Thermotolerant plant growth-promoting bacteria enhance growth and nutrient uptake of lettuce under heat stress conditions by altering stomatal movement and chlorophyll fluorescence.

This study investigates the effects of selected PGPB on lettuce growth performance under heat-stress conditions. Bacterial plant growth-promoting potentials have been characterized and identified successfully in ongoing studies. Based on in vitro plant growth-promoting potential, the top five bacteria were ranked and identified as Acinetobacter sp. GRB12, Bacillus sp. GFB04, Klebsiella sp. LFB06, Klebsiella sp. GRB10, and Klebsiella sp. GRB04. They were mixed to inoculate on lettuce (Lactuca sativa L.) in temperature-controlled greenhouses. Another in-vivo chamber experiment was conducted by using Bacillus sp. GFB04 and Klebsiella sp. GFB10. Plant physiological traits (chlorophyll fluorescence and transpiration) and nutrient contents were measured at harvest, along with growth, development, and yield component analyses. Uninoculated plants under heat-stress condition showed poor growth performance. In contrast, plants with PGPB inoculation showed improved growth under heat-stress conditions, as the uptake of nutrients was facilitated by the symbionts. Inoculation also improved lettuce photosystem II efficiency and decreased total water use under heat stress. In conclusion, the current study suggests that PGPB inoculation successfully enhances lettuce heat-tolerance. PGPB application could potentially help improve sustainable production of lettuce with less fertilization under increasing temperatures. Supplementary Information: The online version contains supplementary material available at 10.1007/s12298-024-01470-5.

Endophytes alleviate the elevated CO2-dependent decrease in photosynthesis in rice, particularly under nitrogen limitation.

The positive effects of high atmospheric CO2 concentrations [CO2] decrease over time in most C3 plants because of down-regulation of photosynthesis. A notable exception to this trend is plants hosting N-fixing bacteria. The decrease in photosynthetic capacity associated with an extended exposure to high [CO2] was therefore studied in non-nodulating rice that can establish endophytic interactions. Rice plants were inoculated with diazotrophic endophytes isolated from the Salicaceae and CO2 response curves of photosynthesis were determined in the absence or presence of endophytes at the panicle initiation stage. Non-inoculated plants grown under elevated [CO2] showed a down-regulation of photosynthesis compared to those grown under ambient [CO2]. In contrast, the endophyte-inoculated plants did not show a decrease in photosynthesis associated with high [CO2], and they exhibited higher photosynthetic electron transport and mesophyll conductance rates than non-inoculated plants under high [CO2]. The endophyte-dependent alleviation of decreases in photosynthesis under high [CO2] led to an increase in water-use efficiency. These effects were most pronounced when the N supply was limited. The results suggest that inoculation with N-fixing endophytes could be an effective means of improving plant growth under high [CO2] by alleviating N limitations.

Do Endophytes Promote Growth of Host Plants Under Stress? A Meta-Analysis on Plant Stress Mitigation by Endophytes.

Endophytes are microbial symbionts living inside plants and have been extensively researched in recent decades for their functions associated with plant responses to environmental stress. We conducted a meta-analysis of endophyte effects on host plants' growth and fitness in response to three abiotic stress factors: drought, nitrogen deficiency, and excessive salinity. Ninety-four endophyte strains and 42 host plant species from the literature were evaluated in the analysis. Endophytes increased biomass accumulation of host plants under all three stress conditions. The stress mitigation effects by endophytes were similar among different plant taxa or functional groups with few exceptions; eudicots and C4 species gained more biomass than monocots and C3 species with endophytes, respectively, under drought conditions. Our analysis supports the effectiveness of endophytes in mitigating drought, nitrogen deficiency, and salinity stress in a wide range of host species with little evidence of plant-endophyte specificity.

Distinct interactions of ericoid mycorrhizae and plant growth-promoting bacteria: impacts on blueberry growth and heat resilience.

Blueberries confront substantial challenges from climate change, such as rising temperatures and extreme heat, necessitating urgent solutions to ensure productivity. We hypothesized that ericoid mycorrhizal fungi (ErM) and plant growth-promoting bacteria (PGPB) would establish symbiotic relationships and increase heat stress tolerance in blueberries. A growth chamber study was designed with low (25/20°C) and high temperature (35/30°C) conditions with micropropagated blueberry plantlets inoculated with ErM, PGPB, and both. Gas exchange and chlorophyll fluorescence properties of the leaves were monitored throughout the growth. At harvest, biochemical assays and biomass analysis were performed to evaluate potential oxidative stress induced by elevated temperatures. ErM application boosted root biomass under 25/20°C conditions but did not impact photosynthetic efficiency. In contrast, PGPB demonstrated a dual role: enhancing photosynthetic capacity and reducing stomatal conductance notably under 35/30°C conditions. Moreover, PGPB showcased conflicting effects, reducing oxidative damage under 25/20°C conditions while intensifying it during 47°C heat shock. A significant highlight lies in the opposing effects of ErM and PGPB on root growth and stomatal conductance, signifying their reciprocal influence on blueberry plant behavior, which may lead to increased water uptake or reduced water use. Understanding these complex interactions holds promise for refining sustainable strategies to overcome climate challenges.

Photosynthetic response and antioxidative activity of 'Hass' avocado cultivar treated with short-term low temperature.

To investigate the effects of short-term low temperatures, three-year-old avocado (Persea americana cv. Hass) seedlings were treated with 1, - 2, or - 5 °C for 1 h and subsequently recovered in ambient condition for 24 h. Leaf color changes were investigated with chlorophyll, carotenoid, and phenolic contents. Photosynthetic responses were examined using gas exchange analysis. With H2O2 contents as oxidative stresses, enzymatic (ascorbate peroxidase, APX; glutathione reductase, GR; catalase, CAT; peroxidase, POD) and non-enzymatic antioxidant activities were determined using spectrophotometry. Leaves in the avocado seedlings started to be discolored with changes in the contents of chlorophyll a, carotenoids, and phenolics when treated with - 5 °C. However, the H2O2 content was not different in leaves treated with low temperatures. Photosynthetic activities decreased in leaves in the seedlings treated with - 5 °C. Of antioxidant enzymes, APX and GR have high activities in leaves in the seedlings treated with 1 and - 2 °C. In leaves in the seedlings treated with - 5 °C, the activities of all enzymes decreased. Non-enzymatic antioxidant activity was not different among leaves treated with low temperatures. These results indicated that APX and GR would play a critical role in withstanding chilling stress in 'Hass' avocado seedlings. However, under lethal temperature, even for a short time, the plants suffered irreversible damage with the breakdown of photosystem and antioxidant system.

Endophytes Increased Fruit Quality with Higher Soluble Sugar Production in Honeycrisp Apple (Malus pumila).

Endophytes are fungi, bacteria, or yeast symbionts that live in the intercellular spaces or vascular tissues of host plants. Investigations indicate that endophytes isolated from the Salicaceae family (Populus and Salix) hosts provide several benefits that promote plant growth, including but not limited to di-nitrogen fixation, plant hormone production, nutrient acquisition, stress tolerance, and defense against phytopathogens. In exchange, the microorganisms receive domicile and photosynthates. Considering the known characteristics of nitrogen fixation and plant hormone production, we hypothesized that apple trees grown under nitrogen-limited conditions would show improved biometrics with endophyte inoculation. Our research objectives were to investigate the endophyte effects on plant physiology and fruiting. We examined these effects through ecophysiology metrics involving rates of photosynthesis, stomatal conductance and density, transpiration, biomass accretion, chlorophyll content and fluorescence, and fruit soluble sugar content and biomass. Our results showed evidence of the endophytes' colonization in apple trees, decreased stomatal density, delayed leaf senescence, and increased lateral root biomass with endophytes. A highlight of the findings was a significant increase in both fruit soluble sugar content and biomass. Future research into the mechanistic underpinnings of this phenomenon stands to offer novel insights on how microbiota may alter carbohydrate metabolism under nitrogen-deficient conditions.

Estimating microbial respiratory CO2 from endophytic bacteria in rice.

Endophytes are symbiotic microbes that live inside host plants. These endophytic symbionts receive photosynthesized carbohydrates from host plants while conferring symbiotic benefits to their host. During photosynthate-fueled respiration, endophytes release CO2 into the intercellular spaces of their host plants in which they reside. We evaluated the possibility for host plants' re-assimilation of microbial respiratory CO2. In planta and in vitro assays were conducted to examine respiratory characteristics of endophyte-symbiotic plants. Endophyte-inoculated plants had a greater in planta respiration rate. In vitro data demonstrated that respiration rates of endophytes are dependent on the total amount of endophytes and the concentration of carbohydrate supply. Assuming the host plant offers sufficient carbohydrates, we estimate that CO2 produced during microbial respiration in planta accounts for about 57% of the CO2 assimilated by the photosynthetic pathways of the symbiotic plant. This suggests that endophytes can produce significant amounts of CO2, which could then be re-assimilated by host plants.

Salicaceae Endophytes Modulate Stomatal Behavior and Increase Water Use Efficiency in Rice.

Bacterial and yeast endophytes isolated from the Salicaceae family have been shown to promote growth and alleviate stress in plants from different taxa. To determine the physiological pathways through which endophytes affect plant water relations, we investigated leaf water potential, whole-plant water use, and stomatal responses of rice plants to Salicaceae endophyte inoculation under CO2 enrichment and water deficit. Daytime stomatal conductance and stomatal density were lower in inoculated plants compared to controls. Leaf ABA concentrations increased with endophyte inoculation. As a result, transpirational water use decreased significantly with endophyte inoculation while biomass did not change or slightly increased. This response led to a significant increase in cumulative water use efficiency at harvest. Different endophyte strains produced the same results in host plant water relations and stomatal responses. These stomatal responses were also observed under elevated CO2 conditions, and the increase in water use efficiency was more pronounced under water deficit conditions. The effect on water use efficiency was positively correlated with daily light integrals across different experiments. Our results provide insights on the physiological mechanisms of plant-endophyte interactions involving plant water relations and stomatal functions.