3.1. ALA Priming Alleviates Salt Stress
Priming with ALA increases the transcripts and protein accumulations of SOS1 (Na
+/H
+ antiporter) and HA3 (proton pump) on the plasma membrane (PM) as well as NHX1 (Na
+/H
+ antiporter) and VHA-A (proton pump) on the vesicle membrane compared with the unprimed cucumber (
Cucumis sativus) in response to salt stress. The ion transporter proteins SOS1 and NHX1, with the energy provided by proton pump HA3 and VAH-A, help cucumber excrete Na
+ from the cytoplasm or transfer it to the vesicles, resulting in a high-low-high osmotic potential in the vesicle-protoplast-exosome, and thus alleviating ion toxicity induced by salt stress. Pretreatment with ALA upregulates the expression of high-affinity K
+ transporter protein 1 (HKT1) that regulates Na
+/K
+ homeostasis in cucumber cells and maintains normal metabolic activities in cells under salt stress conditions
[27,28][22][23]. Proline accumulates in response to salinity and is a common compatible osmolyte in higher plants. Exogenous application of ALA upregulates delta-1-pyrroline-5-carboxylate synthase (P5CS) that controls the rate-limiting step of glutamate-derived proline biosynthesis in Oilseed rape (
Brassica napus) and enhances tolerance to salt stress
[26,29][24][25]. In addition, priming with ALA relieves cell oxidation stress caused by salt stress by improving the activity of antioxidant enzymes, including superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD), and promoting the activity of enzymes involved in the ascorbate-glutathione cycle (AsA-GSH), including ascorbic acid oxidase (AAO), ascorbate peroxidase (APX), glutathione reductase (GR), dehydroascorbic acid reductase (DHAR), and monodehydroascorbic acid reductase (MDHAR)
[30,31,32,33][26][27][28][29].
In addition to coping with osmotic stress and oxidative stress caused by salt stress, priming with ALA improves plant salt tolerance by increasing photosynthetic assimilation and promoting nitrogen metabolism. Cassia seed (
Cassia obtusifolia), peach (
Prunnus persica), and oilseed rape treated with ALA showed an increase in the net photosynthetic rate (Pn) and transpiration rate (Tr), as well as the photochemical efficiency of photosystem II (Fv/Fm) and the non-photochemical quenching (NPQ) during salt stress
[29,31,34][25][27][30].
The induction of salt tolerance in plants by ALA may be achieved through nitric oxide (NO). ALA treatment increased NO and NOS activity in leaves, suggesting that ALA triggers NO synthesis by activating NOS, and thus improves salt tolerance in maize (
Zea mays)
[38][31].
3.2. ALA Priming Increases Plant Tolerance to Extreme Temperature
ALA-pretreated cucumber leaves had higher antioxidant enzyme activity, higher levels of proline and soluble sugar content, and weaker growth inhibition under high-temperature stress conditions
[43][32]. Priming with ALA increases germination and seedling emergence in red pepper (
Capsicum annuum) and reduces tissue electrolyte leakage in rice (
Oryza sativa) under cold stress
[44,45][33][34]. Pretreatment with ALA also increases chlorophyll content and photosynthetic capacity of cucumber and enhances ribulose-1,5-bisphosphate (RuBP) carboxylase activity in maize under cold stress conditions
[46,47,48][35][36][37]. Furthermore, ALA treatment also improved the antioxidant capacity of plants in response to cold stress by increasing the activities of SOD, APX, GR, CAT, and heme oxygenase-1 (HO-1) in red pepper, drooping wild ryegrass (
Elymus nutans), and soybean plants (
Glycine max)
[49,50,51][38][39][40]. Interestingly, ALA priming upregulates the expression levels of
respiratory burst oxidase homologue1 (RBOH1) in tomato (
Solanum lycopersicum) and leads to the production of H
2O
2, which serves as a signaling molecule to activate defense against cold stress
[52][41].
3.3. ALA Priming Mitigates Drought-Induced Damage
ALA pretreatment can maintain moisture in the seedlings of oilseed rape and Kentucky bluegrass (
Poa pratensis), thus enhancing leaf relative water content (RWC)
[58,59][42][43]. It can also increase the contents of proline and foliar N in wheat (
Triticum aestivum), as well as Ca
2+ in the roots under drought conditions
[60,61,62][44][45][46]. In addition, in studies with Kentucky bluegrass and sunflower (
Helianthus annuus), priming with ALA increases the activities of antioxidant enzymes such as catalase (CAT), superoxide dismutase (SOD), ascorbate peroxidase (APX), and glutathione reductase (GR), which reduce the production of ROS, including H
2O
2 content and O
2•− production, thereby improving tolerance against drought stress
[58,63][42][47]. Priming with ALA also preserves plant photosynthesis in oilseed rape, wheat, and sunflower by suppressing chlorophyll degradation and increasing photosynthetic rate (Pn) during drought stress
[59,61,64,65][43][45][48][49]. Furthermore, pretreatment with ALA induces the expressions of enzymes involved in the Calvin cycle such as triose-3-phosphate isomerase (TPI) and fructose-1,6-bisphosphate aldolase (FBPA)
[66][50]. Interestingly, in addition to enhance the drought resistance of plants, ALA priming also improves waterlogging tolerance in Fig (
Ficus carica), with higher levels of antioxidant enzyme activity, photosynthetic efficiency, and root respiration
[67][51].
3.4. ALA Priming Attenuates UV-B-Induced Damage
Ultraviolet-B (UV-B) radiation is a component of sunlight that induces several plant photomorphogenic responses, including hypocotyl growth inhibition and cotyledon curling [68][52]. High-intensity UV-B injures plants by damaging DNA, impaired photosynthesis, and cell death, and triggering the accumulation of ROS [69][53]. Priming with ALA was reported to significantly reduce plant damage from UV-B radiation by promoting photosynthesis, enhancing antioxidant capacity, and improving nitrogen metabolism. 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), Mg-chelatase (CHLH), and protochlorophyllide oxidoreductase (POR) in pigeon pea (Cajanus cajan), thus promoting plant photosynthesis during UV-B stress [8,70][8][54]. In addition, ALA priming-increased activities of antioxidant enzymes are essential for lettuce (Lactuca sativa) resistance to UV-B stress [71][55]. In addition to enzymatic antioxidants, ALA also increases the content of non-enzymatic antioxidants such as flavonoids and phenolics [8]. Under UV-B stress conditions, ALA priming significantly improves the activities of nitrate reductase (NR), nitrite reductase (NiR), glutamine synthetase (GS), and glutamate synthase (GOGAT), and then increases the levels of NO3− and NO2− in the seedlings of pigeon pea [70][54]. Collectively, ALA priming contributes to UV-B tolerance by regulating photosynthesis, antioxidant, and nitrogen metabolism in plants.