Soil Drench Treatment with ?-Aminobutyric Acid Increases Drought Tolerance of Potato

The non-protein amino acid β-aminobutyric acid (BABA) is known to be a priming agent for a more efficient activation of cellular defence responses and a potent inducer of resistance against biotic and abiotic stresses in plants. Nevertheless, most of the studies on priming have been carried out in Arabidopsis. In potato, the effect of BABA was demonstrated only on biotic stress tolerance. We investigated the effect of BABA on the drought tolerance of potato and found that soil drenched with BABA at a final concentration of 0.3 mM improves the drought tolerance of potato. Water loss from the leaves of the primed plants is attenuated and the yield is increased compared to the unprimed drought-stressed plants. The metabolite composition of the tubers of the BABA-treated plants is less affected by drought than the tuber composition of the non-treated plants. Nitric oxide and ROS (reactive oxygen species) production is increased in the BABA-treated roots but not in the leaves. In the leaves of the BABA-treated plants, the expression of the drought-inducible gene StDS2 is delayed, but the expression of ETR1, encoding an ethylene receptor, is maintained for a longer period under the drought conditions than in the leaves of the non-treated, drought-stressed control plants. This result suggests that the ethylene-inducible gene expression remains suppressed in primed plants leading to a longer leaf life and increased tuber yield compared to the non-treated, drought-stressed plants. The priming effect of BABA in potato, however, is transient and reverts to an unprimed state within a few weeks.     

Enhancing Arabidopsis salt and drought stress tolerance by chemical priming for its abscisic acid responses

Drought and salt stress tolerance of Arabidopsis (Arabidopsis thaliana) plants increased following treatment with the nonprotein amino acid beta-aminobutyric acid (BABA), known as an inducer of resistance against infection of plants by numerous pathogens. BABA-pretreated plants showed earlier and higher expression of the salicylic acid-dependent PR-1 and PR-5 and the abscisic acid (ABA)-dependent RAB-18 and RD-29A genes following salt and drought stress. However, non-expressor of pathogenesis-related genes 1 and constitutive expressor of pathogenesis-related genes 1 mutants as well as transgenic NahG plants, all affected in the salicylic acid signal transduction pathway, still showed increased salt and drought tolerance after BABA treatment. On the contrary, the ABA deficient 1 and ABA insensitive 4 mutants, both impaired in the ABA-signaling pathway, could not be protected by BABA application. Our data demonstrate that BABA-induced water stress tolerance is based on enhanced ABA accumulation resulting in accelerated stress gene expression and stomatal closure. Here, we show a possibility to increase plant tolerance for these abiotic stresses through effective priming of the preexisting defense pathways without resorting to genetic alterations.     

β-Aminobutyric acid increases abscisic acid accumulation and desiccation tolerance and decreases water use but fails to improve grain yield in two spring wheat cultivars under soil drying

A pot experiment was conducted to investigate the effect of the non-protein amino acid, β-aminobutyric acid (BABA), on the homeostasis between reactive oxygen species (ROS) and antioxidant defence during progressive soil drying, and its relationship with the accumulation of abscisic acid (ABA), water use, grain yield, and desiccation tolerance in two spring wheat (Triticum aestivum L.) cultivars released in different decades and with different yields under drought. Drenching the soil with 100 μM BABA increased drought-induced ABA production, leading to a decrease in the lethal leaf water potential (Ψ) used to measure desiccation tolerance, decreased water use, and increased water use efficiency for grain (WUE(G)) under moderate water stress. In addition, at severe water stress levels, drenching the soil with BABA reduced ROS production, increased antioxidant enzyme activity, and reduced the oxidative damage to lipid membranes. The data suggest that the addition of BABA triggers ABA accumulation that acts as a non-hydraulic root signal, thereby closing stomata, and reducing water use at moderate stress levels, and also reduces the production of ROS and increases the antioxidant defence enzymes at severe stress levels, thus increasing the desiccation tolerance. However, BABA treatment had no effect on grain yield of wheat when water availability was limited. The results suggest that there are ways of effectively priming the pre-existing defence pathways, in addition to genetic means, to improve the desiccation tolerance and WUE(G) of wheat.     

β-aminobutyric acid-mediated tobacco tolerance to potassium deficiency

??-Aminobutyric acid (BABA) is known as a nonprotein amino acid that can enhance the capacity of plants to resist various biotic and abiotic stresses. However, the role of BABA in protecting tobacco (Nicotiana tabacum L.) against potassium deficiency is not well understood. In this study, we demonstrated that the appropriate concentration of BABA promoted tobacco growth and enhanced plant tolerance to potassium deficiency. BABA protected tobacco from the reduction in the primary root length and chlorophyll content and increased K⁺ uptake via the regulation of expression of potassium up-taking-related NtHAK, NtLKS, NKT1, and NtKC genes. In addition, BABA affected the balancing between hydrogen peroxide level and peroxidase activity under low-potassium stress. Together, our results suggest that BABA plays a role in enhancing potassium deficiency stress tolerance by increasing K⁺ uptake, at least in part, via a ROS-dependent mechanism.     

Preparing plants for improved cold tolerance by priming

Cold is a major stressor, which limits plant growth and development in many parts of the world, especially in the temperate climate zones. A large number of experimental studies has demonstrated that not only acclimation and entrainment but also the experience of single short stress events of various abiotic or biotic kinds (priming stress) can improve the tolerance of plants to chilling temperatures. This process, called priming, depends on a stress “memory”. It does not change cold sensitivity per se but beneficially modifies the response to cold and can last for days, months, or even longer. Elicitor factors and antagonists accumulate due to increased biosynthesis or decreased degradation either during or after the priming stimulus. Comparison of priming studies investigating improved tolerance to chilling temperatures highlighted key regulatory functions of ROS/RNS and antioxidant enzymes, plant hormones, especially jasmonates, salicylates, and abscisic acid, and signalling metabolites, such as β‐ and γ‐aminobutyric acid (BABA and GABA) and melatonin. We conclude that these elicitors and antagonists modify local and systemic cold tolerance by integration into cold‐induced signalling cascades.     

The effects of a plant defence priming compound, β-aminobutyric acid,on multitrophic interactions with an insect herbivore and a hymenopterous parasitoid

Biocontrol of aphids by natural enemies is utilized in many organic and integrated pest management schemes. β-aminobutyric acid (BABA), a non-protein amino acid, is a plant defence primer that suppresses growth of some insect herbivores when applied as a root drench. This investigation examined how applying BABA to host plants via the roots may impact on a parasitoid wasp of aphids. Female Aphidius ervi (Haliday) did not discriminate against pea aphids (Acyrthosiphon pisum (Harris)) reared on BABA-treated beans (Vicia faba L.) or show any modified responses to volatiles released from BABA-treated plants. BABA reduced the size of emerging wasps, primarily by inhibiting the growth of the host aphid. Metabolomic analysis revealed BABA in both aphids and emergent wasps indicating some potential for direct physiological inhibition to have occurred. Survival of the parasitoids was only reduced at doses of BABA likely to produce phytotoxic effects in many plant species, thus there may be potential to incorporate plant defence primers like BABA into integrated pest management practices. However, the precise mechanisms of BABA-inhibition of insects still require elucidation.     

Crop Improvement and Abiotic Stress Tolerance Promoted by Moringa Leaf Extract

Moringa leaf extract (MLE) has been shown to promote beneficial outcomes in animals and plants. It is rich in amino acids, antioxidants, phytohormones, minerals, and many other bioactive compounds with nutritional and growth-promoting potential. Recent reports indicated that MLE improved abiotic stress tolerance in plants. Our understanding of the mechanisms underlying MLE-mediated abiotic stress tolerance remains limited. This review summarizes the existing literature on the role of MLE in promoting plant abiotic stress acclimation processes. MLE is applied to plants in a variety of ways, including foliar spray, rooting media, and seed priming. Exogenous application of MLE promoted crop plant growth, photosynthesis, and yield under both nonstress and abiotic stress conditions. MLE treatment reduced the severity of osmotic and oxidative stress in plants by regulating osmolyte accumulation, antioxidant synthesis, and secondary metabolites. MLE also improves mineral homeostasis in the presence of abiotic stress. Overall, this review describes the potential mechanisms underpinning MLE-mediated stress tolerance.     

Treating seeds with activators of plant defence generates long-lasting priming of resistance to pests and pathogens

? Priming of defence is a strategy employed by plants exposed to stress to enhance resistance against future stress episodes with minimal associated costs on growth. Here, we test the hypothesis that application of priming agents to seeds can result in plants with primed defences. ? We measured resistance to arthropod herbivores and disease in tomato (Solanum lycopersicum) plants grown from seed treated with jasmonic acid (JA) and/or β-aminobutryric acid (BABA). ? Plants grown from JA-treated seed showed increased resistance against herbivory by spider mites, caterpillars and aphids, and against the necrotrophic fungal pathogen, Botrytis cinerea. BABA seed treatment provided primed defence against powdery mildew disease caused by the biotrophic fungal pathogen, Oidium neolycopersici. Priming responses were long-lasting, with significant increases in resistance sustained in plants grown from treated seed for at least 8 wk, and were associated with enhanced defence gene expression during pathogen attack. There was no significant antagonism between different forms of defence in plants grown from seeds treated with a combination of JA and BABA. ? Long-term defence priming by seed treatments was not accompanied by reductions in growth, and may therefore be suitable for commercial exploitation.     

β-Aminobutyric Acid-induced Resistance in Cucumber against Biotrophic and Necrotrophic Pathogens

The non-protein amino acid β-aminobutyric acid (BABA) protects plants against a wide range of pathogens. Protection of cucumber plants by BABA depends on the potentiation of pathogen specific defence responses. To contribute to the analyses of the mode of action of BABA, we established a protocol for a fast and reproducible leaf disc assay to evaluate the effect of this chemical compound on cucumbers infected with either the biotrophic downy mildew fungus Pseudoperonospora cubensis, or the necrotrophic microbial pathogen Colletotrichum lagenarium . Accumulation of callose could be found in interactions with both pathogens after BABA-treatment. Furthermore, a localized rapid cell death and the production of reactive oxygen intermediates were detected after downy mildew attack. In contrast to this, degenerated primary hyphae were found in BABA induced tissue after inoculation with C. lagenarium .     

An Appraisal of Ancient Molecule GABA in Abiotic Stress Tolerance in Plants,and Its Crosstalk with Other Signaling Molecules

Gamma-aminobutyric acid (GABA), a non-proteinaceous amino acid, is reported in prokaryotes and eukaryotes, since ancient times. However, it has gained attention in the present time because of its rapid accumulation during stressed conditions in plants as well as in the cyanobacteria. In plants, it regulates the number of physiological processes such as pollen tube growth, root growth, TCA cycle, N₂-metabolism, and osmoregulation. Several biotic and abiotic stresses prevail in the environment, which lead to enhanced accumulation of reactive oxygen species (ROS) thus causing oxidative damage. However, a rapid increase in the accumulation of GABA during stress in various plant forms like bacteria, cyanobacteria, fungi, and plants indicates its putative role in stress regulation and acclimation. This review summarizes the biosynthesis of GABA, its role in abiotic stress tolerance, and its crosstalk with ROS, nitric oxide, Ca⁺² ions, phytohormones, and polyamines in stress acclimation.

     

γ-Aminobutyric acid and related amino acids in plant immune responses: Emerging mechanisms of action

The entanglement between primary metabolism regulation and stress responses is a puzzling and fascinating theme in plant sciences. Among the major metabolites found in plants, γ-aminobutyric acid (GABA) fulfils important roles in connecting C and N metabolic fluxes through the GABA shunt. Activation of GABA metabolism is known since long to occur in plant tissues following biotic stresses, where GABA appears to have substantially different modes of action towards different categories of pathogens and pests. While it can harm insects thanks to its inhibitory effect on the neuronal transmission, its capacity to modulate the hypersensitive response in attacked host cells was proven to be crucial for host defences in several pathosystems. In this review, we discuss how plants can employ GABA's versatility to effectively deal with all the major biotic stressors, and how GABA can shape plant immune responses against pathogens by modulating reactive oxygen species balance in invaded plant tissues. Finally, we discuss the connections between GABA and other stress-related amino acids such as BABA (β-aminobutyric acid), glutamate and proline.     

Does exogenous application of ascorbic acid modulate growth,photosynthetic pigments and oxidative defense in okra (<Emphasis Type="Italic">Abelmoschus esculentus</Emphasis> (L.) Moench) under lead stress?

Seed priming increases tolerance of plants against various environmental stresses. Although ample literature is available that depicts the beneficial effects of priming under different environmental stresses, the information on induction of tolerance to Pb stress through seed priming with ascorbic acid (AsA) is limited. Therefore, this study was performed to examine the effect of seed priming with AsA (50 and 100 mg L^?1), hydropriming and without priming (control) on physiochemical processes of okra cultivars (Subz-Pari and Arka Anamika) under Pb stress (0, 100 mg L^?1). Pb stress caused a considerable decline in plant growth and photosynthetic pigments. Contrarily, Pb stress exhibited rise in the contents of total amino acids, free proline, total soluble proteins and AsA. The POD, CAT, and SOD activities were recorded highest at 100 mg L^?1 of Pb. Moreover, Pb stress markedly increased H₂O₂ and MDA levels that triggered oxidative stress. However, plants raised from seed primed with AsA and water exhibited better growth and had higher chlorophylls, free proline, total proteins, total amino acids, AsA and activities of enzymatic antioxidants. Priming with AsA (50 mg L^?1) induced better tolerance to Pb stress in okra plants. Plants of cv. Arka Anamika exhibited greater tolerance to Pb than that of cv. Subz-Pari as was evident from higher plant fresh and dry masses.     

Priming for enhanced defence responses by specific inhibition of the Arabidopsis response to coronatine

The priming agent β-aminobutyric acid (BABA) is known to enhance Arabidopsis resistance to the bacterial pathogen Pseudomonas syringae pv. tomato (Pst) DC3000 by potentiating salicylic acid (SA) defence signalling, notably PR1 expression. The molecular mechanisms underlying this phenomenon remain unknown. A genome-wide microarray analysis of BABA priming during Pst DC3000 infection revealed direct and primed up-regulation of genes that are responsive to SA, the SA analogue benzothiadiazole and pathogens. In addition, BABA was found to inhibit the Arabidopsis response to the bacterial effector coronatine (COR). COR is known to promote bacterial virulence by inducing the jasmonic acid (JA) response to antagonize SA signalling activation. BABA specifically repressed the JA response induced by COR without affecting other plant JA responses. This repression was largely SA-independent, suggesting that it is not caused by negative cross-talk between SA and JA signalling cascades. Treatment with relatively high concentrations of purified COR counteracted BABA inhibition. Under these conditions, BABA failed to protect Arabidopsis against Pst DC3000. BABA did not induce priming and resistance in plants inoculated with a COR-deficient strain of Pst DC3000 or in the COR-insensitive mutant coi1-16. In addition, BABA blocked the COR-dependent re-opening of stomata during Pst DC3000 infection. Our data suggest that BABA primes for enhanced resistance to Pst DC3000 by interfering with the bacterial suppression of Arabidopsis SA-dependent defences. This study also suggests the existence of a signalling node that distinguishes COR from other JA responses.     

Signal transduction and biotechnology in response to environmental stresses

Providing sufficient food to burgeoning population from the steadily shrinking arable land seems to be very difficult in near future and is one of the foremost challenges for plant scientists. In addition, there are several biotic and abiotic stresses which frequently encounter crop plants during various stages of life cycle, resulting in considerable yield losses. Environmental stresses, including drought, flooding, salinity, temperature (both low and high), high radiation, and xenobiotics induce toxicity, membrane damage, excessive reactive oxygen species (ROS) production, reduced photosynthesis, and altered nutrient acquisition. Several indigenous defence mechanisms (physiological and molecular) are triggered in plants on exposure to environmental cues. Enhancement of resistance of crop plants to environmental stresses has been the topic of prime interest for agriculturalists and plant scientists since long. Development of water and salinity stress-tolerant crops through genetic engineering provides an avenue towards the reclamation of farmlands that have been lost due to salinity and lack of irrigation water/rainfall. Understanding the complexity of stress tolerance mechanisms in orthodox or model plants at the genetic and molecular levels improves feasibility of enhancing tolerance of sensitive crop plants.     

Metabolism and functions of gamma-aminobutyric acid

Gamma-aminobutyric acid (GABA), a four-carbon non-protein amino acid, is a significant component of the free amino acid pool in most prokaryotic and eukaryotic organisms. In plants, stress initiates a signal-transduction pathway, in which increased cytosolic Ca²⁺ activates Ca²⁺/calmodulin-dependent glutamate decarboxylase activity and GABA synthesis. Elevated H⁺ and substrate levels can also stimulate glutamate decarboxylase activity. GABA accumulation probably is mediated primarily by glutamate decarboxylase. However, more information is needed concerning the control of the catabolic mitochondrial enzymes (GABA transaminase and succinic semialdehyde dehydrogenase) and the intracellular and intercellular transport of GABA. Experimental evidence supports the involvement of GABA synthesis in pH regulation, nitrogen storage, plant development and defence, as well as a compatible osmolyte and an alternative pathway for glutamate utilization. There is a need to identify the genes of enzymes involved in GABA metabolism, and to generate mutants with which to elucidate the physiological function(s) of GABA in plants.