Physiological and transcriptomic responses of Pinellia ternata to continuous cropping

Pinellia ternata (Thunb.) Breit. is one of the popular medicinal plants and industrial crops in China, but its production has been severely limited by continuous cropping obstacles (CCO). In order to elucidate the reaction of P. ternata to successive cultivation and the potentially uncover the possible causes of CCO, the research group examined P. ternata relevant indexes, including morphological, physio-biochemical, as well as its transcriptome, under continuous cropping (CC) conditions. Based on the results, the influences of CC were multilevel. Specifically, CC led to chlorophyll content in leaves and antioxidant enzymes (peroxidase and superoxide dismutase, i.e. POD & SOD) activities significant reduced, reactive oxygen species (ROS) and malondialdehyde (MDA) in cells increased significantly. And the obvious changes were also observed in osmoregulatory substances such as proline (Pro), soluble protein (SP) and soluble sugars (SS). As a result, the plants growth was hindered and had the appearance of a "sprout tumble" prematurely. By combining SMRT full-length and Illumina transcriptome sequencing, a database of the root and leaf transcriptome of P. ternata at different years of CC years was established, and 77,783 full-length non-redundant transcripts were assembled. 412 differentially expressed genes (DEGs) were identified in the root during the first planting year (FPR) vs. the root after 2 years of Continuous Cropping (CC2R), of which 138 were up-regulated, meanwhile 274 were down-regulated. Moreover, 5020 DEGs were identified in the leaves during the first planting year (FPL) vs. the leaves after 2 years of Continuous Cropping (CC2L), of which 1894 were up-regulated, and 3126 were down-regulated. The analysis of DEGs showed that photosynthesis and chlorophyll biosynthesis, fatty acid and wax biosynthesis, cell wall synthesis and modification, defense response and plant-pathogen interaction, as well as phytohormone signaling were the key genes differentially expressed. Weighted gene correlation network analysis (WGCNA) analysis revealed that the hub genes in the roots were enriched in pathways associated with sugar synthesis and metabolism, phosphate ion transport, jasmonate metabolism, and fungal defense. At the same time, phosphate ion transport, positive regulation of cellular response to phosphate starvation, chlorophyll biosynthetic process, photosynthesis, and stress response pathways were enriched in the leaves. As a result, it was inferred that phosphorus depletion and the alteration of the soil microbiota by the previous planting cycles, which resulted in nutrient element deficiencies, as well as the damage of roots by pathogenic fungi, were the main CCO causes of P. ternata, and the corresponding changes in the physiological and transcriptional homeostasis of the leaves due to a generalized stress response.