A single hemoglobin gene in Myrica gale retains both symbiotic and non-symbiotic specificity

Abstract

Here, a hemoglobin gene from the nitrogen-fixing actinorhizal plant Myrica gale was isolated, cloned and sequenced. The gene (MgHb) was a class I hemoglobin with strong sequence homology to non-symbiotic hemoglobin genes. MgHb is highly expressed in symbiotic root nodules, but transcripts and protein were also detected in leaves of M. gale. In Arabidopsis thaliana the MgHb promoter, linked to a beta-glucuronidase coding region, directed expression in the vascular tissue, in shoot meristem and at root branch point--a pattern very similar to the combined expression pattern of the two non-symbiotic A. thaliana hemoglobin promoters AHb1 and AHb2. The results points to a symbiotic as well as a non-symbiotic specificity of MgHb similar to a hemoglobin gene identified in Parasponia andersonii, but different from the situation in Casuarina glauca--a close actinorhizal relative of M. gale.

Nodules and oxygen

“…This is not consistent with the fact that hypoxia increases NO production and that the transcription of many plant class I hb genes, e.g., the one from M. gale, is induced by hypoxia (Heckmann et al 2006). Yet, it would be consistent with the fact that the transcription of the Arabidopsis trHb gene, encoding another putative NO scavenger, is reduced by hypoxia (Watts et al 2001).…”

“…The hbI gene of M. gale was expressed at high levels in nodules and at low levels in non-symbiotic organs, and the expression pattern of a promoter-GUS fusion in Arabidopsis had similarities with those of the endogenous class I and class II hemoglobin genes (Heckmann et al 2006). It was induced by hypoxia and by ethylene.…”

“…Tjepkema 1983) with envelopes of similar thickness as in the well-aerated nodules of Alnus (Berg 1994). Furthermore, while Myrica nodules, like C. glauca nodules, contain high concentrations of a hemoglobin (Pathirana and Tjepkema 1995), this is not a class II hemoglobin (Heckmann et al 2006).…”

“…The Alnus hemoglobin was not present in nodules in amounts comparable to that of the symbiotic hemoglobin from Casuarina nodules, but the Myrica hemoglobin was. Cloning of the corresponding genes from A. firma (Sasakura et al 2006) and M. gale (Heckmann et al 2006) revealed that both proteins represented class I hemoglobins, and for the A. firma hemoglobin a function in NO scavenging could be demonstrated experimentally.…”

“…However, studies on oxygen dissociation rates and regulation of gene expression in culture have been performed (Beckwith et al 2002;Schwintzer and Tjepkema 2005), and for Frankia trHbO, a role in the facilitation of oxygen diffusion has been suggested based on nodule oximetry data (Pawlowski et al 2007). Earlier studies on CO-reactive heme contents of actinorhizal nodules demonstrate that if a class I hemoglobin is present in D. glomerata nodules as shown for A. firma (Sasakura et al 2006) and M. gale (Heckmann et al 2006), it is only present at extremely low levels.…”