Plants construct complex defense systems through their metabolic products to cope with environmental stresses. These metabolic products can be divided into primary metabolites and secondary metabolites, each playing a specific role while also cooperating in plant defense. Secondary metabolites are the leading force in direct defense, while primary metabolites provide the material and energy basis and participate in indirect defense. Understanding their interactions helps in cultivating stress-resistant crops through metabolic engineering.
Under stress, plants respond by regulating their metabolic networks. Primary metabolites primarily provide basic osmotic protection against abiotic stress. In contrast, secondary metabolites have evolved into a more specialized chemical defense system, mainly used to resist biotic threats, but also assisting in resistance to abiotic stress.
This article provides a brief overview of the defensive roles of primary and secondary metabolites.
The Defensive Role of Primary Metabolites
Although primary metabolites are mainly involved in basic plant metabolism, increasing evidence suggests they also play a crucial role in defense:
- Osmotic Regulation: Under abiotic stress, primary metabolites such as proline and glycine betaine act as osmotic agents and protectants, helping plants maintain water balance. Khan P et al., through greenhouse experiments, found that exogenous application of proline significantly alleviated drought stress in maize. Under drought conditions, proline treatment increased maize root fresh weight by 247% and aboveground fresh weight by 97%, effectively scavenging hydrogen peroxide (reducing it by 38%) by increasing antioxidant enzyme activity (SOD activity increased by 144%) and reducing membrane lipid damage (malondialdehyde decreased by 67%). Simultaneously, proline enhanced leaf water retention capacity, chlorophyll content, and nitrogen, phosphorus, and potassium nutrient absorption, and maintained osmotic balance by accumulating endogenous proline and soluble sugars. Ultimately, this demonstrates that proline can synergistically enhance maize's drought resistance at multiple levels, including growth, physiology, and biochemistry.
- Direct defense function: Studies have found that galactitol, as a synthetic precursor of the raffinose family, accumulates significantly in plants infected with pathogens and participates in the systemic induction of plant resistance to pathogens. Dopamine, as a primary metabolite, can affect sugar metabolism and plays an important role in responses to abiotic stresses. Mashabela MD et al. used a non-targeted metabolomics approach based on LC-MS, combined with multivariate data analysis, to study the dynamic metabolic changes in resistant and susceptible wheat varieties four weeks after infection with stripe rust fungus (Pst). The study found that pathogen infection triggered time-dependent metabolic reprogramming in wheat, involving multiple metabolites such as phenylpropanes (e.g., flavonoids), amino acids, lipids, and benzoxazines. Resistant varieties (Koonap) rapidly accumulated specific defensive phenolic substances (e.g., rutin, luteolin derivatives), while these metabolites were downregulated in susceptible varieties (Gariep). These differentially expressed metabolites can serve as biomarkers for plant resistance responses.
- Enhanced defense through exogenous application: Experiments have shown that exogenous treatment of rice infected with Strombus haematous using six primary metabolite standards, including dopamine hydrochloride, L-glutamine, and tyramine, can reduce the symptoms of rice blast disease. For example, Dehghanian Z et al. demonstrated that dopamine is a key defense signaling molecule in plant responses to biotic and abiotic stresses. It functions through a dual mechanism: directly as an antioxidant to scavenge reactive oxygen species and reduce oxidative damage (e.g., reducing H2O2 by 30% and MDA by 15%); and indirectly regulating the expression of numerous stress-related genes, affecting aquaporins, nutrient absorption, hormone metabolism (e.g., auxin), and antioxidant enzyme systems (increasing SOD, CAT, and POD activities by more than 2 times), thereby enhancing the overall stress resistance of plants. Studies have shown that exogenous application of dopamine can effectively alleviate damage caused by various stresses such as drought and salt damage.
The role of dopamine in plant stress responses (Dehghanian Z et al., 2024)
For more information on primary plant metabolites, please refer to "Primary Plant Metabolites: Types, Functions, and Analysis".
Defense Mechanisms and Experimental Evidence of Secondary Metabolites
Secondary metabolites (PSMs) are a core component of plant defense systems, protecting plants through various mechanisms:
- Direct toxicity: Terpenes are among the most important secondary metabolites in plant defense. Approximately 25,000 terpenes are known, exhibiting feeding deterrence, direct toxicity, or oviposition deterrence. For example, the sesquiterpene lactone cnicin accumulates in Centaurea maculosa and has a significant defensive effect against herbivores. Song W et al., using in vitro antibacterial screening, plant infection models, and RNA-Seq transcriptome analysis, discovered that the plant secondary metabolite citral can effectively inhibit Phytophthora capsici. Its mechanism of action is concentration-dependent: high concentrations of citral directly disrupt the pathogen's hyphal growth, spore germination, and cell membrane integrity; while low concentrations, although not directly bactericidal or inducing plant immunity, weaken its pathogenicity by significantly downregulating the expression of key virulence factors (such as RxLR effector proteins). This study presents a novel approach to controlling Phytophthora capsici as an antibacterial agent by directly targeting the pathogen's virulence system, offering new insights for the development of environmentally friendly pesticides.
- Diverse defense mechanisms: PSMs protect plants through direct defense (e.g., producing toxic substances, antifeedants, growth inhibitors) and indirect defense (e.g., releasing volatile substances to attract herbivorous predators).
- Multi-target mechanisms of action: PSMs act on herbivores at the molecular level through various mechanisms:
- Nervous system: For example, alkaloids (nicotine, caffeine) interfere with nerve signal transduction.
- Digestion and metabolism: For example, certain phenolic compounds and protease inhibitors affect protein synthesis and function.
- Cellular structure: Lipophilic terpenes disrupt biological membranes, and specific compounds inhibit cell division.
- Genetic material: Some alkaloids can directly insert into DNA, interfering with its replication and leading to cell death.
- Application potential: Researchers have pointed out that many PSMs (such as pyrethrin and azadirachtin) have been developed into environmentally friendly biopesticides, demonstrating their application prospects in sustainable agriculture (Divekar PA et al., 2022).
Functional integration of plant secondary metabolites shapes plant–herbivore and tritrophic interactions (Erb M et al., 2020)
- Induced defense: Studies have shown that when Centaurea maculosa is adjacent to other plants of the same species and stimulated by methyl jasmonate (a mimicking herbivore attack), the total phenolic content in the leaves increases significantly. Under low nutrient conditions, this increase reaches 17% (approximately 5 mg gallic acid equivalents/g dry weight) (Broz AK et al., 2010).
- Regulation of defense signaling pathways: Genetic studies have confirmed that indole-deficient mutants in Arabidopsis thaliana no longer produce callose defense responses after Flg22 treatment, while the addition of 4-methoxy-indole-3-ylmethylthiogluconate restores callose formation. The study also found that volatile terpenes and indoles can act as signaling molecules to initiate systemic resistance. These findings break through the traditional understanding of secondary metabolites as merely "toxins," establishing a new paradigm of their role as endogenous signaling molecules regulating immune responses, and revealing the high complexity and precision of plant defense networks (Erb M et al., 2020).
For more information on secondary plant metabolites, see "Secondary Plant Metabolites: Pathways, Functions, and Research Methods".
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How Plants Manage Their Defence Budget: Growth vs. Protection
For researchers in plant science and natural product discovery, understanding plant defence mechanisms is key. Plants constantly face a strategic dilemma: invest limited resources in growth or in defence? A fascinating study on Centaurea maculosa (spotted knapweed) reveals how they make this choice based on their social circle.
The Neighbourhood Effect on Defence Spending
The research shows this plant can identify its neighbours and adjust its strategy accordingly.
- With Relatives (Same Species): When growing near kin, the plant prioritises defence. It significantly ramps up production of defensive secondary metabolites after being challenged.
- With Strangers (Different Species): When surrounded by non-kin, it shifts resources towards competitive growth, investing less in chemical defences.
The Metabolic Evidence: A Shift in Priorities
This strategic reallocation is clearly visible in the plant's metabolic profile. Detailed metabolic profiling uncovered a distinct chemical signature in plants growing with same-species neighbours:
- Reduced Primary Metabolism: Levels of small molecules involved in core processes like glycolysis, the citric acid cycle, and amino acid synthesis were notably lower.
- Boosted Secondary Metabolism: Meanwhile, defensive compounds like inositols, chlorogenic acid, and quinic acid showed a significant increase.
This trade-off demonstrates that a plant's chemical arsenal is dynamically managed, not static. For drug discovery, this means a plant's environment directly impacts the potency and yield of its valuable bioactive compounds.
A Comparative Guide to Plant Metabolites
For researchers in plant science and agri-tech, understanding the distinct roles of primary and secondary metabolites is crucial. This knowledge directly impacts natural product discovery and sustainable crop development.
| Comparison Dimension | Primary Metabolites | Secondary Metabolites |
|---|---|---|
| Primary Role | Sustains fundamental life processes like growth, development, and energy metabolism. | Supports ecological adaptation, including defense against herbivores and pathogens, and attracting pollinators. |
| Defense Role | Indirect Role: Supports survival under stress by maintaining cell structure and osmotic balance. Compounds like proline and glycine betaine act as osmotic regulators. | Direct Role: Often toxic, repellent, or anti-nutritive to herbivores and microbes. Alkaloids, tannins, and terpenes can directly inhibit insect feeding or pathogen growth. |
| Biosynthesis | Universal in all plants, with relatively conserved pathways. | Distribution is often species-specific. Synthesis is frequently induced by biotic (e.g., insect herbivory) or abiotic stress. |
| Key Examples | Sugars, amino acids, proteins, lipids, nucleic acids. | Alkaloids (e.g., quinine, nicotine), Terpenes (e.g., monoterpenes, sesquiterpenes), Phenolics (e.g., tannins, flavonoids). |
| Response Speed | Typically rapid, aiming to maintain cellular homeostasis. | Induced synthesis can be slower, but the effects are often more specific. |
| Resource Allocation | Plants face a constant resource allocation trade-off. Under grazing stress, for example, a plant may shift carbon and nitrogen resources to secondary metabolite production, potentially slowing short-term growth. |
This functional division shows how plants intelligently manage their resources. They balance immediate survival needs against long-term growth and defense investments.
Harnessing Plant Chemistry for Smarter Agriculture
For researchers and agri-tech developers, a new frontier is emerging in sustainable crop management. By decoding how plants naturally balance growth and defence, we can now cultivate more resilient harvests. Advanced metabolomics tools are key to unlocking this potential, providing unprecedented insights into a plant's inner workings. These analytical techniques are revolutionising how we approach sustainable crop development.
Key Metabolomics Tools Driving Innovation
Modern labs rely on sophisticated instrumentation to map plant metabolic pathways. The most powerful platforms for this analysis include:
- GC-MS & LC-MS: These systems excel at identifying and quantifying a vast range of plant compounds.
- NMR Spectroscopy: This technique is invaluable for determining the precise structures of novel metabolites.
From Insight to Application: Developing Next-Gen Crops
This deeper understanding allows for targeted crop improvement strategies. The goal is to breed or engineer plants with optimised metabolic networks. We can now develop varieties that possess enhanced disease resistance without compromising their growth potential or yield. This approach minimises reliance on synthetic pesticides, creating new opportunities for eco-conscious farming.
The Bigger Picture: A Dynamic Metabolic System
The old view of plant metabolites in rigid categories is fading. We now see a highly integrated and dynamic system. Secondary metabolites act as versatile tools—they provide defence, regulate growth, and can even be repurposed as building blocks. This sophisticated chemical toolkit allows plants to adapt to challenges in real-time, offering a blueprint for the future of agriculture.
For more information on the applications of plant metabolomics in crop improvement and stress response, please refer to "Plant Metabolomics for Crop Improvement and Stress Response".
People Also Ask
What is the role of secondary metabolites in plant defense mechanisms?
The role of secondary metabolites in defence may involve deterrence/anti-feedant activity, toxicity or acting as precursors to physical defence systems.
What is the difference between primary and secondary metabolites in plants PPT?
Primary metabolites are involved in growth and development through processes like respiration and photosynthesis. They maintain physiological functions and include compounds like amino acids and vitamins. Secondary metabolites are not required for primary metabolism but can be important ecologically.
What are the two types of plant defense mechanisms?
Plants have both structural and biochemical defense mechanisms against pathogens.
What is the role of plant secondary metabolites in Defence and transcriptional regulation in response to biotic stress?
Under stress conditions, plants secondary metabolites mount up to an enhanced level and behave as signalling molecule to increase the expression of defence-related genes.
What is the purpose of plants producing secondary metabolites?
Plants produce a high diversity of natural products or secondary metabolites which are important for the communication of plants with other organisms. A prominent function is the protection against herbivores and/or microbial pathogens.
What are plant secondary metabolites for defense against herbivores?
Plant secondary metabolites (PSMs) are synthesized to provide defensive functions and regulate defense signaling pathways to safeguard plants against herbivores. Herbivore injury initiates complex reactions which ultimately lead to synthesis and accumulation of PSMs.
References
- Salam U, Ullah S, Tang ZH, Elateeq AA, Khan Y, Khan J, Khan A, Ali S. Plant Metabolomics: An Overview of the Role of Primary and Secondary Metabolites against Different Environmental Stress Factors. Life (Basel). 2023 Mar 6;13(3):706.
- Khan P, Abdelbacki AMM, Albaqami M, Jan R, Kim KM. Proline Promotes Drought Tolerance in Maize. Biology (Basel). 2025 Jan 7;14(1):41.
- Mashabela MD, Tugizimana F, Steenkamp PA, Piater LA, Dubery IA, Mhlongo MI. Metabolite profiling of susceptible and resistant wheat (Triticum aestivum) cultivars responding to Puccinia striiformis f. sp. tritici infection. BMC Plant Biol. 2023 Jun 1;23(1):293.
- Dehghanian Z, Ahmadabadi M, Asgari Lajayer B, Bagheri N, Chamani M, Gougerdchi V, Hamedpour-Darabi M, Shu W, Price GW, Dell B. Role of Neurotransmitters (Biomediators) in Plant Responses to Stress. Plants (Basel). 2024 Nov 7;13(22):3134.
- Divekar PA, Narayana S, Divekar BA, Kumar R, Gadratagi BG, Ray A, Singh AK, Rani V, Singh V, Singh AK, Kumar A, Singh RP, Meena RS, Behera TK. Plant Secondary Metabolites as Defense Tools against Herbivores for Sustainable Crop Protection. Int J Mol Sci. 2022 Feb 28;23(5):2690.
- Song W, Yin Z, Lu X, Shen D, Dou D. Plant secondary metabolite citral interferes with Phytophthora capsici virulence by manipulating the expression of effector genes. Mol Plant Pathol. 2023 Aug;24(8):932-946.
- Broz AK, Broeckling CD, De-la-Peña C, Lewis MR, Greene E, Callaway RM, Sumner LW, Vivanco JM. Plant neighbor identity influences plant biochemistry and physiology related to defense. BMC Plant Biol. 2010 Jun 17;10:115.
- Erb M, Kliebenstein DJ. Plant Secondary Metabolites as Defenses, Regulators, and Primary Metabolites: The Blurred Functional Trichotomy. Plant Physiol. 2020 Sep;184(1):39-52.
