Insights Into Microbially Induced Salt Tolerance and Endurance Mechanisms (STEM) in Plants

Antioxidant Responses and Redox Regulation Within Plant-Beneficial Microbe Interaction

The increase in extreme climate events associated with global warming is a great menace to crop productivity nowadays. In addition to abiotic stresses, warmer conditions favor the spread of infectious diseases affecting plant performance. Within this context, beneficial microbes constitute a sustainable alternative for the mitigation of the effects of climate change on plant growth and productivity. Used as biostimulants to improve plant growth, they also increase plant resistance to abiotic and biotic stresses through the generation of a primed status in the plant, leading to a better and faster response to stress. In this review, we have focused on the importance of a balanced redox status for the adequate performance of the plant and revisited the different antioxidant mechanisms supporting the biocontrol effect of beneficial microbes through the adjustment of the levels of reactive oxygen species (ROS). In addition, the different tools for the analysis of antioxidant responses and redox regulation have been evaluated. The importance of redox regulation in the activation of the immune responses through different mechanisms, such as transcriptional regulation, retrograde signaling, and post-translational modification of proteins, emerges as an important research goal for understanding the biocontrol activity of the beneficial microbes.

Strategies for combating plant salinity stress: the potential of plant growth-promoting microorganisms

Global climate change and the decreasing availability of high-quality water lead to an increase in the salinization of agricultural lands. This rising salinity represents a significant abiotic stressor that detrimentally influences plant physiology and gene expression. Consequently, critical processes such as seed germination, growth, development, and yield are adversely affected. Salinity severely impacts crop yields, given that many crop plants are sensitive to salt stress. Plant growth-promoting microorganisms (PGPMs) in the rhizosphere or the rhizoplane of plants are considered the "second genome" of plants as they contribute significantly to improving the plant growth and fitness of plants under normal conditions and when plants are under stress such as salinity. PGPMs are crucial in assisting plants to navigate the harsh conditions imposed by salt stress. By enhancing water and nutrient absorption, which is often hampered by high salinity, these microorganisms significantly improve plant resilience. They bolster the plant's defenses by increasing the production of osmoprotectants and antioxidants, mitigating salt-induced damage. Furthermore, PGPMs supply growth-promoting hormones like auxins and gibberellins and reduce levels of the stress hormone ethylene, fostering healthier plant growth. Importantly, they activate genes responsible for maintaining ion balance, a vital aspect of plant survival in saline environments. This review underscores the multifaceted roles of PGPMs in supporting plant life under salt stress, highlighting their value for agriculture in salt-affected areas and their potential impact on global food security.

NtHD9 modulates plant salt tolerance by regulating the formation of glandular trichome heads in Nicotiana tabacum

Salt stress is one of the main abiotic factor affecting plant growth. We have previously identified a key gene (NtHD9) in Nicotiana tabacum L. that positively regulates the formation of long glandular trichomes (LGTs). Here, we verified that both abiotic stress (aphids, drought and salt stress) could restore the phenotype lacking LGTs in NtHD9-knockout (NtHD9-KO) plants. The abiotic stress response assays indicated that NtHD9 is highly sensitive to salt stress. Compared with cultivated tobacco "K326" (CK) plants, NtHD9-overexpressing (NtHD9-OE) plants with more LGTs exhibited stronger salt tolerance, whereas NtHD9-KO with no LGTs showed weaker tolerance to salt. The densities and sizes of the glandular heads gradually increased with increasing NaCl concentrations in NtHD9-KO plants. Mineral element determination showed that leaves and trichomes of NtHD9-OE plants accumulated less Na+ but had higher K+ contents under salt stress, thus maintaining ion homeostasis in plants, which could contribute to a robust photosynthetic and antioxidant system under salt stress. Therefore, NtHD9-OE plants maintained a larger leaf area and root length under high-salt conditions than CK and NtHD9-KO plants. We verified that NtHD9 could individually interact with NtHD5, NtHD7, NtHD12, and NtJAZ10 proteins. Salt stress led to an increase in jasmonic acid (JA) levels and activated the expression of NtHDs while inhibiting the expression of NtJAZ. This study suggests that the glandular heads play an important role in plant resistance to salt stress. The activation of JA signaling leading to JAZ protein degradation may be key factors regulating the glandular heads development under salt stress.

Comparative physiological, biochemical, metabolomic, and transcriptomic analyses reveal the formation mechanism of heartwood for Acacia melanoxylon

Acacia melanoxylon is well known as a valuable commercial tree species owing to its high-quality heartwood (HW) products. However, the metabolism and regulatory mechanism of heartwood during wood development remain largely unclear. In this study, both microscopic observation and content determination proved that total amount of starches decreased and phenolics and flavonoids increased gradually from sapwood (SW) to HW. We also obtained the metabolite profiles of 10 metabolites related to phenolics and flavonoids during HW formation by metabolomics. Additionally, we collected a comprehensive overview of genes associated with the biosynthesis of sugars, terpenoids, phenolics, and flavonoids using RNA-seq. A total of ninety-one genes related to HW formation were identified. The transcripts related to plant hormones, programmed cell death (PCD), and dehydration were increased in transition zone (TZ) than in SW. The results of RT-PCR showed that the relative expression level of genes and transcription factors was also high in the TZ, regardless of the horizontal or vertical direction of the trunk. Therefore, the HW formation took place in the TZ for A. melanoxylon from molecular level, and potentially connected to plant hormones, PCD, and cell dehydration. Besides, the increased expression of sugar and terpenoid biosynthesis-related genes in TZ further confirmed the close connection between terpenoid biosynthesis and carbohydrate metabolites of A. melanoxylon. Furthermore, the integrated analysis of metabolism data and RNA-seq data showed the key transcription factors (TFs) regulating flavonoids and phenolics accumulation in HW, including negative correlation TFs (WRKY, MYB) and positive correlation TFs (AP2, bZIP, CBF, PB1, and TCP). And, the genes and metabolites from phenylpropanoid and flavonoid metabolism and biosynthesis were up-regulated and largely accumulated in TZ and HW, respectively. The findings of this research provide a basis for comprehending the buildup of metabolites and the molecular regulatory processes of HW formation in A. melanoxylon.

Exogenous melatonin improves germination rate in buckwheat under high temperature stress by regulating seed physiological and biochemical characteristics

The germinations of three common buckwheat (Fagopyrum esculentum) varieties and two Tartary buckwheat (Fagopyrum tataricum) varieties seeds are known to be affected by high temperature. However, little is known about the physiological mechanism affecting germination and the effect of melatonin (MT) on buckwheat seed germination under high temperature. This work studied the effects of exogenous MT on buckwheat seed germination under high temperature. MT was sprayed. The parameters, including growth, and physiological factors, were examined. The results showed that exogenous MT significantly increased the germination rate (GR), germination potential (GP), radicle length (RL), and fresh weight (FW) of these buckwheat seeds under high-temperature stress and enhanced the content of osmotic adjustment substances and enzyme activity. Comprehensive analysis revealed that under high-temperature stress during germination, antioxidant enzymes play a predominant role, while osmotic adjustment substances work synergistically to reduce the extent of damage to the membrane structure, serving as the primary key indicators for studying high-temperature resistance. Consequently, our results showed that MT had a positive protective effect on buckwheat seeds exposed to high temperature stress, providing a theoretical basis for improving the ability to adapt to high temperature environments.

Partial substitution of chemical fertilizer by Trichoderma biofertilizer improved nitrogen use efficiency in wolfberry (Lycium chinense) in coastal saline land

A two-year field trial was conducted to investigate the effects of partial substitution of chemical fertilizer (CF) by Trichoderma biofertilizer (TF) on nitrogen (N) use efficiency and associated mechanisms in wolfberry (Lycium chinense) in coastal saline land. As with plant biomass and fruit yield, apparent N use efficiency and plant N accumulation were also higher with TF plus 75% CF than 100% CF, indicating that TF substitution promoted plant growth and N uptake. As a reason, TF substitution stabilized soil N supply by mitigating steep deceases in soil NH4 +-N and NO3 -N concentrations in the second half of growing seasons. TF substitution also increased carbon (C) fixation according to higher photosynthetic rate (Pn) and stable 13C abundance with TF plus 75% CF than 100% CF. Importantly, leaf N accumulation significantly and positively related with Pn, biomass, and fruit yield, and structural equation modeling also confirmed the importance of the causal relation of N accumulation coupled with C fixation for biomass and yield formation. Consequently, physiological and agronomical N use efficiencies were significantly higher with TF plus 75% CF than 100% CF. Overall, partial substitution of CF by TF improved N use efficiency in wolfberry in coastal saline land by stabilizing soil N supply and coupling N accumulation with C fixation.

Comparative Secretome Analyses of Trichoderma/Arabidopsis Co-cultures Identify Proteins for Salt Stress, Plant Growth Promotion, and Root Colonization

Numerous Trichoderma strains are beneficial for plants, promote their growth, and confer stress tolerance. A recently described novel Trichoderma strain strongly promotes the growth of Arabidopsis thaliana seedlings on media with 50 mM NaCl, while 150 mM NaCl strongly stimulated root colonization and induced salt-stress tolerance in the host without growth promotion. To understand the dynamics of plant-fungus interaction, we examined the secretome from both sides and revealed a substantial change under different salt regimes, and during co-cultivation. Stress-related proteins, such as a fungal cysteine-rich Kp4 domain-containing protein which inhibits plant cell growth, fungal WSC- and CFEM-domain-containing proteins, the plant calreticulin, and cell-wall modifying enzymes, disappear when the two symbionts are co-cultured under high salt concentrations. In contrast, the number of lytic polysaccharide monooxygenases increases, which indicates that the fungus degrades more plant lignocellulose under salt stress and its lifestyle becomes more saprophytic. Several plant proteins involved in plant and fungal cell wall modifications and root colonization are only found in the co-cultures under salt stress, while the number of plant antioxidant proteins decreased. We identified symbiosis- and salt concentration-specific proteins for both partners. The Arabidopsis PYK10 and a fungal prenylcysteine lyase are only found in the co-culture which promoted plant growth. The comparative analysis of the secretomes supports antioxidant enzyme assays and suggests that both partners profit from the interaction under salt stress but have to invest more in balancing the symbiosis. We discuss the role of the identified stage- and symbiosis-specific fungal and plant proteins for salt stress, and conditions promoting root colonization and plant growth.