Microbial Biosynthesis of Ethylene and Its Influence on Plant Growth. In: Marshall KC, editor. Advances in Microbial Ecology. Boston: Springer US; 1992. pp. 69-111. [DOI: 10.1007/978-1-4684-7609-5_2]

Separating the effects of air and soil temperature on silver birch. Part I. Does soil temperature or resource competition determine the timing of root growth?

The aboveground parts of boreal forest trees mostly grow earlier, and the roots later, in the growing season. We aimed to experimentally test whether the extrinsic driver of soil temperature or the intrinsic driver (resource competition between plant parts) is a more important control for the root and shoot growth of silver birch (Betula pendula Roth) seedlings. Sixteen two-year-old seedlings were grown in controlled environment rooms for two simulated growing seasons (GS1, GS2). In GS1, all the seedlings were acclimatized under the same conditions, but in GS2, the soil temperature treatments were: (i) constant 10 °C (Cool); (ii) constant 18 °C (Warm); (iii) early growing season at 10 °C, switched to 18 °C later (Early Cool Late Warm, ECLW) and (iv) early growing season 18 °C, switched to 10 °C later (Early Warm Late Cool, EWLC). The treatments did not affect growth allocation between shoots and roots. Warm soil benefitted shoot elongation as it slowed down in EWLC and accelerated in ECLW after the soil temperature switch. However, whole-tree biomasses were similar to Cool and the seedlings grew largest in Warm. Phenology was not strongly affected by soil temperature, and root and shoot growth did not usually peak simultaneously. Short root mortality increased strongly in ECLW and decreased in EWLC after the soil temperature switch. Long root longevity was not significantly affected but long root growth ceased earliest in ECLW. Soil warming increased foliar nutrient contents. Growth dynamics were not solely driven by soil temperature, but resource competition also played a significant role. The study showed the importance of soil temperature for fine root dynamics not only through root growth but also via root mortality, as soil warming increased mortality even more than growth. Soil temperature has complex effects on tree and soil functioning, which further affects carbon dynamics in forest ecosystems that have a climate feedback.

Let it snow! Winter conditions affect growth of birch seedlings during the following growing season

Air temperatures and precipitation are predicted to increase in the future, especially at high latitudes and particularly so during winter. In contrast to air temperatures, changes in soil temperatures are more difficult to predict, as the fate of the insulating snow cover is crucial in this respect. Soil conditions can also be affected by rain-on-snow events and warm spells during winter, resulting in freeze-thaw cycles, compacted snow, ice encasement and local flooding. These adverse conditions during winter could counteract the otherwise positive effects of climate change on forest growth and productivity. For studying the effects of different winter and snow conditions on young Downy birch (Betula pubescens Ehrh.) seedlings, we carried out a laboratory experiment with birch seedlings subjected to four different winter scenarios: snow covering the seedlings (SNOW), compressed snow and ice encasement (ICE), flooded and frozen soil (FLOOD) and no snow at all (NO SNOW). After the winter treatments we simulated a spring and early summer period of 9.5 weeks, and monitored the growth by measuring shoot and root biomass of the seedlings, and starch and soluble sugar concentrations. We also assessed the stress experienced by the seedlings by measuring leaf chlorophyll fluorescence and gas exchange. Although no difference in mortality was observed between the treatments, the seedlings in the SNOW and ICE treatments had significantly higher shoot and root biomass compared with those in the FLOOD and NO SNOW treatments. We found higher starch concentrations in roots of the seedlings in the SNOW and ICE treatments, compared with those in the FLOOD and NO SNOW treatments, although photosynthesis did not differ. Our results suggest a malfunction of carbohydrate distribution in the seedlings of the FLOOD and NO SNOW treatments, probably resulting from decreased sinks. The results underline the importance of an insulating and protecting snow cover for small tree seedlings, and that future winters with changed snow pattern might affect the growth of tree seedlings and thus possibly species composition and forest productivity.

Non-Targeted Metabolomics Reveals Sorghum Rhizosphere-Associated Exudates are Influenced by the Belowground Interaction of Substrate and Sorghum Genotype

Root exudation is an important plant process by which roots release small molecules into the rhizosphere that serve in overall plant functioning. Yet, there is a major gap in our knowledge in translating plant root exudation in artificial systems (i.e., hydroponics, sterile media) to crops, specifically for soils expected in field conditions. Sorghum (Sorghum bicolor L. Moench) root exudation was determined using both ultra-performance liquid chromatography and gas chromatography mass spectrometry-based non-targeted metabolomics to evaluate variation in exudate composition of two sorghum genotypes among three substrates (sand, clay, and soil). Above and belowground plant traits were measured to determine the interaction between sorghum genotype and belowground substrate. Plant growth and quantitative exudate composition were found to vary largely by substrate. Two types of changes to rhizosphere metabolites were observed: rhizosphere-enhanced metabolites (REMs) and rhizosphere-abated metabolites (RAMs). More REMs and RAMs were detected in sand and clay substrates compared to the soil substrate. This study demonstrates that belowground substrate influences the root exudate profile in sorghum, and that two sorghum genotypes exuded metabolites at different magnitudes. However, metabolite identification remains a major bottleneck in non-targeted metabolite profiling of the rhizosphere.

Beijerinckia derxii releases plant growth regulators and amino acids in synthetic media independent of nitrogenase activity

AIMS

This study aims at evaluating the ability of Beijerinckia derxii, a free-living nitrogen (N)-fixing bacterium frequently isolated from tropical soils, to release certain plant growth regulators [indoleacetic acid (IAA), ethylene, polyamines] and amino acids into the growth medium.

METHODS AND RESULTS

The production of those substances was compared using both cultures in which nitrogenase was active (N-free medium) and cultures in which nitrogenase was repressed (combined-N cultures). Those cultures were grown under agitation and in absence of agitation. Total IAA production was higher in agitated, N-free cultures but specific production was greater in combined-N cultures under agitation. Putrescine and spermidine were detected under all conditions tested. Ethylene was produced in both N-free and combined-N cultures. A greatest diversity of amino acids was released in N-free cultures.

CONCLUSIONS

There was no inhibition of the production of the analysed substances under conditions where nitrogenase was inactive.

SIGNIFICANCE AND IMPACT OF THE STUDY

Beijerinckia derxii is potentially a producer of plant-active substances; its presence in the natural environment suggests that this bacterium may contribute to the development of other living organisms.

Microbial species associated with different sections of broccoli harvested from three regions in Australia

The microbial populations associated with the different sections of broccoli harvested from three locations in Australia were studied during storage at 5, 15 and 20 degrees C. Bacterial and yeast populations associated with the outer florets and cut surfaces of the stem were generally 10-fold or more higher than those associated with inner florets or non-cut stems, respectively. The predominating bacterial species varied with the origin of the broccoli. Pseudomonas fluorescens, Ps. corrugata and Ps. viridiflava predominated at populations of 10(5)-10(7) cfu/g on broccoli harvested from Victoria, Ps. fluorescens, Ps. mendocina and Ps. fragii and Arthrobacter spp. (10(-3) 10(6) cfu/g) were prevalent on broccoli harvested from Queensland. Broccoli harvested from New South Wales exhibited a predominance of Ps. fluorescens, Arthrobacter spp. and Enterobacteragglomerans (10(3)-10(5) cfu/g). Most species grew on broccoli during storage. Similar species were found at the different sections of broccoli, although, for some species there was evidence of strain variation at the different locations and for different temperature of storage.

A novel procedure for rapid isolation of plant growth promoting pseudomonads

A rapid and novel procedure for the isolation of plant growth promoting rhizobacteria (PGPR) is described. This method entails screening soil bacteria for the ability to utilize the compound 1-aminocyclopropane-1-carboxylate (ACC) as a sole N source, a trait that is a consequence of the presence of the activity of the enzyme ACC deaminase. This trait appears to be limited to soil bacteria that are also capable of stimulating plant growth. Seven different soil samples from two geographically disparate locations were found to contain pseudomonads that were able to to utilize ACC as a N source. Each of the seven strains was shown, by the ability of the bacterium to promote canola seedling root elongation under gnotobiotic conditions, to be a PGPR. The method described here may be used to replace the otherwise slow and tedious process of testing individual bacterial strains for their ability to promote plant growth, thereby significantly speeding up the process of finding new PGPR.Key words: plant growth promoting rhizobacteria, PGPR, 1-aminocyclopropane-1-carboxylate, ACC, ACC deaminase, bacterial fertilizer, soil bacteria.