Growth-Defense Trade-Offs Induced by Long-term Overgrazing Could Act as a Stress Memory

Abstract

Long-term overgrazing (OG) is one of the key drivers of global grassland degradation with severe loss of productivity and ecosystem functions, which may result in stress memory such as smaller stature of grassland plants. However, how the OG-induced stress memory could be regulated by phytohormones is unknown. In this study, we investigated the changes of four phytohormones of cloned offspring of Leymus chinensis that were developed from no-grazing (NG) plants and OG plants with a grazing history of 30 years. The concentrations of auxin (IAA) and gibberellic acid (GA) in OG plant leaves were 45% and 20% lower than control, respectively. Meanwhile, the level of abscisic acid (ABA) in OG leaves nearly doubled compared with that in NG leaves. The situation was quite similar in roots. Unexpectedly, no significant changes in the jasmonic acid (JA) level were observed between OG and NG plants. The changes in gene expression patterns between OG and NG plants were also investigated by transcriptomic analysis. In total, 302 differentially expressed genes (DEGs) were identified between OG and NG plants, which were mainly classified into the functions of synthesis, receptor, and signal transduction processes of phytohormones. The expression of 24 key genes related to the biosynthesis and signal transduction of IAA and GA was downregulated in OG plants. Among them, OASA1 and AO1 (regulating the biosynthesis of IAA and ABA, respectively) were reduced significantly by 88 and 92%, respectively. In addition, the content of secondary metabolites related to plant defense such as flavonoids and phenols was also increased in leaves. Taken together, the decrease of positive plant growth-related hormones (IAA and GA) together with the increase of plant stress-related hormones or factors (ABA, flavonoids, and phenols) induced the growth-defense trade-offs for L. chinensis adaptation to long-term OG stress. The findings reported in this study shed new light on the mechanism of plant–animal interaction in the grassland ecosystem and provide a deeper insight into optimizing grazing management and sustainable utilization of grassland.

Biomass and Leaf Nutrition Contents of Selected Grass and Legume Species in High Altitude Rangelands of Kashmir Himalaya Valley (Jammu & Kashmir), India

“…In this regard, Dai et al [ 16 ] reported that the moderate grazing promoted the root biomass of Kobresia meadow (BGB) in the northern Qinghai-Tibet pastures, which corroborates with our findings. Thus, the adaptive response of plants might occur in which they tend to increase their root development to survive [ 17 ]. Also, during the April-May period, snow melting occurs in the pasturelands of Kashmir Valley which naturally favors and promotes the growth and development of BGB.…”

Clonal Transgenerational Effects of Parental Grazing Environment on Offspring Shade Avoidance

“…Finally, its supernatant was used for the determination of three endogenous hormone (GA3, IAA, and ABA) contents using HPLC-MS/MS. The chromatographic conditions were set according to Qu et al [17].…”

“…For example, livestock overgrazing reduces individual plant height, shortens and narrows leaves, and decreases stem length [14][15][16]. Except for the immediate responses to trampling and herbivory in the field, some studies have demonstrated that these dwarf characteristics contribute to the regulation of photosynthetic and hormonal pathways [15,17]. The dwarf characteristics may help plants to escape herbivores as much as possible, which is supposed to be a grazing avoidance strategy.…”

“…The dwarf characteristics may help plants to escape herbivores as much as possible, which is supposed to be a grazing avoidance strategy. Moreover, plants can "remember" the grazing stress history and the dwarf phenotype across clonal generations even when the grazing pressure is removed [17,18], a phenomenon that is called stress memory or clonal transgenerational effects [19][20][21]. Among the published studies that show such transgenerational dwarfism effects are induced by livestock herbivory (see references above), very few have directly tested the effects on key ecological interactions such as the light environment.…”