High-resolution profile of transcriptomes reveals a role of alternative splicing for modulating response to nitrogen in maize

The fluctuation of nitrogen (N) contents profoundly affects the root growth and architecture in maize by altering the expression of thousands of genes. The differentially expressed genes (DEGs) in response to N have been extensively reported. However, information about the effects of N variation on the alternative splicing in genes is limited. To reveal the effects of N on the transcriptome comprehensively, we studied the N-starved roots of B73 in response to nitrate treatment, using a combination of short-read sequencing (RNA-seq) and long-read sequencing (PacBio-sequencing) techniques. Samples were collected before and 30 min after nitrate supply. RNA-seq analysis revealed that the DEGs in response to N treatment were mainly associated with N metabolism and signal transduction. In addition, we developed a workflow that utilizes the RNA-seq data to improve the quality of long reads, increasing the number of high-quality long reads to about 2.5 times. Using this workflow, we identified thousands of novel isoforms; most of them encoded the known functional domains and were supported by the RNA-seq data. Moreover, we found more than 1000 genes that experienced AS events specifically in the N-treated samples, most of them were not differentially expressed after nitrate supply-these genes mainly related to immunity, molecular modification, and transportation. Notably, we found a transcription factor ZmNLP6, a homolog of AtNLP7-a well-known regulator for N-response and root growth-generates several isoforms varied in capacities of activating downstream targets specifically after nitrate supply. We found that one of its isoforms has an increased ability to activate downstream genes. Overlaying DEGs and DAP-seq results revealed that many putative targets of ZmNLP6 are involved in regulating N metabolism, suggesting the involvement of ZmNLP6 in the N-response. Our study shows that many genes, including the transcription factor ZmNLP6, are involved in modulating early N-responses in maize through the mechanism of AS rather than altering the transcriptional abundance. Thus, AS plays an important role in maize to adapt N fluctuation.