Impact of PGPR inoculation on root morphological traits and root exudation in rapeseed and camelina: interactions with heat stress

Synergistic effects of PGPRs and fertilizer amendments on improving the yield and productivity of Canola (Brassica napus L.)

BACKGROUND

Organic fertilizers are safer and more eco-friendly than chemical fertilizers; hence, organic fertilizers can be used to support sustainable farming. The effects of PGPRs are manifold in agriculture, especially in monoculture crops, where the soil needs to be modified to increase germination, yield, and disease resistance. The objective of this study was to assess the effects of PGPRs combined with fertilizer on the yield and productivity of canola. Canola was chosen for its global importance as an oilseed crop and its responsiveness to soil amendments, making it ideal for evaluating the synergistic effects of PGPRs and fertilizers on yield and soil health.

METHODOLOGY

This research, which was carried out over two years, was aimed at establishing the effectiveness of PGPRs together with organic and inorganic fertilizers on canola yields and was performed with a two-factorial RCBD design under field conditions. We applied Azotobacter salinestris and Bacillus subtilis with biochar, compost, animal manure, poultry manure, and NPK fertilizer. Insect pest management and other agronomic practices were carried out to maintain the experiment.

RESULTS

Canola yield and agronomic traits were enhanced by the combination of Bacillus subtilis with the fully recommended N: P:K ratio (140:55:40 kg/ha). Additionally, the application of Bacillus subtilis with biochar at 2 tons/ha improved the yield and quality of canola, as well as the structure and nutrient regulation of the soil.

CONCLUSION

In light of these results, we recommend the application of Bacillus subtilis to canola seeds along with either 2 t/ha biochar or the entire recommended dose of N: P:K (140:55:40 kg/ha). These strategies are sustainable and help producers and the environment increase the productivity of canola. Combining PGPRs with fertilizers for canola enhances nutrient efficiency, promotes sustainable growth, and boosts stress resilience, addressing agricultural and environmental challenges.

CLINICAL TRIAL NUMBER

Not Applicable.

Effects of thermopriming and bacteria-mediated heat-stress acclimation strategies on seed yield and quality criteria in Brassica napus cv Aviso and Camelina sativa cv Calena

The effects of intense heat during the reproductive phase of two Brassica species-B. napus and C. sativa-could be alleviated by a prior gradual increase exposure and/or PGPR inoculation. Abct. Among extreme weather events caused by climate change, heat waves are one of the most threatening issues for food security. Heat stress is known to be particularly penalizing at the reproductive stage for oleaginous crops, such as oilseed rape and camelina, and is responsible for crop failures as a consequence of yield losses and lower quality of harvest plants parts. In this context, our study aims to analyze two acclimation strategies that rely on the induction of signals prior to an intense heat stress event, i.e., thermopriming (herein, a gradual increase in temperature) and bacteria inoculations (herein, two Plant Growth-Promoting Rhizobacteria (PGPR) were tested). In the two experiments, we assessed the expected beneficial effects of these two acclimation strategies on yield components, seed quality criteria (nutritional and related to dormancy). While thermopriming improved heat stress tolerance in B. napus cv Aviso by maintaining yield, seed nutritional quality and seed dormancy, the effects of the gradual increase prior to the heat stress were even more negative than the later intense heat stress event in C. sativa cv Calena which resulted in cumulated negative effects. The experimentation based on PGPR inoculation highlighted similar trends to thermopriming in B. napus cv Aviso but to a lesser extent. However, in C. sativa cv Calena, very weak effects of PGPR inoculation upon heat stress were observed. Finally, these two acclimation strategies were shown to help alleviate the impacts of intense heat stress but in a species-dependent manner. This study should be deepened by exploring the behaviors of more cultivars of oilseed rape and camelina in the perspective to generalize these results at the species scale.

A "love match" score to compare root exudate attraction and feeding of the plant growth-promoting rhizobacteria Bacillus subtilis, Pseudomonas fluorescens, and Azospirillum brasilense

Introduction

The rhizosphere is the zone of soil surrounding plant roots that is directly influenced by root exudates released by the plant, which select soil microorganisms. The resulting rhizosphere microbiota plays a key role in plant health and development by enhancing its nutrition or immune response and protecting it from biotic or abiotic stresses. In particular, plant growth-promoting rhizobacteria (PGPR) are beneficial members of this microbiota that represent a great hope for agroecology, since they could be used as bioinoculants for sustainable crop production. Therefore, it is necessary to decipher the molecular dialog between roots and PGPR in order to promote the establishment of bioinoculants in the rhizosphere, which is required for their beneficial functions.

Methods

Here, the ability of root exudates from rapeseed (Brassica napus), pea (Pisum sativum), and ryegrass (Lolium perenne) to attract and feed three PGPR (Bacillus subtilis, Pseudomonas fluorescens, and Azospirillum brasilense) was measured and compared, as these responses are directly involved in the establishment of the rhizosphere microbiota.

Results

Our results showed that root exudates differentially attracted and fed the three PGPR. For all beneficial bacteria, rapeseed exudates were the most attractive and induced the fastest growth, while pea exudates allowed the highest biomass production. The performance of ryegrass exudates was generally lower, and variable responses were observed between bacteria. In addition, P. fluorescens and A. brasilense appeared to respond more efficiently to root exudates than B. subtilis. Finally, we proposed to evaluate the compatibility of each plant-PGPR couple by assigning them a "love match" score, which reflects the ability of root exudates to enhance bacterial rhizocompetence.

Discussion

Taken together, our results provide new insights into the specific selection of PGPR by the plant through their root exudates and may help to select the most effective exudates to promote bioinoculant establishment in the rhizosphere.

Heat Stress and Plant-Biotic Interactions: Advances and Perspectives

Climate change presents numerous challenges for agriculture, including frequent events of plant abiotic stresses such as elevated temperatures that lead to heat stress (HS). As the primary driving factor of climate change, HS threatens global food security and biodiversity. In recent years, HS events have negatively impacted plant physiology, reducing plant's ability to maintain disease resistance and resulting in lower crop yields. Plants must adapt their priorities toward defense mechanisms to tolerate stress in challenging environments. Furthermore, selective breeding and long-term domestication for higher yields have made crop varieties vulnerable to multiple stressors, making them more susceptible to frequent HS events. Studies on climate change predict that concurrent HS and biotic stresses will become more frequent and severe in the future, potentially occurring simultaneously or sequentially. While most studies have focused on singular stress effects on plant systems to examine how plants respond to specific stresses, the simultaneous occurrence of HS and biotic stresses pose a growing threat to agricultural productivity. Few studies have explored the interactions between HS and plant-biotic interactions. Here, we aim to shed light on the physiological and molecular effects of HS and biotic factor interactions (bacteria, fungi, oomycetes, nematodes, insect pests, pollinators, weedy species, and parasitic plants), as well as their combined impact on crop growth and yields. We also examine recent advances in designing and developing various strategies to address multi-stress scenarios related to HS and biotic factors.

A plant's perception of growth-promoting bacteria and their metabolites

Many recent studies have highlighted the importance of plant growth-promoting (rhizo)bacteria (PGPR) in supporting plant's development, particularly under biotic and abiotic stress. Most focus on the plant growth-promoting traits of selected strains and the latter's effect on plant biomass, root architecture, leaf area, and specific metabolite accumulation. Regarding energy balance, plant growth is the outcome of an input (photosynthesis) and several outputs (i.e., respiration, exudation, shedding, and herbivory), frequently neglected in classical studies on PGPR-plant interaction. Here, we discuss the primary evidence underlying the modifications triggered by PGPR and their metabolites on the plant ecophysiology. We propose to detect PGPR-induced variations in the photosynthetic activity using leaf gas exchange and recommend setting up the correct timing for monitoring plant responses according to the specific objectives of the experiment. This research identifies the challenges and tries to provide future directions to scientists working on PGPR-plant interactions to exploit the potential of microorganisms' application in improving plant value.