It is now more than 80 years since boron was convincingly demonstrated to be essential for normal growth of higher plants. However, its biochemical role is not well understood at the moment. Several recent reviews propose that B is implicated in three main processes: keeping cell wall structure, maintaining membrane function, and supporting metabolic activities. However, in the absence of conclusive evidence, the primary role of boron in plants remains elusive. Besides plants, growth of specific bacteria, such as heterocystous cyanobacteria and the recently reported actinomycetes of the genus Frankia, requires B, particularly for the stability of the envelopes that control the access of the nitrogenase-poisoning oxygen when they grow under N2-fixing conditions. Likewise, a role for B for animal embryogenesis and other developmental processes is being established. Finally, a new feature of the role of boron comes from signaling mechanisms for communication among bacteria and among legumes and rhizobia leading to N2-fixing symbiosis, and it is possible that new roles for B, based on its special chemistry and its interaction with Ca would appear in the world of signal transduction pathways. In conclusion, the diversity of roles played by B might indicate that either the micronutrient is involved in numerous processes or that its deficiency has a pleiotropic effect. The arising question is why such an element? Since all of the roles clearly established for B are related to its capacity to form diester bridges between cis-hydroxyl-containing molecules, we propose that the main reason for B essentiality is the stabilization of molecules with cis-diol groups turning them effective, irrespectively of their function.

During colonization of the alfalfa rhizosphere, Pseudomonas fluorescens F113 undergoes phenotypic variation, resulting in the appearance of colonies with different morphology. Among phenotypic variants, three isolates, C, F, and S were selected, with the C variant showing colony morphology identical to that of the inoculated wild-type strain and F and S having a translucent and diffuse morphology. Phenotypic variants F and S were shown to preferentially colonize distal parts of the roots and showed alterations in motility, swimming faster than the C variant and swarming under conditions that did not allow swarming of the C variant. The motility behavior correlated with overproduction of the fliC-encoded protein flagellin but not with hyperflagellation. Flagella of the F and S variants were several times longer than those of the C variant, and overproduction of flagellin was regulated at the transcriptional level. Variant F showed alterations in traits that have been shown to be important for rhizosphere colonization, such as siderophore, cyanide, and exoprotease production, and these phenotypes were complemented by a cloned gacA. Sequence analysis of the gacA alelle in variant F suggested selection of the phenotype in the rhizosphere. Variant F was also affected in other phenotypes, such as lipopolysaccharide structure and flocculation in unshaken liquid medium, which were not complemented by the gacA or gacS gene. Mutation of the F113 sss gene, encoding a site-specific recombinase, showed that most of the phenotypic variation was due to the activity of this recombinase, indicating that phase variation occurs during rhizosphere colonization.Bacterial phase variation consists of the diversification of a population into subpopulations which present genotypic and phenotypic differences. It has been described for a variety of species, mostly within gram-negative bacteria (reviewed in reference 26). Phase variation is typical of bacteria that occupy heterogeneous ecological niches and has been related to adaptation to different environmental situations and to sudden changes in the ecosystem (49). Under these conditions, phase variation would generate a mixed population able to colonize different parts of the ecosystem, and in the case of rapid environmental changes, part of the initial population would survive (17).Phase variation predominantly affects surface components of cells, such as membrane antigens, flagella, and fimbriae, causing morphological alterations in colonies that allow easy detection (26). Typical examples are fimbrial variation in Escherichia coli, flagellar variation in Salmonella spp., and variation in surface antigens in several species of the genus Neisseria. From a genetic point of view, phase variation often results from genomic rearrangements that can be caused by several molecular mechanisms. The best-studied mechanisms are the action of a site-specific recombinase that can produce inversions or deletions of specific sites of the genome (1), mutations in homopolymeric tracts that produce fr...

We investigated the possibility of Ca 2ϩ signaling in cyanobacteria (blue-green algae) by measuring intracellular free Ca 2ϩ levels ([Ca 2ϩ ] i ) in a recombinant strain of the nitrogen fixing cyanobacterium Anabaena strain sp. PCC7120, which constitutively expresses the Ca 2ϩ -binding photoprotein apoaequorin. The homeostasis of intracellular Ca 2ϩ in response to increasing external Ca 2ϩ has been studied in this strain. The resting level of free Ca 2ϩ in Anabaena was found to be between 100 and 200 nm. Additions of increasing concentrations of external Ca 2ϩ gave a transient burst of [Ca 2ϩ ] i followed by a very quick decline, reaching a plateau within seconds that brought the level of [Ca 2ϩ ] i back to the resting value. These results indicate that Anabaena strain sp. PCC7120 is able to regulate its internal Ca 2ϩ levels. We also monitored Ca 2ϩ transients in our recombinant strain in response to heat and cold shock. The cell's response to both stresses was dependent on the way they were induced. The use of inhibitors suggests that heat shock mobilizes cytosolic Ca 2ϩ from both intracellular and extracellular sources, while the Ca 2ϩ source for cold shock signaling is mostly extracellular.

Boron (B) is an essential micronutrient for the development of nitrogen-fixing root nodules in pea (Pisum sativum). By using monoclonal antibodies that recognize specific glycoconjugate components implicated in legume root-nodule development, we investigated the effects of low B on the formation of infection threads and the colonization of pea nodules by Rhizobium leguminosarum bv viciae. In B-deficient nodules the proportion of infected host cells was much lower than in nodules from plants supplied with normal quantities of B. Moreover, the host cells often developed enlarged and abnormally shaped infection threads that frequently burst, releasing bacteria into damaged host cells. There was also an overproduction of plant matrix material in which the rhizobial cells were embedded during their progression through the infection thread. Furthermore, in a series of in vitro binding studies, we demonstrated that the presence of B can change the affinity with which the bacterial cell surface interacts with the peribacteroid membrane glycocalyx relative to its interaction with intercellular plant matrix glycoprotein. From these observations we suggest that B plays an important role in mediating cell-surface interactions that lead to endocytosis of rhizobia by host cells and hence to the correct establishment of the symbiosis between pea and Rhizobium.B plays an essential role in nodule development. Brenchley and Thornton (1925) described ineffective nodules in B-deficient Vicia faba, and more recently we reported that pea (Pisum sativum) nodules that developed under low-B conditions were not functional and became prematurely senescent (Bolafios et al., 1994). Three-and 4-week-old B-deprived nodules showed a generalized degeneration of cell walls and membranes, including the PBM, which surrounds intracellular bacteroids. Because of these impairments in nodule development, rhizobia inside nodules showed little or no ability to fix N2. This leads to N, deficiency and to the necrosis of nodulated pea plants (Bolafios et al., 1994).The physiological role of B has been related to the chemistry of borate ions: boric acid acts as a Lewis acid and forms anions by accepting hydroxyl ions. The special struc-

The effect of boron deficiency on symbiotic nitrogen fixation in pea (Pisum safivum) was examined. l h e absence of boron in the culture medium resulted in a decrease of the number of nodules and an alteration of nodule development leading to an inhibition of nitrogenase activity. Examination of boron-deficient nodules showed dramatic changes in cell walls and in both peribacteroid and infection thread membranes, suggesting a role for boron in the stability of these structures. These results indicate that boron is a requirement for normal nodule development and functionality.

Quinoa cultivation has been expanded around the world in the last decade and is considered an exceptional crop with the potential of contributing to food security worldwide. The exceptional nutritional value of quinoa seeds relies on their high protein content, their amino acid profile that includes a good balance of essential amino acids, the mineral composition and the presence of antioxidants and other important nutrients such as fiber or vitamins. Although several studies have pointed to the influence of different environmental stresses in certain nutritional components little attention has been paid to the effect of the agroecological context on the nutritional properties of the seeds what may strongly impact on the consumer food’s quality. Thus, aiming to evaluate the effect of the agroecological conditions on the nutritional profile of quinoa seeds we analyzed three quinoa cultivars (Salcedo-INIA, Titicaca and Regalona) at different locations (Spain, Peru and Chile). The results revealed that several nutritional parameters such as the amino acid profile, the protein content, the mineral composition and the phytate amount in the seeds depend on the location and cultivar while other parameters such as saponin or fiber were more stable across locations. Our results support the notion that nutritional characteristics of seeds may be determined by seed’s origin and further analysis are needed to define the exact mechanisms that control the changes in the seeds nutritional properties.

Sinorhizobium fredii HH103 is a fast-growing rhizobial strain infecting a broad range of legumes including both American and Asiatic soybeans. In this work, we present the sequencing and annotation of the HH103 genome (7.25 Mb), consisting of one chromosome and six plasmids and representing the structurally most complex sinorhizobial genome sequenced so far. Comparative genomic analyses of S. fredii HH103 with strains USDA257 and NGR234 showed that the core genome of these three strains contains 4,212 genes (61.7% of the HH103 genes). Synteny plot analysis revealed that the much larger chromosome of USDA257 (6.48 Mb) is colinear to the HH103 (4.3 Mb) and NGR324 chromosomes (3.9 Mb). An additional region of the USDA257 chromosome of about 2 Mb displays similarity to plasmid pSfHH103e. Remarkable differences exist between HH103 and NGR234 concerning nod genes, flavonoid effect on surface polysaccharide production, and quorum-sensing systems. Furthermore a number of protein secretion systems have been found. Two genes coding for putative type III-secreted effectors not previously described in S. fredii, nopI and gunA, have been located on the HH103 genome. These differences could be important to understand the different symbiotic behavior of S. fredii strains HH103, USDA257, and NGR234 with soybean.

B-deficient bean (Phaseolus vulgaris L.) nodules examined by light microscopy showed dramatic anatomical changes, mainly in the parenchyma region. Western analysis of total nodule extracts examined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed that one 116-kD polypeptide was recognized by antibodies raised against hydroxyproline-rich glycoproteins (HRGPs) from the soybean (Glycine max) seed coat. A protein with a comparable molecular mass of 116 kD was purified from the cell walls of soybean root nodules. The amino acid composition of this protein is similar to the early nodulin (ENOD2) gene. Immunoprecipitation of the soybean ENOD2 in vitro translation product showed that the soybean seed coat anti-HRGP antibodies recognized this early nodulin. Furthermore, we used these antibodies to localize the ENOD2 homolog in bean nodules. Immunocytochemistry revealed that in B-deficient nodules ENOD2 was absent in the walls of the nodule parenchyma. The absence of ENOD2 in B-deficient nodules was corroborated by performing hydroxyproline assays. Northern analysis showed that ENOD2 mRNA is present in B-deficient nodules; therefore, the accumulation of ENOD2 is not affected by B deficiency, but its assembly into the cell wall is. B-deficient nodules fix much less N2 than control nodules, probably because the nodule parenchyma is no longer an effective O2 barrier.