Transcriptome profiling of Eutrema salsugineum under low phosphate and low sulfur

Abstract

Improving the efficiency by which crops use nutrients is critical for maintaining high crop productivity while reducing fertility management costs and eutrophication related to fertilizer runoff. The native crucifer and halophyte, Yukon Eutrema salsugineum, was used in this study. Yukon E. salsugineum is closely related to important Brassica crops and thrives in its native habitat on soil that is low in available phosphate (Pi) and high in sulfur (S). To determine how Yukon E. salsugineum copes with low Pi, leaf transcriptomes were prepared from four week-old plants grown in controlled environment chambers using soil lacking or supplemented with Pi and/or S. This thesis focused on using bioinformatic approaches to assemble, analyze and compare the transcriptome profiles produced by the Yukon E. salsugineum plants undergoing four nutrient combinations of high and/or low Pi and S. The objective of the study was to identify traits associated with altered S and/or Pi with the prediction based on other species that low Pi, in particular, would pose the greatest stress and hence elicit the greatest transcriptional reprogramming. Transcriptome libraries were generated from four treatment groups with three biological replicates each. Reads in each library were mapped to 23,578 genes in the E. salsugineum transcriptome with an average unique read mapping ratio of 99.52%. Surprisingly, pairwise comparisons of the transcriptomes showed little evidence of Pi-responsive reprogramming whereas treatments differing in soil S content showed a clear S-responsive transcriptome profile. Principal Component Analysis revealed that the low variance quaternary Principal Component distinguished the transcriptomes of plants undergoing low versus high Pi treatments with differential gene expression analysis only finding 11 Pi-responsive genes. This outcome suggests that leaf transcriptomes of Yukon E. salsugineum plants under low Pi are largely undifferentiated from plants provided with Pi and is consistent with Yukon E. salsugineum maintaining Pi homeostasis through fine-tuning the expression of protein-coding and non-coding RNA rather than large-scale transcriptomic reprogramming. Previous research has shown Yukon E. salsugineum to be very efficient in its use of Pi and this work suggests that the altered expression of relatively few genes may be needed to develop Pi-efficient crops to sustain the crop demand of a growing population.