Rapid degradation of a large fraction of newly synthesized proteins by proteasomes

Agrobacterium tumefaciens and related Agrobacterium species have been known as plant pathogens since the beginning of the 20th century. However, only in the past two decades has the ability of Agrobacterium to transfer DNA to plant cells been harnessed for the purposes of plant genetic engineering. Since the initial reports in the early 1980s using Agrobacterium to generate transgenic plants, scientists have attempted to improve this “natural genetic engineer” for biotechnology purposes. Some of these modifications have resulted in extending the host range of the bacterium to economically important crop species. However, in most instances, major improvements involved alterations in plant tissue culture transformation and regeneration conditions rather than manipulation of bacterial or host genes. Agrobacterium-mediated plant transformation is a highly complex and evolved process involving genetic determinants of both the bacterium and the host plant cell. In this article, I review some of the basic biology concerned with Agrobacterium-mediated genetic transformation. Knowledge of fundamental biological principles embracing both the host and the pathogen have been and will continue to be key to extending the utility of Agrobacterium for genetic engineering purposes.

/pdf/agrobacterium-mediated-plant-transformation-the-biology-4ez5vf6vl1.pdf

Agrobacterium-Mediated Plant Transformation: the Biology behind the “Gene-Jockeying” Tool

Arabidopsis MAP Kinase 4 Negatively Regulates Systemic Acquired Resistance

https://arca.igc.gulbenkian.pt/bitstream/10400.7/67/4/Slotkin%20et%20al%202009%20post-print.pdf

Epigenetic Reprogramming and Small RNA Silencing of Transposable Elements in Pollen

The crucifer Arabidopsis thaliana has been used widely as a model organism for the study of plant development. We describe here the development of an efficient insertional mutagenesis system in Arabidopsis that permits identification of genes by their patterns of expression during development. Transposable elements of the Ac/Ds system carrying the GUS reporter gene have been designed to act as enhancer traps or gene traps. A novel selection scheme maximizes recovery of unlinked transposition events. In this study 491 plants carrying independent transposon insertions were generated and screened for expression patterns. One-half of the enhancer trap insertions and one-quarter of the gene trap insertions displayed GUS expression in seedlings or flowers, including expression patterns specific to organs, tissues, cell types, or developmental stages. The patterns identify genes that act during organogenesis, pattern formation, or cell differentiation. Transposon insertion lines with specific GUS expression patterns provide valuable markers for studies of Arabidopsis development and identify new cell types or subtypes in plants. The diversity of gene expression patterns generated suggests that the identification and cloning of Arabidopsis genes expressed in any developmental process is feasible using this system.

/pdf/patterns-of-gene-action-in-plant-development-revealed-by-3t17mymmku.pdf

Samuel Haward

Papers

Patterns of gene action in plant development revealed by enhancer trap and gene trap transposable elements.