Amino Acid-Originated Growth Regulators: Functions and Regulation of Plant Developmental Responses under Changing Environmental Conditions

Dear Colleagues,

Stress management in plants is an intricately connected network of various growth regulators that activate defense responses in plants. Among a variety of growth regulators, there is generally an interdependence or interaction that influences plants' growth potential under normal or stressful conditions. Apparently, the reason for such interaction could lie either in their origin or their metabolism that adapts a common strategy for adaptation and growth.

A large variety of growth regulators are derived from amino acids including ethylene, auxin, melatonin, serotonin, nitric oxide, and polyamine. Various studies have shown interaction between these growth regulators for the growth and tolerance of plants to environmental stress.

Plant signaling networks exhibit an integrated crosstalk among their signaling pathways. Auxin derived from the amino acid tryptophan plays an important role during abiotic stress-induced changes in the root through local auxin maxima and minima. Auxin transport, biosynthesis, conjugation, perception, and signaling regulate auxin-mediated stress responses. In the same biosynthesis pathway from tryptophan, melatonin, with its precursors and derivatives, acts as a powerful growth regulator, biostimulator, and antioxidant, which delays leaf senescence, photosynthesis inhibition, and improves redox homeostasis and the antioxidant system through a direct scavenging of reactive oxygen species (ROS) and reactive nitrogen species (RNS) under abiotic and biotic stress conditions. The standard pathway of melatonin biosynthesis occurs from tryptophan in plants involving four steps, decarboxylation by tryptophan decarboxylase, amine hydroxylation by tryptamine 5-hydroxylase to serotonin, and O-methylation to melatonin via N-acetylserotonin O-methyltransferase. Thus, during melatonin formation, serotonin is produced and both these indoleamines possess antioxidative and growth-inducing properties, thus proving beneficial for stress acclimatization. Additionally, the regulation of auxin and serotonin biosynthesis through the modulation of tryptophan levels seems to be associated with stress signals. Serotonin regulates gene expression associated with auxin responsive pathways. Abiotic stress-induced inhibition of auxin biosynthesis is likely to increase serotonin accumulation in plant tissues. Further, the evidence the ethylene biosynthesis gene (ACC synthase) is upregulated upon melatonin supplementation suggests its role in the induction of ethylene biosynthesis.

Ethylene is formed from methionine, another amino acid, and is known to regulate plant stress responses. The accumulating evidence suggests that ethylene regulates metabolite production, nutrient homeostasis, the antioxidant system, and photosynthesis in optimal and stressful environments. It is known as the emerging potential growth regulator for stress management in plants. Its interaction with auxin and melatonin for stress tolerance has been reported.

Polyamines are another category of growth regulators that help in the adaptation to stress responses. Polyamines are synthesized from the amino acids arginine, ornithine, and methionine and they also interact with ethylene for stress tolerance.  The research on the role of polyamine and crosstalk with ethylene in the adaptation of plants to environmental stress has gained momentum.

For the synthesis of nitric oxide (NO), a group of enzymes called nitric oxide synthases convert arginine into citrulline, producing NO in the process. Nitric oxide is a dynamic gaseous molecule involved in signaling, crosstalk with stress regulators, and plant abiotic-stress responses. Nitric oxide interacts with ethylene, auxin, polyamines, and melatonin for stress tolerance in plants. The pathways for ethylene, polyamine, and NO are again connected via arginine and methionine.

Thus, the amino acid-derived growth regulators work either alone or in coordination, and there exists a crosstalk for the tolerance of plants to environmental stress. Deciphering the interrelationship between them considering their common origin or pathway for stress tolerance would be beneficial for crop improvement under various environmental stress conditions.

This Special Issue aims to explore the individual function and interaction among these growth regulators for the adaptation of plants to environmental stress.

The main submitted topics could include but are not limited to:

• Mechanisms involved in biosynthesis and signaling of amino acid-derived plant growth regulators under stressful and unstressed conditions.
• Implications of the interaction between auxin and melatonin under different environmental conditions.
• Interaction between ethylene and melatonin for adaptation to stress.
• Polyamines and ethylene regulatory interaction for stress adaptation.
• Studying the major metabolic processes for stress tolerance through different growth regulators derived from amino acids.
• Nitric oxide and ethylene in the maintenance of plant growth attributes under stress.
• Serotonin in stress management in plants under adverse environmental conditions
• Serotonin, melatonin, and auxin: does the common biosynthesis pathway influence the stress survival strategy?
• Nitric oxide and polyamines interaction for stress adaptation.
• Crosstalk between methionine-derived growth regulators: optimal and stressful conditions.
• Crosstalk between arginine-derived growth regulators under optimal or stressful environmental conditions.
• Crosstalk between tryptophan-derived growth regulators under optimal or stressful environmental conditions.
• Revealing the role of growth regulators under environmental stress conditions using omics technologies.
• Modulation of growth regulators using genetical approaches to develop stress resistance in plants.

Prof. Dr. Nafees A. Khan
Dr. Noushina Iqbal
Dr. Peter Poór
Guest Editors

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