Plant growth promoting rhizobacteria produce chemical compounds with different benefits for the plant. Among them, HCN is recognized as a biocontrol agent, based on its ascribed toxicity against plant pathogens. Based on several past studies questioning the validity of this hypothesis, we have re-addressed the issue by designing a new set of in vitro experiments, to test if HCN-producing rhizobacteria could inhibit the growth of phytopathogens. The level of HCN produced by the rhizobacteria in vitro does not correlate with the observed biocontrol effects, thus disproving the biocontrol hypothesis. We developed a new concept, in which HCN does not act as a biocontrol agent, but rather is involved in geochemical processes in the substrate (e.g., chelation of metals), indirectly increasing the availability of phosphate. Since this scenario can be important for the pioneer plants living in oligotrophic alpine environments, we inoculated HCN producing bacteria into sterile mineral sand together with germinating plants and showed that the growth of the pioneer plant French sorrel was increased on granite-based substrate. No such effect could be observed for maize, where plantlets depend on the nutrients stored in the endosperm. To support our concept, we used KCN and mineral sand and showed that mineral mobilization and phosphate release could be caused by cyanide in vitro. We propose that in oligotrophic alpine environments, and possibly elsewhere, the main contribution of HCN is in the sequestration of metals and the consequential indirect increase of nutrient availability, which is beneficial for the rhizobacteria and their plant hosts.
The germination of surface-sterilized maize kernels under aseptic conditions proved to be a suitable method for isolation of kernel-associated bacterial endophytes. Bacterial strains identified by partial 16S rRNA gene sequencing as Pantoea sp., Microbacterium sp., Frigoribacterium sp., Bacillus sp., Paenibacillus sp., and Sphingomonas sp. were isolated from kernels of 4 different maize cultivars. Genus Pantoea was associated with a specific maize cultivar. The kernels of this cultivar were often overgrown with the fungus Lecanicillium aphanocladii; however, those exhibiting Pantoea growth were never colonized with it. Furthermore, the isolated bacterium strain inhibited fungal growth in vitro.
The cytokinins of developing maize caryopsis may originate from both local syntheses as well as by transport. High levels of fertilization-dependent cytokinins in the pedicel suggest filial control on metabolism in the maternal tissue; they may also trigger developmental programmed cell death in the pedicel.
We report here on a comparative developmental profile of plant hormone cytokinins in relation to cell size, cell number and endoreduplication in developing maize caryopsis of a cell wall invertase-deficient miniature1 (mn1) seed mutant and its wild type, Mn1, genotype. Both genotypes showed extremely high levels of total cytokinins during the very early stages of development, followed by a marked and genotype specific reduction. While the decrease of cytokinins in Mn1 was associated with their deactivation by 9-glucosylation, the absolute and the relative part of active cytokinin forms was higher in the mutant. During the exponential growth phase of endosperm between 6 d after pollination and 9 d after pollination, the mean cell doubling time, the absolute growth rate and the level of endoreduplication were similar in the two genotypes. However, the entire duration of growth was longer in Mn1 compared with mn1, resulting in a significantly higher cell number in the Mn1 endosperm. These data correlate with the previously reported peak levels of the Mn1-encoded cell wall invertase-2 (INCW2) at 12 d after pollination in the Mn1 endosperm. A model showing possible crosstalk among cytokinins, cell cycle and cell wall invertase as causal to increased cell number and sink strength of the Mn1 developing endosperm is discussed.Key words: 9-glucosylation; cell wall invertase; cytokinins; maize caryopsis; miniature1. Rijavec T, Kovač M, Kladnik A, Chourey PS, Dermastia M (2009).A comparative study on the role of cytokinins in caryopsis development in the maize miniature1 seed mutant and its wild type. J. Integr. Plant Biol. 51(9), 840-849.Available online at www.jipb.netIn the caryopsis, the unique fruit of plants from the grass family, a single seed contains an embryo surrounded by an endosperm. Following fertilization, the embryo and endosperm act as "utilization sinks" in which cell division activity governs sink strength, but at maturity, the endosperm constitutes a storage sink (Ho 1988). It then develops into a nutritive tissue used at germination. Molecular mechanisms involved in these processes are not well understood. However, regulation of cell divisions and sink strength requires a network that links various individual signals at different regulatory levels. The hydrolytic enzyme, cell wall invertase (CWI) and plant hormone cytokinins are likely participants in this system. Cytokinins play a crucial role in regulating proliferation and differentiation of plant cells, and also control various processes in plant growth and development (reviewed in Sakakibara 2006). CWI localized in the apoplast and ionically bound to the cell wall catalyses the irreversible cleavage of sucrose. The resulting hexoses are then imported into sink cells by hexose transporters and are crucial in establishing sink strength. Notably, in cultured cells of Chenopodium rubrum, the expression of CWI and of plasma membrane hexose uptake receptors is enhanced by cytokinins. The latter could also alter the source-sink balance in SAG12-I...
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