Alfalfa Responses to Intensive Soil Compaction: Effects on Plant and Root Growth, Phytohormones and Internal Gene Expression

The perennial legume alfalfa (Medicago sativa L.) is of high value in providing cheap and high-nutritive forages. Due to a lack of tillage during the production period, the soil in which alfalfa grows prunes to become compacted through highly mechanized agriculture. Compaction deteriorates the soil's structure and fertility, leading to compromised alfalfa development and productivity. However, the way alfalfa responses to different levels of soil compaction and the underlying molecular mechanism are still unclear. In this study, we systematically evaluated the effects of gradient compacted soil on the growth of different cultivars of alfalfa, especially the root system architecture, phytohormones and internal gene expression profile alterations. The results showed that alfalfa growth was facilitated by moderate soil compaction, but drastically inhibited when compaction was intensified. The inhibition effect was universal across different cultivars, but with different severity. Transcriptomic and physiological studies revealed that the expression of a set of genes regulating the biosynthesis of lignin and flavonoids was significantly repressed in compaction treated alfalfa roots, and this might have resulted in a modified secondary cell wall and xylem vessel formation. Phytohormones, like ABA, are supposed to play pivotal roles in the regulation of the overall responses. These findings provide directions for the improvement of field soil management in alfalfa production and the molecular breeding of alfalfa germplasm with better soil compaction resilience.