The mechanisms by which many plant growth promoting rhizobacteria (PGPR) affect plants are unknown. We recently isolated a rhizosphere bacterium (Bacillus thuringiensis NEB17), that promotes soybean growth and screened the liquid growth medium in which it grew for plant growth stimulating materials. We have also shown that it produces a bacteriocin (named by us as thuricin-17 and a member of the recently described class IId bacteriocins). Here we show that application of this bacteriocin to leaves (spray) or roots (drench) directly stimulates the growth of both a C(3) dicot (soybean) and a C(4) monocot (corn). This growth stimulation is similar in nature to that previously seen when plants are treated with Nod factors. Strain NEB17 contains three copies of the gene for thuricin 17 that code for identical amino acid sequences. These two lines of evidence suggest that the dual functions of these proteins may have constrained their evolution. This is the first report of direct plant growth enhancement by a bacteriocin.

Climate change has led to increased temperatures, and simulation models suggest that this should affect crop production in important agricultural regions of the world. Nations at higher latitudes, such as Canada, will be most affected. We studied the relationship between climate variability (temperature and precipitation) and corn yield trends over a period of 33 years for the Monteregie region of south-western Quebec using historical yield and climate records and statistical models. Growing season mean temperature has increased in Monterregie, mainly due to increased September temperature. Precipitation did not show any clear trend over the 33 year period. Yield increased about 118 kg ha −1 year −1 from 1973 to 2005 (under normal weather conditions) due mainly to changes in technology (genetics and management). Two climate variables were strongly associated with corn yield variability: July temperature and May precipitation. These two variables explain more than a half of yield variability associated with climate. In conclusion, July temperatures below normal and May precipitation above normal have negative effects on corn yield, and the growing seasons have warmed, largely due to increases in the September temperature.

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Jasmonic acid (JA) and methyl jasmonate, collectively known as jasmonates, are naturally occurring in plants; they are important signal molecules involved in induced disease resistance and mediate many physiological activities in plants. We studied the effect of JA and its methyl ester, methyl jasmonate (MeJA), on the induction of nod genes in Bradyrhizobium japonicum GG4 (USDA3) carrying a plasmid with a translational fusion between B. japonicum nodY and lacZ of Escherichia coli, and the expression activity was measured by b-galactosidase activity. Both JA and MeJA strongly induced the expression of nod genes. They have little or no deleterious effects on the growth of B. japonicum cells, while genistein (Gen) showed inhibitory effects. We further studied the effect of JA-and MeJA-induced B. japonicum on soybean nodulation and nitrogen fixation under optimal (25 C) and suboptimal (17 C) root zone temperature (RZT) conditions. B. japonicum cells were grown in liquid yeast extract mannitol media and induced with a range of Gen, JA, and MeJA concentrations, including a treatment control with no inducer added. Soybean seedlings were grown at 25 or 17 C RZT with a constant air temperature (25 C) and inoculated, at the vegetative cotyledonary stage, with various B. japonicum induction treatments. Addition of Gen or jasmonates to B. japonicum, prior to inoculation, enhanced nodulation, nitrogen fixation, and plant growth at suboptimal RZT conditions. A higher concentration of Gen was inhibitory at 25 C, while this same concentration was stimulatory at 17 C. Interestingly, pre-incubation of B. japonicum with JA and MeJA enhanced soybean nodulation and nitrogen fixation under both optimal and suboptimal RZTs. We show that jasmonates are thus a new class of signaling molecules in the B. japonicum-soybean symbiosis and that pre-induction of B. japonicum with jasmonates can be used to enhance soybean nodulation, nitrogen fixation, and early plant growth.

Mabood, F., Zhou, X. and Smith, D. L. 2014. Microbial signaling and plant growth promotion. Can. J. Plant Sci. 94: 1051–1063. The rhizosphere offers a complex microhabitat where root exudates provide a diverse mixture of organic compounds that are used as nutrients or signals by the soil microbial population. On the other hand, these soil microorganisms produce compounds that directly or indirectly assist in plant growth promotion. The widely recognized mechanisms of plant growth promotion are biofertilization, production of phytohormones, suppression of diseases through biocontrol, induction of disease resistance and production of volatile signal compounds. During the past few decades our understanding of the interaction between rhizobacteria and plants has expanded enormously and this has resulted in application of microbial products used as crop inoculants (as biofertilizers), for increased crop biomass and disease suppression. However, this plant–microbe interaction is affected by adverse environmental conditions, and recent work has suggested that inoculants carrying plant-to-bacteria or bacteria-to-plant signals can overcome this and promote plant productivity under stressful environmental conditions. Very recent work has also shown that some plant growth-promoting rhizobacteria secrete novel signaling molecules that also promote plant growth. The use of rhizobacterial signaling in promoting plant growth offers a new window of opportunity, especially when we are looking at plants to provide biofuels and novel bioproducts. Developing technologies that can enhance plant growth and productivity is imperative.