Two EguCBF1 genes overexpressed in Eucalyptus display a different impact on stress tolerance and plant development

Two C-repeat binding factor genes (EguCBF1a/b), isolated from E. gunnii and differentially cold-regulated, were constitutively overexpressed in a cold-sensitive Eucalyptus hybrid. In addition to the expected improvement on freezing tolerance, some resulting transgenic lines (EguCBF1a-OE and EguCBF1b-OE) exhibited a decrease in stomata density and an over-accumulation of anthocyanins also observed to a lesser extent in a cold-acclimated control plant. Given that the induction of five putative CBF target genes was observed in CBF-overexpressing lines as well as in the cold-acclimated control line, these phenotypes might be related to cold acclimation. In comparison with the control plant, the most altered transgenic line (EguCBF1a-OE A1 line), exhibited reduced growth and better water retention capacity. This modified phenotype includes reduced leaf area and thickness associated with a decrease in cell size, as well as a higher oil gland density and a wax deposition on the cuticle. Surprisingly, the EguCBF1b-OE B9 line, with a level of transgene expression equivalent to the A1 line, showed a less marked phenotype, suggesting a difference in transactivation efficiency between EguCBF1A and B factors. The features of these transgenic lines provide the first signs of adaptive mechanisms controlled by CBF transcription factors in an evergreen broad-leaved tree. These data also open new prospects towards genetic improvement on Eucalyptus for freezing tolerance.

Complementary regulation of four Eucalyptus CBF genes under various cold conditions

CBF transcription factors play central roles in the control of freezing tolerance in plants. The isolation of two additional CBF genes, EguCBF1c and EguCBF1d, from E. gunnii, one of the cold-hardiest Eucalyptus species, is described. While the EguCBF1D protein sequence is very similar to the previously characterized EguCBF1A and EguCBF1B sequences, EguCBF1C is more distinctive, in particular in the AP2-DBD (AP2-DNA binding domain). The expression analysis of the four genes by RT-qPCR reveals that none of them is specific to one stress but they are all preferentially induced by cold, except for the EguCBF1c gene which is more responsive to salt. The calculation of the transcript copy number enables the quantification of constitutive CBF gene expression. This basal level, significant for the four genes, greatly influences the final EguCBF1 transcript level in the cold. A cold shock at 4 degrees C, as well as a progressive freezing which mimics a natural frost episode, trigger a fast and strong response of the EguCBF1 genes, while growth at acclimating temperatures results in a lower but more durable induction. The differential expression of the four EguCBF1 genes under these cold regimes suggests that there is a complementary regulation. The high accumulation of the CBF transcript, observed in response to the different types of cold conditions, might be a key for the winter survival of this evergreen broad-leaved tree.

Medicago truncatula DMI1 required for bacterial and fungal symbioses in legumes

Legumes form symbiotic associations with both mycorrhizal fungi and nitrogen-fixing soil bacteria called rhizobia. Several of the plant genes required for transduction of rhizobial signals, the Nod factors, are also necessary for mycorrhizal symbiosis. Here, we describe the cloning and characterization of one such gene from the legume Medicago truncatula. The DMI1 (does not make infections) gene encodes a novel protein with low global similarity to a ligand-gated cation channel domain of archaea. The protein is highly conserved in angiosperms and ancestral to land plants. We suggest that DMI1 represents an ancient plant-specific innovation, potentially enabling mycorrhizal associations.