Abstract
Priming is an adaptive strategy that improves plant defenses against biotic and abiotic stresses. Stimuli from chemicals, abiotic cues, and pathogens can trigger the establishment of priming state. Priming with 5-aminolevulinic acid (ALA), a potential plant growth regulator, can enhance plant tolerance to the subsequent abiotic stresses, including salinity, drought, heat, cold, and UV-B. However, the molecular mechanisms underlying the remarkable effects of ALA priming on plant physiology remain to be elucidated. Here, we summarize recent progress made in the stress tolerance conferred by ALA priming in plants and provide the underlying molecular and physiology mechanisms of this phenomenon. Priming with ALA results in changes at the physiological, transcriptional, metabolic, and epigenetic levels, and enhances photosynthesis and antioxidant capacity, as well as nitrogen assimilation, which in turn increases the resistance of abiotic stresses. However, the signaling pathway of ALA, including receptors as well as key components, is currently unknown, which hinders the deeper understanding of the defense priming caused by ALA. In the future, there is an urgent need to reveal the molecular mechanisms by which ALA regulates plant development and enhances plant defense with the help of forward genetics, multi-omics technologies, as well as genome editing technology.
Highlights
Academic Editors: Lam-SonDefense priming refers to a physiological state that is induced after the plants perceive a variety of stimuli, such as pathogens, arthropods, and abiotic cues, as well as chemicals (Figure 1)
aminolevulinic acid (ALA) treatment significantly increases the activities of nitrate reductase (NR), glutamine synthetase (GS), glutamate synthase (GOGAT), and glutamate dehydrogenase (GDH) and decreases the activity of nitrite reductase (NiR) in watermelon (Citrullus lanatus)
As a key precursor of chlorophyll biosynthesis, ALA alleviated the deficiency of chlorophyll biosynthesis during UV-B stress; ALA pretreatment upregulates the expression of genes involved in chlorophyll biosynthesis such as glutamyl-tRNA reductase (HEMA1), Mgchelatase (CHLH), and protochlorophyllide oxidoreductase (POR) in pigeon pea (Cajanus cajan), promoting plant photosynthesis during UV-B stress [8,70]
Summary
Defense priming refers to a physiological state (a state of readiness for defense) that is induced after the plants perceive a variety of stimuli, such as pathogens, arthropods, and abiotic cues, as well as chemicals (Figure 1). In this state, the defense responses are deployed in a faster, stronger, and/or more sustained manner, defense priming is considered an adaptive and low-cost defensive strategy [1]. Pretreatment with low concentrations of chemicals such as hydrogen peroxide (H2 O2 ), sodium hydrosulfide (NaHS), sodium chloride (NaCl), sodium nitroprusside (SNP), γ-aminobutyric acid (GABA), melatonin, polyamines (PAs), as well as 5-aminolevulinic acid (ALA) induces plants into defense priming status [6–8], in which plants respond to biological and abiotic stresses through faster and stronger defensive activation [9].
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