Regulation of the boron transporter EgBor1 in Eucalyptus globulus: a plausible model

Abstract

Boron is a micronutrient that plays an important role in plant cell wall biosynthesis. Nevertheless, an excess of boron in the soil causes severe damage to the respiratory tissue of the plant. In Arabidopsis thaliana, the Atbor1 gene encodes a boron transporter that distributes this element throughout the plant. A cDNA sequence encoding a bor1 transporter was isolated from a Eucalyptus globulus cDNA library. This sequence contains several stop codons within the coding region. Initial bioinformatic analyses suggest that this interruption corresponds to an intron that may generate a truncated protein. Egbor1was overexpressed in Saccharomyces cereviseae to assess whether it was capable of restoring the phenotype of a mutant strain that lacks the boron transporter. It was also overexpressed in a wild type strain as a control. In both cases a significant increase in boron tolerance was observed, suggesting that the encoded transporter is functional. Subsequently, a western blot analysis showed that the expressed transporter corresponds to the product of the full-length protein, rather than the truncated protein. Additional bioinformatic analyses showed that the intron presents several regulatory elements, therefore it may function as a promoter for a small protein encoded in the 3’ region of Egbor1. Transient expression of GUS under the control of the intron sequence proved its capability to activate gene expression. Thus, we have identified three ORFs in the Egbor1 sequence: a full length protein encoded by the spliced mRNA named fragment C, and two smaller proteins encoded by the 5’ and the 3’ regions of the non spliced mRNA named fragments A and B. We are currently challenging the yeast mutant strain with all the putative encoded proteins to understand their specific role in boron transport. Funded by PFB-016 and DI-UNAB.