Genetic analysis of plant disease resistance pathways.

Plant disease resistance (R) genes are introduced into high yielding crop varieties to improve resistance to agronomically important pathogens. The R gene-encoded proteins are recognitionally specific, interacting directly or indirectly with corresponding pathogen avirulence (avr) determinants, and are therefore under strong diversifying selection pressure to evolve new recognition capabilities. Genetic analyses in different plant species have also revealed more broadly recruited resistance signalling genes that provide further targets for manipulation in crop improvement strategies. Understanding the processes that regulate both plant-pathogen recognition and the induction of appropriate defences should provide fresh perspectives in combating plant disease. Many recent studies have utilized the model plant, Arabidopsis thaliana. Here, mutational screens have identified genes that are required for R gene function and for restriction of pathogen growth in compatible plant-pathogen interactions. Genetic analyses of these plant mutants suggest that whilst signalling pathways are conditioned by particular R protein structural types they are also influenced by pathogen lifestyle. Two Arabidopsis defence signalling genes, EDS1 and PAD4, are required for the accumulation of salicylic acid, a phenolic molecule required for systemic immunity. The cloning, molecular and biochemical characterization of these components suggests processes that may be important in their disease resistance signalling roles.

Dissecting embryonic and seedling morphogenesis in Arabidopsis by promoter trap insertional mutagenesis.

Development can be considered to comprise the co-ordinated regulation of patterning at different levels: patterning of cells to form tissues, patterning of tissues to form organs, and patterning of organs to generate the characteristic architecture of the organism. These processes are expected, in turn, to be mediated by the precise spatial and temporal regulation of patterns of gene expression during development, which depend on appropriate signalling mechanisms. In order to investigate molecular events of morphogenesis in plants, we have utilized a system of promoter trap insertional mutagenesis in Arabidopsis, to generate both phenotypic mutants and gene fusions that represent markers useful in studying the regulation of patterning. A screen of transgenic seedlings containing a T-DNA promoter trap has led to the identification of mutants defective in seedling shape and embryonic development, and of GUS fusion genes that are expressed in spatially restricted patterns. Mutants have been crossed with marker lines expressing cell type-specific GUS activities, to investigate their cellular organization. For example, the POLARIS marker gene is expressed in the embryonic and seedling root tip. When crossed with hydra, which lacks an embryonic root, and with emb30, which lacks both embryonic and seedling roots, it is nevertheless expressed in the correct relative position, and we hypothesize that it represents a novel marker of root positional information, independent of root morphogenesis.

Mutations in the HYDRA1 gene of Arabidopsis perturb cell shape and disrupt embryonic and seedling morphogenesis.

Mutations in the HYDRA1 (HYD1) gene of Arabidopsis thaliana can prevent normal morphological development of embryos and seedlings. Three allelic mutants (hydra 1-1, hydra1-2 and hydra1-3) have been identified, and in each the seedling is characterized by having a variable number of cotyledons, a short and wide hypocotyl and a much reduced root system. hydra1 embryos appear to develop normally to the octant stage, but fail to establish a distinct protoderm and lack bilateral symmetry, developing multiple cotyledonary primordia of irregular size and shape. Cells of the embryo proper, but not the suspensor, exhibit abnormalities in size and shape. The hydra1 embryo fails to develop an embryonic root, but embryos and seedlings express molecular markers of apical-basal polarity. Mutant seedlings produce leaves to form a small cabbage-like habit and may occasionally produce sterile flowers, though the mutation is commonly seedling-lethal. hydra1 seedlings exhibit abnormal radial patterning, but nevertheless express at least one molecular marker of vascular cell differentiation. A model is proposed in which the HYDRA1 protein functions as an essential component of the cell expansion system.