Abstract
Iron deficiency chlorosis (IDC) is a global crop production problem, significantly impacting yield. However, most IDC studies have focused on model species, not agronomically important crops. Soybean is the second largest crop grown in the United States, yet the calcareous soils across most of the upper U.S. Midwest limit soybean growth and profitability. To understand early soybean iron stress responses, we conducted whole genome expression analyses (RNA-sequencing) of leaf and root tissue from the iron efficient soybean (Glycine max) cultivar Clark, at 30, 60 and 120 min after transfer to iron stress conditions. We identified over 10,000 differentially expressed genes (DEGs), with the number of DEGs increasing over time in leaves, but decreasing over time in roots. To investigate these responses, we clustered our expression data across time to identify suites of genes, their biological functions, and the transcription factors (TFs) that regulate their expression. These analyses reveal the hallmarks of the soybean iron stress response (iron uptake and homeostasis, defense, and DNA replication and methylation) can be detected within 30 min. Furthermore, they suggest root to shoot signaling initiates early iron stress responses representing a novel paradigm for crop stress adaptations.
Highlights
Iron Deficiency Chlorosis (IDC) limits growth of soybeans grown in calcareous soils across most of the upper U.S Midwest
To identify genes differentially expressed in response to iron stress in roots and leaves, data were normalized across time points within each tissue using Trimmed Mean of M-values (TMM) normalization
Buckhout et al [24] used microarray analyses to measure Arabidopsis root responses to iron stress across time (30 min, 1, 6 and 24 h). They concluded that the 90 differentially expressed genes (DEGs) identified at 30 min and 1 h after iron stress were not specific to the iron stress response because they were only expressed in a single time point
Summary
Iron Deficiency Chlorosis (IDC) limits growth of soybeans grown in calcareous soils across most of the upper U.S Midwest. The primary symptom of IDC is interveinal chlorosis, caused by insufficient iron needed for chlorophyll production. Yield losses due to IDC are estimated to be USD 120 million per year [3]. On-farm management recommendations for IDC are limited to choosing varieties resistant/tolerant to IDC or the application of expensive, and not highly effective, iron foliar sprays. IDC-prone fields are not uniform and IDC tolerant lines tend to have low yield on non-IDC prone soil, so farmers often choose to use higher yielding lines with higher seeding rates to overcome potential yield loss [4]. One goal for soybean breeders is to generate lines that are both IDC tolerant and high yielding. To do so will require better understanding of whole plant responses to IDC
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