2.1. Asp is an Endogenous Metabolic Limitation for Cell Proliferation

Cytosolic Asp has profound importance to the proliferating cells, as it determines the cell’s survival, especially when Gln is limited [13]. The drop in cytosolic Asp resulting from the knockdown of aspartate–glutamate carrier 1 (AGC1, known as ARALAR) leads to the reduction of the proliferation of several cell lines [15]. On the contrary, the supply of exogenous Asp or overexpression of an Asp transporter can bypass the need for an electron transport chain to support cell proliferation [6], demonstrating that Asp biosynthesis is a golden requirement for cell proliferation [13]. This has been further confirmed by the finding that TCA can only fully restore cell growth if it partners with Asp biosynthesis, thus, when AspAT is activated [15]. Further DNA content analysis by propidium iodide staining and flow cytometry reveals that the requirement of Asp for cell growth is at least partially because it sustains nucleotide biosynthesis [11,13].

2.2. Asp in Plants Coordinates Nitrogen Assimilation into Amino Acids

Asp and Glu and their amides make up more than one-third of the free amino acids in Arabidopsis [3]. They link the in vivo metabolism of amino acids to the relevant organic acids in the TCA cycle and the carbon metabolism in the glycolysis pathway [67,68]. When carbon skeletons are limited, Asp is amidated to form Asn, which serves as an efficient nitrogen transport and storage compound due to its relatively high N:C ratio (2:4) [69,70]. Under nitrogen stress, Asp appears to be one of the most importantamino acids [71,72,73,74,75]. It has been found that when N is sufficient, as a predominant amino acid translocated in plant phloem, Asp supplied by the phloem is converted in the root to Asn to export N to the shoot via xylem as part of the process of nitrogen assimilation, whereas, when N is absent, Asp supplied by the phloem is diverted to the formation of malate to support the metabolism cycle back to the shoot [74]. In a very recent study, higher Asp and Asn contents were observed to be positively coordinated with the nitrogen use efficiency (NUE) trait in potatoes with low N supply [75]. The above results suggest that Asp is imperative for amino acid and organic acid biosynthesis, especially under fluctuating N conditions. Asp coordinates nitrogen assimilation into amino acids such that the available carbon skeleton is mobilized [27,76]. Further targeted regulation of Asp metabolism might be a useful strategy to improve the NUE traits in plants.

2.3. Asp is a Drought Stress-Specific Responsive Metabolite

One of the most critical processes that affects plants under drought conditions is the accumulation of solutes, including amino acids in the leaf tissues and the roots. Asp concentration was recorded to increase by more than twofold in drought treatment in Brassica napus [77], Astragalus membranaceus [78], and Triticeae [79]. Similarly, Asp has shown the second-highest concentration (the second most activated compound) after ABA in root exudates of the holm oak (Quercus ilex) upon drought treatment [80]. Additionally, in chickpea plants treated with a plant growth-promoting rhizobacterium (PGPR) and plant growth regulator (PGRs) consortium and grown under drought stress conditions, a higher accumulation of Asp in the leaf of the tolerant variety was recorded as compared to the sensitive variety [81]. In addition, a significant change of Asp has been recorded in kale [82] and Caragana korshinskii [83], though its content declined upon drought stress. Regardless, the great range of variation of Asp content upon drought exposure suggests that Asp can serve as a drought-responsive biomarker.

2.5. Asp Acts as a Biomarker of Biotic Stress and Environment-Induced Exposure

As biochemically active compounds, free amino acids strongly respond to increased amounts of toxic substances in the environment [84]. In agreement with this, the free amino acid content increased in tomato plants grown in soil contaminated with arsenate (As(V)). Among these free amino acids, Asp and Glu were intensively accumulated [98]. The content of Asp was significantly reduced in aluminum (Al)-treated citrus roots, although most of the amino acids, as well as some sugars (i.e., raffinose and trehalose), were increased [99]. Asp, together with other amino acids relating to nitrogen metabolism, showed high accumulation in response to the arbuscular colonization of Medicago truncatula Gaertn. cv. Jemalong (A17) [100]. The level of Asp concentration was reported to be elevated in the root, stem, and leaf tissue of Fusarium wilt-symptomatic watermelon compared to the asymptomatic plants. As a consequence, it promotes the growth and development of Fusarium oxysporum f. sp. niveum. [101]. In addition, a higher concentration of Asp was detected in tomato seedlings grown from seeds primed with 100 nM jasmonic acid (JA) treatments under nematode infection, displaying the vital role of Asp in the fight against nematodes during JA treatment in tomatoes [102]. However, disease suppression of Fusarium crown rot showed a positive change of Asp in mycorrhizal asparagus, indicating the possible implication of Asp in Fusarium infection [103].

Taken together, notable stress-related responses of Asp to variable environmental exposures in plants have been observed, indicating an essential role of Asp in response to stress (Table 1). Since Asp is a key precursor for the biosynthesis of many fundamental metabolites, it can be reasonably inferred that the change of concentration of Asp fine-tunes the availability of downstream metabolites that are indispensable for plants to grow and to counteract various stresses. For instance, it fine-tunes central metabolism with glycolysis (sucrose, hexose, pyruvate, etc.), the citric acid cycle (2-OG, succinate, etc.), NAD, and nucleotides to support cell survival [9,11,78,79,83,90,91,96]; the essential amino acids Met, Lys, Thr, Ile, and other amino acids to adjust the total amino acid pool and protein metabolism [89,104]; and the organic N carriers Glu, Asn, and Gln to regulate N mobilization, storage, and recycling [71,72,105,106,107]. In addition, alteration of Asp levels can also be linked with malate activity as a consequence of stomatal opening, Ca²⁺ uptake inhibition, and amino acid transformation [74,86]. In addition, increasing Asp concentration in plants during anaerobic stress decreases the cytoplasmic pH, which affects the production of other intracellular metabolites, such as alanine and GABA [108].