A review by El Sabagh et al. (2022) published on the Frontiers in Agronomy updates the role of phytohormones (plant hormones) and microbes in helping plants tolerate stresses, such as drought, pathogens, salinity, temperatures, and metals. The review discusses why phytohormones are crucial for regulating growth and survival strategies in response to these stresses while commenting on microbes' essential role in these pathways.
We will dive into the review, as it provides key learnings on how microbes are essential in plant stress tolerance.
Key Phytohormones and Their Roles:
Abscisic Acid (ABA):
Function: ABA plays a vital role in drought stress tolerance by regulating stomatal closure to minimize water loss. It also influences seed dormancy and germination.
Mechanism: Under water deficit conditions, ABA is synthesized in roots and transported to leaves, triggering a signaling cascade that closes stomata and reduces water loss.
Ethylene (ET):
Function: Ethylene is involved in plant responses to multiple stresses such as flooding, heavy metals, and pathogen attacks. It helps modulate stress responses like leaf senescence and fruit ripening.
Mechanism: ET levels increase under stress conditions, triggering defense mechanisms that help mitigate damage.
Salicylic Acid (SA):
Function: SA plays a dual role in plant defense against pathogens and alleviates osmotic stress under salinity conditions.
Mechanism: SA helps induce systemic acquired resistance and modulates plant responses to stress by influencing various biochemical pathways.
Jasmonates (JA):
Function: JAs are involved in plant defense against biotic and abiotic stresses, including drought, salinity, and cold.
Mechanism: JAs activate defense responses and signaling pathways that help the plant manage stress.
Cytokinins (CK):
Function: CKs promote cell division and growth. They interact with other hormones like ABA to enhance stress tolerance, particularly under conditions of water scarcity.
Mechanism: CKs influence root architecture and nutrient uptake, helping plants better cope with stress.
Gibberellins (GA):
Function: GAs are involved in growth processes such as seed germination, flowering, and fruit development. They help plants manage stress by regulating growth patterns.
Mechanism: GAs modulate stress responses through interactions with other hormonal pathways and by influencing redox homeostasis.
Auxins:
Function: Auxins regulate root and shoot growth and play a role in stress-induced morphogenic responses.
Mechanism: Auxins interact with other hormones to modify growth patterns and enhance stress tolerance.
Melatonin:
Function: Melatonin, a recently recognized hormone, acts as an antioxidant and protects plants against both abiotic and biotic stresses.
Mechanism: Melatonin regulates ROS and other stress-related responses, contributing to overall plant resilience.
Strigolactones (SL) and Brassinosteroids (BR):
Function: SLs and BRs regulate architecture and development under stress conditions. They also enhance symbiotic relationships with beneficial microbes.
Mechanism: SLs and BRs modulate gene expression and signaling pathways that support stress adaptation.
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The Beneficial Effects of Microbe Phytohormones
Plant Growth-Promoting Rhizobacteria or PGPR: beneficial soil bacteria that colonize plant roots or the rhizosphere and enhance growth by improving nutrient uptake, producing phytohormones, and increasing stress tolerance.
Role of Plant Growth-Promoting Rhizobacteria (PGPR):
PGPR are beneficial bacteria that colonize plant roots and promote growth by a variety of mechanisms, including the production of phytohormones. These bacteria significantly contribute to plant stress tolerance, particularly under abiotic stresses such as drought, salinity, heavy metals, and extreme temperatures.
Mechanisms of Action of Microbes and Phytohormones on Plant Stress:
Phytohormone Production:
Phytohormones Produced by PGPR: PGPR can produce several phytohormones, including indole-3-acetic acid (IAA, a type of auxin), cytokinins (CK), salicylic acid (SA), gibberellins (GA), and abscisic acid (ABA). These hormones directly influence plant growth and stress response mechanisms.
Impact on Plant Growth: These hormones help plants endure stress by:
Enhancing antioxidant potential and reducing oxidative stress-induced damage
Improving photosynthetic capacity and membrane stability
Promoting cell division and stomatal regulation
Stimulating root system growth
Improving water and nutrient acquisition
Enhancement of Nutrient Uptake:
PGPR can improve plant nutrient availability and uptake, which is particularly beneficial when nutrient access is limited due to abiotic stresses. For example, they can solubilize phosphorus and fix atmospheric nitrogen, making these nutrients more available to plants.
PGPRs like Rhizobium, Bacillus, and Pseudomonas are examples mentioned by El Sabagh et al. as key players in these processes.
Production of Siderophores and Iron Acquisition:
Under iron-limiting conditions, PGPR produces siderophores, which are molecules that bind to iron and make it more available to plants. This process is essential for plants growing in iron-deficient soils and helps mitigate the adverse effects of such nutrient stress.
Stress Relief:
Ethylene Modulation: Under stress, plants often produce higher levels of ethylene, inhibiting plant growth. PGPR that can produce the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase can degrade ACC, the precursor of ethylene, thereby lowering ethylene levels in the plant and reducing its inhibitory effects on growth.
Antioxidants: PGPR can increase the antioxidant capacity of plants by enhancing the production of antioxidant enzymes like superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX). This reduces oxidative stress caused by reactive oxygen species (ROS) during abiotic stress conditions.
Induction of Systemic Resistance:
PGPR can induce systemic resistance in plants, known as Induced Systemic Resistance (ISR), which provides broad-spectrum protection against a range of stresses. This is achieved by priming the plant's immune responses via jasmonic acid and ethylene signaling pathways, leading to faster and stronger activation of defense mechanisms when exposed to stress.
Must Read on Phytohormones and Microbes
El Sabagh, A., Islam, M. S., Hossain, A., Iqbal, M. A., Mubeen, M., Waleed, M., Reginato, M., Battaglia, M., Ahmed, S., Rehman, A., Arif, M., Athar, H.-U.-R., Ratnasekera, D., Danish, S., Raza, M. A., Rajendran, K., Mushtaq, M., Skalicky, M., Brestic, M., ... Abdelhamid, M. T. (2022). Phytohormones as growth regulators during abiotic stress tolerance in plants. Frontiers in Agronomy, 4, Article 765068. https://doi.org/10.3389/fagro.2022.765068
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