Differential expression of members of the annexin multigene family in Arabidopsis

The Nod factor-elicited annexin MtAnn1 is preferentially localised at the nuclear periphery in symbiotically activated root tissues of Medicago truncatula

The Medicago truncatula MtAnn1 gene, encoding a putative annexin, is transcriptionally activated in root tissues in response to rhizobial Nod factors. To gain further insight into MtAnn1 function during the early stages of nodulation, we have examined in detail both spatio-temporal gene expression patterns and MtAnn1 activity and localisation in root tissues. Analysis of transgenic Medicago plants expressing a pMtAnn1-GUS fusion has revealed a novel pattern of transcription in both outer and inner cell layers of the root following either Nod factor-treatment or rhizobial inoculation. The highest gene expression levels were observed in the endodermis and outer cortex. These transgenic plants also revealed that MtAnn1 expression is associated with lateral root development and cell differentiation in the root apex independent of nodulation. By purifying recombinant MtAnn1 we were able to demonstrate that this plant annexin indeed possesses the calcium-dependent binding to acidic phospholipids typical of the annexin family. Antisera against recombinant MtAnn1 were then used to show that tissue-specific localisation of the MtAnn1 protein in Medicago roots matches the pMtAnn1-GUS expression pattern. Finally, both immunolabelling and in vivo studies using MtAnn1-GFP reporter fusions have revealed that MtAnn1 is cytosolic and in particular localises to the nuclear periphery in cortical cells activated during the early stages of nodulation. In the light of our findings, we discuss the possible role of this annexin in root tissues responding to symbiotic rhizobial signals.

Microarray analysis of chitin elicitation in Arabidopsis thaliana

Summary Chitin oligomers, released from fungal cell walls by endochitinase, induce defence and related cellular responses in many plants. However, little is known about chitin responses in the model plant Arabidopsis. We describe here a large-scale characterization of gene expression patterns in Arabidopsis in response to chitin treatment using an Arabidopsis microarray consisting of 2375 EST clones representing putative defence-related and regulatory genes. Transcript levels for 71 ESTs, representing 61 genes, were altered three-fold or more in chitin-treated seedlings relative to control seedlings. A number of transcripts exhibited altered accumulation as early as 10 min after exposure to chitin, representing some of the earliest changes in gene expression observed in chitin-treated plants. Included among the 61 genes were those that have been reported to be elicited by various pathogen-related stimuli in other plants. Additional genes, including genes of unknown function, were also identified, broadening our understanding of chitin-elicited responses. Among transcripts with enhanced accumulation, one cluster was enriched in genes with both the W-box promoter element and a novel regulatory element. In addition, a number of transcripts had decreased abundance, encoding several proteins involved in cell wall strengthening and wall deposition. The chalcone synthase promoter element was identified in the upstream regions of these genes, suggesting that pathogen signals may suppress the expression of some genes. These data indicate that Arabidopsis should be an excellent model to elucidate the mechanisms of chitin elicitation in plant defence.

Genes for calcium-permeable channels in the plasma membrane of plant root cells

In plant cells, Ca(2+) is required for both structural and biophysical roles. In addition, changes in cytosolic Ca(2+) concentration ([Ca(2+)](cyt)) orchestrate responses to developmental and environmental signals. In many instances, [Ca(2+)](cyt) is increased by Ca(2+) influx across the plasma membrane through ion channels. Although the electrophysiological and biochemical characteristics of Ca(2+)-permeable channels in the plasma membrane of plant cells are well known, genes encoding putative Ca(2+)-permeable channels have only recently been identified. By comparing the tissue expression patterns and electrophysiology of Ca(2+)-permeable channels in the plasma membrane of root cells with those of genes encoding candidate plasma membrane Ca(2+) channels, the genetic counterparts of specific Ca(2+)-permeable channels can be deduced. Sequence homologies and the physiology of transgenic antisense plants suggest that the Arabidopsis AtTPC1 gene encodes a depolarisation-activated Ca(2+) channel. Members of the annexin gene family are likely to encode hyperpolarisation-activated Ca(2+) channels, based on their corresponding occurrence in secretory or elongating root cells, their inhibition by La(3+) and nifedipine, and their increased activity as [Ca(2+)](cyt) is raised. Based on their electrophysiology and tissue expression patterns, AtSKOR encodes a depolarisation-activated outward-rectifying (Ca(2+)-permeable) K(+) channel (KORC) in stelar cells and AtGORK is likely to encode a KORC in the plasma membrane of other Arabidopsis root cells. Two candidate gene families, of cyclic-nucleotide gated channels (CNGC) and ionotropic glutamate receptor (GLR) homologues, are proposed as the genetic correlates of voltage-independent cation (VIC) channels.

Plant annexins form calcium-independent oligomers in solution

The oligomeric state in solution of four plant annexins, namely Anx23(Ca38), Anx24(Ca32), Anx(Gh1), and Anx(Gh2), was characterized by sedimentation equilibrium analysis and gel filtration. All proteins were expressed and purified as amino-terminal His(n) fusions. Sequencing of the Anx(Gh1) construct revealed distinct differences with the published sequence. Sedimentation equilibrium analysis of Anx23(Ca38), Anx24(Ca32), and Anx(Gh1) suggests monomer-trimer equilibria for each protein with association constants in the range of 0.9 x 10(10)-1.7 x 10(11) M(-2). All four proteins were subjected to analytical gel filtration under different buffer conditions. Observations from this experiment series agree quantitatively with the ultracentrifugation results, and strongly suggest calcium independence of the annexin oligomerization behavior; moreover, binding of calcium ions to the proteins seems to require disassembly of the oligomers. Anx(Gh2) showed a different elution profile than the other plant annexins; while having only a very small trimer content, this annexin seems to exist in a monomer-dimer equilibrium in solution.