Full-length transcriptome sequencing of the short-rotation woody crop Salix integra reveals a time series response to Pb stress

The short-rotation woody crop Salix integra is a low-cost, renewable feedstock that can be used in the medical and energy industry sectors. Lead (Pb) is a highly toxic, nondegradable heavy metal that accumulates in the soil, can reduce the biomass productivity of plants and threatens human health. Some genotypes of S. integra show strong tolerance to and high accumulation potential for Pb. Here, a highly tolerant S. integra clone was subjected to 0.3 mM Pb in a hydroponic system for 14 d, and roots were sampled at 9 time points. Cytotoxic substance contents and antioxidant enzyme activities were affected by Pb. Transcriptome data revealed 11039 differentially expressed transcripts (DETs); the expression levels of representative DETs were verified by RTqPCR. Weighted gene coexpression network analysis (WGCNA) and enrichment analyses showed that the response of S. integra to Pb stress could be divided into two main stages. In the early stage (mainly within 24 h), DETs were involved in the "sulfur assimilation and GSH synthesis" process and "ABC transporters" pathway. In the late stage (mainly after 3 d), the differential expression of DETs that regulate the synthesis of multiple secondary metabolites in the "phenylpropanoid biosynthesis" pathway was the main factor. Additionally, the continuous differential expression of DETs involved in the "plant hormone signal transduction" pathway suggested a complex response pattern. The results deeply revealed the Pb response pattern in S. integra driven by the coregulation of multiple pathways at different stages instead of the single Pb response pathway found in other species. This study provides new ideas for studying the Pb tolerance mechanisms of energy crops.