γ-aminobutyric acid (γ-aminobutyric acid, GABA) is a signal substance widely used in regulating plant stress adaptation in recent years.
This study analyzes the physiological mechanism of GABA production under environmental stress, and how GABA regulates plant seed germination, seedling morphological development, plant antioxidant activity, photosynthesis, and nitrogen metabolism under environmental stress.
1. Effects of GABA on Plant Seed Germination
Seed germination is an important transitional period in plant life activities and is easily affected by environmental stress.
Under stress conditions such as salt, low temperature, and aluminum, the emergence rate of seeds and the growth of seedlings will be affected.
It is manifested as uneven emergence, low emergence rate, weak seedling growth or underdeveloped radicle and embryo, etc., which hinders the normal growth of plants.
Under salt stress, the conversion rate of seed storage substances increased after GABA soaking, effectively alleviating the poor seed development caused by the stress, improving seed vigor, promoting radicle development, and increasing the radicle length of early seedlings.
At low temperatures, GABA-soaking seeds can provide energy substances for seed germination by accelerating the metabolism of soluble sugar and other storage substances, thereby effectively alleviating environmental stress and enhancing plant seed resistance.
Amylase is an important enzyme that converts starch into small molecular nutrients during seed germination. This process can provide energy for seed germination. When plants are subjected to environmental stress, starch catabolism is inhibited, which will seriously affect seed germination.
After soaking seeds with GABA, the activity of amylase (α-amylase and β-amylase) was significantly enhanced, thereby accelerating the rate of starch catabolism and providing more carbohydrates for seed germination.
2. Regulation of GABA on Plant Morphogenesis
An important performance of plants under environmental stress is slow growth, resulting in decreased plant height, stem diameter, fresh weight and dry weight of aboveground and underground plants.
Long-term exposure to adverse environments will seriously inhibit the normal growth and development of seedlings, and even cause death, hindering the establishment of seedlings.
Under drought conditions, GABA spraying alleviated the growth inhibition of leaves and roots of melon seedlings, and increased chlorophyll content, leaf area ratio, and various growth indicators, thereby promoting the extension of leaves and the development of roots.
Under salt stress, 0.5 mmol·L-1 GABA can promote the production of root hairs, thereby increasing root length, root surface area, and root volume, and promoting root development
Under high-temperature stress, the application of GABA can promote stem and leaf growth and increase root length.
GABA is an amino acid nitrogen source that is easily absorbed by the root system. It participates in the process of nitrate absorption. After applying an appropriate concentration of GABA, it can promote the absorption of nitrate in plants and stimulate root elongation.
Afterwards, GABA is absorbed by the roots and transferred to the leaves, effectively promoting the synthesis of polyamines in plants and relieving environmental stress.
3. Regulation of GABA on Antioxidative Effects of Plants
The role of GABA in regulating plant antioxidants, it can reduce the cell damage caused by the massive accumulation of ROS by activating the antioxidant defense system (increasing antioxidant enzymes, increasing non-enzymatic antioxidants), and can also directly inhibit ROS synthesis-related enzymes activity, maintain plant cell membranes from damage, and promote plant growth and development under environmental stress.
Exogenous application of GABA can reduce the inhibition of drought stress on plant growth by reducing the superoxide anion production rate, hydrogen peroxide and malonaldehyde (MDA) content in tomato seedling leaves, and increasing osmotic adjustment substances such as soluble sugar and soluble protein.
4. Regulation of GABA on Plant Photosynthesis
GABA can stabilize and protect thylakoids of chloroplasts by reducing the content of hydroxyl radicals in chloroplasts, so that cells can avoid low-temperature damage.
Spraying an appropriate amount of GABA can increase the net photosynthetic rate (Pn) and stomatal conductance (Gs) of plants under NaCl stress, and reduce the intercellular CO2 concentration (Ci).
This result also indicated that an appropriate amount of exogenous GABA could reduce the rate of decline of photosynthetic activity of mesophyll cells under NaCl stress.
5. Regulation of GABA on Nitrogen Metabolism in Plants
The conversion of nitrate nitrogen (NO3–N) and ammonium nitrogen (NH4+-N) in plants is a key step in plant nitrogen metabolism, and the conversion rate of this process is the limiting factor for plant growth and protein synthesis. The accumulation of ammonium nitrogen (NH4+-N) will cause damage to plants.
When GABA is applied to plants, it can significantly promote the absorption of nitrate by plants, and promote the growth of plants by increasing the soluble protein and total nitrogen content in plants.
The effect of GABA on seed germination is to enhance the activity of amylase by soaking seeds and increase the conversion rate of stored substances, thereby improving the germination index of seeds and promoting early and rapid germination of seeds.
The effect of GABA on the growth of plant seedlings is to adjust the antioxidant system through GABA spraying, reduce the rate of ROS generation, increase the rate of photosynthesis, protect photosynthetic tissue from damage, regulate the degradation of glutamic acid and polyamines, and promote the content of GABA. Increase, provide substrate for TCA cycle, alleviate plant growth and development under adversity, and finally enhance plant stress resistance.