Identification of two groups of leghemoglobin genes in alfalfa (Medicago sativa) and a study of their expression during root nodule development

Metabolism of reactive oxygen species is attenuated in leghemoglobin-deficient nodules of Lotus japonicus

Leghemoglobins together with high rates of respiration are believed to be major sources of reactive oxygen species (ROS) in root nodules of leguminous plants. High capacities of antioxidative systems apparently protect this organ from oxidative damage. Using leghemoglobin-RNA interference (LbRNAi) lines of Lotus japonicus, we found that loss of leghemoglobin results in significantly lower H(2)O(2) levels in nodules. Transcript levels and catalytic activities of ascorbate-glutathione cycle enzymes involved in H(2)O(2) detoxification as well as concentrations of reduced ascorbate were also altered in LbRNAi nodules. Thus, symbiotic leghemoglobins contribute significantly to ROS generation in functional nodules.

Transcript profiling coupled with spatial expression analyses reveals genes involved in distinct developmental stages of an arbuscular mycorrhizal symbiosis

The formation of symbiotic associations with arbuscular mycorrhizal (AM) fungi is a phenomenon common to the majority of vascular flowering plants. Here, we used cDNA arrays to examine transcript profiles in Medicago truncatula roots during the development of an AM symbiosis with Glomus versiforme and during growth under differing phosphorus nutrient regimes. Three percent of the genes examined showed significant changes in transcript levels during the development of the symbiosis. Most genes showing increased transcript levels in mycorrhizal roots showed no changes in response to high phosphorus, suggesting that alterations in transcript levels during symbiosis were a consequence of the AM fungus rather than a secondary effect of improved phosphorus nutrition. Among the mycorrhiza-induced genes, two distinct temporal expression patterns were evident. Members of one group showed an increase in transcripts during the initial period of contact between the symbionts and a subsequent decrease as the symbiosis developed. Defense- and stress-response genes were a significant component of this group. Genes in the second group showed a sustained increase in transcript levels that correlated with the colonization of the root system. The latter group contained a significant proportion of new genes similar to components of signal transduction pathways, suggesting that novel signaling pathways are activated during the development of the symbiosis. Analysis of the spatial expression patterns of two mycorrhiza-induced genes revealed distinct expression patterns consistent with the hypothesis that gene expression in mycorrhizal roots is signaled by both cell-autonomous and cell-nonautonomous signals.

Analysis of Medicago truncatula nodule expressed sequence tags

Systematic sequencing of expressed sequence tags (ESTs) can give a global picture of the assembly of genes involved in the development and function of organs. Indeterminate nodules representing different stages of the developmental program are especially suited to the study of organogenesis. With the vector lambdaHybriZAP, a cDNA library was constructed from emerging nodules of Medicago truncatula induced by Sinorhizobium meliloti. The 5' ends of 389 cDNA clones were sequenced, then these ESTs were analyzed both by sequence homology search and by studying their expression in roots and nodules. Two hundred fifty-six ESTs exhibited significant similarities to characterized data base entries and 40 of them represented 26 nodulin genes, while 133 had no similarity to sequences with known function. Only 60 out of the 389 cDNA clones corresponded to previously submitted M. truncatula EST sequences. For 117 cDNAs, reverse Northern (RNA) hybridization with root and nodule RNA probes revealed enhanced expression in the nodule, 48 clones are likely to code for novel nodulins, 33 cDNAs are clones of already known nodulin genes, and 36 clones exhibit similarity to other characterized genes. Thus, systematic analysis of the EST sequences and their expression patterns is a powerful way to identify nodule-specific and nodulation-related genes.

Organization of non-vertebrate globin genes

The organization of non-vertebrate globin genes exhibits substantially more variability than the three-exon, two-intron structure of the vertebrate globin genes. (1) The structures of genes of the single-domain globin chains of the annelid Lumbricus and the mollusc Anadara, and the globin gene coding for the two-domain chains of the clam Barbatia, are similar to the vertebrate plan. (2) Genes for single-domain chains exist in bacteria and protozoa. Although the globin gene is highly expressed in the bacterium Vitreoscilla, the putative globin gene hmp in E. coli, which codes for a chimeric protein whose N-terminal moiety of 139 residues contains 67 residues identical to the Vitreoscilla globin, may be either unexpressed or expressed at very low levels, despite the presence of normal regulatory sequences. The DNA sequence of the globin gene of the protozoan Paramecium, determined recently by Yamauchi and collaborators, appears to consist of two exons separated by a short intron. (3) Among the lower eukaryotes, the yeasts Saccharomyces and Candida have chimeric proteins consisting of N-terminal globin and C-terminal flavoprotein moieties of about the same size. The structure of the gene for the chimeric protein of Saccharomyces exhibits no introns. According to Riggs, the presence of chimeric proteins in E. coli and other prokaryotes, such as Alcaligenes and Rhizobium, as well as in yeasts, suggests a previously unrecognized evolutionary pathway for hemoglobin, namely that of a multipurpose heme-binding domain attached to a variety of unrelated proteins with diverse functions. (4) The published globin gene sequences of the insect larva Chironomus have an intron-less structure and are present as clusters of multiple copies; the expression of the globin genes is tissue and developmental stage-specific. Furthermore, the expression of many of these genes has not yet been demonstrated despite the presence of apparently normal regulatory sequences in the two flanking regions. Unexpectedly, Bergtrom and collaborators have recently shown that at least three Ctt globin II beta genes contain putative introns. (5) Pohajdak and collaborators have found a seven-exon and six-intron structure for the globin gene of the nematode Pseudoterranova which codes for a two-domain globin chain. Although the second and fourth introns of the N-terminal domain correspond to the two introns found in vertebrate globin genes, the position of the third intron is close to that of the central intron in plant hemoglobins.(ABSTRACT TRUNCATED AT 400 WORDS)