Glycyrrhiza uralensis Fisch., commonly known as licorice, is a kind of resource plant of the legume family with high economic value, and its roots and rhizomes are used in medicine, which not only occupy an important position in traditional Chinese medicine but also show a broad application prospect in modern medicine, health care and food industry. The wild G. uralensis resources are on the verge of depletion due to long-term over-exploitation. Yet, the demand for licorice remains high and cultivated licorice is becoming a substitute for wild G. uralensis. Although cultivated G. uralensis has been planted on a large scale in Xinjiang, , Inner Mongolia, Ningxia and Gansu in China, it is currently facing a number of challenges: On the one hand, due to arable land constraints, fertile farmland is usually used for the cultivation of conventional crops such as grain, vegetables, oil crops, cotton, etc., while salinization abandoned cropland and salt desert, which are intolerable to conventional crops, are often used to grow G. uralensis. On the other hand, although the adult G. uralensis has strong salt tolerance, its salt tolerance in the seed germination and seedling stage is poor, and the phenomenon of fail place, root rot, and seedling death in the field is frequent, and the growth of the individual is strongly inhibited by salt stress, resulting in a significant decrease in the yield and quality of its medicinal materials, which has seriously constrained the healthy development of the cultivated G. uralensis industry.
In this thesis, the mitigation effect of exogenous lanthanum nitrate (La(NO₃)₃) on salt stress in G. uralensis was investigated from multiple dimensions, including seed germination, seedling growth, herb yield and medicinal quality, by combining a pot experiment and a field experiment.
The optimal application dose of La(NO₃)₃ was determined, and the physiological and molecular mechanisms of exogenously applied lanthanum nitrate on the quality and yield enhancement of G. uralensis under salt stress were revealed from the aspects of photosynthetic gas exchange, antioxidant system, osmoregulatory substances, and transcriptome technology, with the specific findings as follows:
La(NO₃)₃ soaking treatment significantly increased the germination rate of G. uralensis seeds and promoted the growth of radicle as well as radicle viability under different concentrations of sodium chloride (NaCl) and sodium sulfate (Na₂SO₄) stress conditions. The promotional effects of the 0.45 mM La(NO₃)₃ treatments were all the most significant, resulting in seed germination rates ranging from 17-25% and 3.8-9.4-fold enhancement of the radicle vigor index under various salt stress conditions.
The results of pot experiments with one-year-old G. uralensis showed that exogenous application of La(NO₃)₃ significantly alleviated the salt-mediated inhibition of root growth under different salt stress treatments, and that root watering with 0.75 mM La(NO₃)₃ solution had the most obvious effect on the growth of the primary roots of G. uralensis and accumulation of medicinal constituents in the primary roots, which significantly contributed to the growth and thickening of the primary roots (primary root length + 21%- 42%, primary root diameter + 16%-58%, primary root biomass (+ 47%-85%, glycyrrhetin content + 75%-85%) and accumulation of medicinal constituents (glycyrrhetinic acid content + 75%-85%, glycyrrhetin content). 42%, main root diameter +16%-58%), main root biomass (+ 47%-85%) and accumulation of medicinal components (glycyrrhizic acid +75%-85%, liquiritigenin +36%-67%, glycyrrhizin +75%-133%).
The results of two consecutive years of field experiments showed that the root irrigation treatment with 0.75 mM La(NO₃)₃ solution substantially increased the length of the primary roots of G. uralensis, the maximum diameter of the primary roots, the average diameter of the primary roots, the biomass, and the contents of glycyrrhizic acid and glycyrrhizin in the primary roots of G. uralensis at two ages, respectively, compared to the control group, which increased by 58.1%, 143.2%, 121.2%, 480.2%, 46.2% and 50.8%, resulting in glycyrrhizic acid and glycyrrhizin contents of 2769.59 μg/g and 1720 μg/g, respectively, in the main roots.
Physiological studies based on three-year pot experiments showed that root application of 0.75 mM La(NO₃)₃ solution significantly improved photosynthetic performance (chlorophyll content, photosynthetic gas exchange parameters, chlorophyll fluorescence parameters), antioxidant enzymes (superoxide dismutase, catalase, and peroxidase), and osmoregulators (proline, soluble sugars, and soluble proteins) and ionic homeostasis (Na+, K+, Mg2+, and Ca2+) in G. uralensis under salt stress, thereby significantly increasing the endogenous hormone levels (zeatin, abscisic acid, and salicylic acid) of the plant, which resulted in a significant increase in the yield of the main root of G. uralensis and the content of its medicinal components under salt stress conditions.
The molecular mechanism of lanthanum nitrate in regulating the salt tolerance of G. uralensis showed that the application of 0.75 mM La(NO₃)₃ significantly enriched the pathways of flavonoid biosynthesis (KEGG: gmx00941) and phenylpropanoid metabolism (KEGG: gmx00940), and the expression of key genes, such as chalcone synthase (CHS) and flavonol synthase (FLS) genes, in the G. uralensis under the conditions of salt stress. The expression of key genes such as the CHS gene and FLS gene significantly promoted the synthesis of antioxidants. In addition, La(NO₃)₃ significantly up-regulated the expression levels of auxin response factor (ARF) and small auxin-up RNA (SAUR), enhanced the activity of the growth hormone signaling pathway, and promoted cell growth and stress recovery. Moreover, the negative effects of salt stress on Glycyrrhiza glabra were mitigated by inhibiting the PIF transcription factor by DELLA to promote stem elongation and photomorphogenesis; Promoting zeatin accumulation by enhancing the activity of zeatin regulatory-related cis-acting elements (CREs); inhibiting the activity of SnRK2 to reduce the concentration of abscisic acid (ABA); and inhibiting the expression of PR-1 gene and TGA transcription factor induced by salt stress to inhibit the salicylic acid-mediated immune response, reducing the burden of disease resistance-related metabolite synthesis, and thus alleviating the negative effects of salt stress on G. uralensis.
The results of lanthanum residues showed that the level of lanthanum in G. uralensis root was well below the safety threshold of the Codex Alimentarius Commission (CODEX) (≤2 mg/kg DW) and complied with the international safety standard (No 1881/2006). Therefore, the application of 0.75 mM La(NO₃)₃ solution did not pose a safety risk to G. uralensis herbs. Moreover, the environmental monitoring results showed that the lanthanum residue in the inter-root soil was only 10.7 mg/kg after continuous root application of La(NO₃)₃, which was far below the ecological safety threshold of 20 mg/kg.
In summary, the seed dipping treatment of 0.45 mM La(NO₃)₃ solution could maximize the salt tolerance performance of G. uralensis seeds under NaCl and Na₂SO₄ stress conditions, and thus improve their germination rate. The best effect of root application of 0.75 mM La(NO₃)₃ on salt tolerance of G. uralensis seedlings under NaCl and Na₂SO₄ stress conditions was achieved through the regulation of physiological pathways, such as photosynthetic performance, antioxidant enzyme activity, osmoregulatory substance accumulation, ionic homeostasis, and hormone levels, as well as enrichment of flavonoid biosynthesis and phenylpropanoid metabolism pathway, and the modulation of the molecular mechanisms, such as expression of key genes of phytohormone signaling pathway, of liquiritin. The molecular mechanisms such as enrichment of flavonoid biosynthesis and phenylpropanoid metabolic pathway, and regulation of key gene expression of phytohormone signaling pathway not only greatly increased the salt tolerance of G. uralensis, but also significantly improved the yield and quality of the medicinal herbs. The research results provide a scientific basis for the practical application of La(NO₃)₃ in the cultivation of G. uralensis to improve the quality and yield, and also contribute to a new way of thinking for the improvement of stress tolerance of important economic crops.
谷歌
火狐
