Calmodulin-binding transcription activator (CAM-TA) 3 (also called SR1) is a calmodulin-binding transcription factor in Arabidopsis. Two homozygous T-DNA insertion mutants (camta3-1, camta3-2) showed enhanced spontaneous lesions. Transcriptome analysis of both mutants revealed 6 genes with attenuated expression and 99 genes with elevated expression. Of the latter, 32 genes are related to defense against pathogens (e.g. WRKY33, PR1 and chitinase). Propagation of a virulent strain of the bacterial pathogen Pseudomonas syringae and the fungal pathogen Botrytis cinerea were attenuated in both mutants. Moreover, both mutants accumulated high levels of H 2 O 2 . We suggest that CAMTA3 regulates the expression of a set of genes involved in biotic defense responses.

SummaryThe sfr6 mutant of Arabidopsis displays a deficit in freezing tolerance after cold acclimation. We previously observed that the transcripts of three cold-, ABA-and drought-inducible genes, each having a C-repeat motif or the drought-responsive element (CRT/DRE) in its promoter, failed to normally accumulate in this mutant. We now report that the effects of sfr6 upon transcript levels are reflected in the levels of the encoded proteins, confirming that the cold-inducible protein expression is affected by the sfr6 mutation. Using microarray analysis, we found not only that this effect may be general to cold-inducible genes with CRT/DRE promoter elements, but also that it extends to some other genes whose promoters lack a CRT/ DRE element. The role of the CRT/DRE has been empirically tested by use of a synthetic promoter, confirming that the CRT/DRE is sufficient to confer the sfr6 effect upon expression. Tolerance of osmotic stress was also found to be reduced in sfr6, consistent with a role in osmotic stress tolerance for the cold-, ABAand drought-inducible genes whose expression is affected by the sfr6 mutation.

Auxin is a key plant hormone that regulates various aspects of plant development. However, the mechanisms integrating auxin growth effects with stress responses are not fully understood. In this study, we investigated the possible role of calmodulin-binding transcription activator 1 (CAMTA1), an Arabidopsis thaliana calcium/calmodulin-binding transcription activator, in auxin signaling and its responses to different stresses. Plants harboring the AtCAMTA1 promoter fused to the GUS reporter gene revealed cell-specific expression patterns reminiscent of auxin responses. The responsiveness of CAMTA1 to auxin was further assessed by chemical disturbances in polar auxin transport, and by RT-PCR analysis of gene expression of dissected leaf sections from plants exposed to the auxin transport inhibitor NPA. Furthermore, the intensity and cell-specific expression patterns of CAMTA1 changed significantly and differentially on exposure to increasing salt concentrations and heat. Transcriptome analysis of a camta1 T-DNA insertion mutant revealed 63 up-regulated genes, of which 17 are associated with auxin signaling. Finally, analysis of hypocotyl elongation in the presence and absence of auxin revealed that camta1 T-DNA insertion mutants and CAMTA1-repressor lines are hyper-responsive to auxin compared to wild-type seedlings. Thus, CAMTA1 participates in auxin signaling and responds to abiotic stresses.

The most abundant proteins in the lumen of the endoplasmic reticulum (ER) are thought to be molecular chaperones, some of which might also be involved in calcium storage and release. We have purified calreticulin from maize by ion exchange and reverse-phase chromatography. Identity with plant and animal calreticulins was confirmed by N-terminal amino acid sequencing and it was shown to bind calcium with a calcium overlay technique. An antiserum raised to the purified protein was used to screen an expression library and the full coding sequence for maize calreticulin was determined from the clones selected. The sequence shows 96% identity to barley calreticulin and 55% identity to animal calreticulins. The three major functional regions are conserved, as are targeting and retention features. When visualized by indirect immunofluorescence microscopy, calreticulin was found to be confined to the ER and nuclear envelope of maize root cells. It was distributed throughout the ER compartment and we found no evidence of calreticulin-enriched areas of ER, such as might be associated with specialized calcium storage domains. Increasing or decreasing extracellular calcium did not induce measurable changes in calreticulin levels. In addition, maize calreticulin, as well as other recognized chaperones, was shown to bind to denatured protein and could be eluted specifically by nucleoside trisphosphates.

The GenBank accession number for sequence reported in this article is U08315. LITERATURE ClTED Briars S-A, Kessler F, Evans D E (1988) The calmodulin-stimulated ATPase of maize coleoptiles is a 140,000 M, polypeptide. Planta Huang L, Franklin AE, Hoffman N E (1993) Primary structure and characterisation of an Arabidopsis thaliana calnexin-like protein. J Biol Chem 268: 6560-6566 Ou W-J, Cameron PH, Thomas DY, Bergeron JJM (1993) Association of folding intermediates with calnexin during protein maturation. Nature 364 771-776