I ron deficiency chlorosis is an abiotic stress prevalent in soybean. Soils with a pH greater than 8 cause Fe to become unavailable for plant uptake. High soil pH is often accompanied in calcareous soils by bicarbonate release, which acts as a pH buffer and exacerbates IDC in soybeans because of the inability of the plant to acidify the rhizosphere (Hansen et al., 2003; Inskeep and Bloom, 1987). This lack of Fe availability reduces chlorophyll production, visually observed as interveinal chlorosis of leaves in soybean, which ultimately hinders photochemical capacity ABSTRACT Iron deficiency chlorosis (IDC) is an important nutrient stress for soybean [Glycine max (L.) Merr.] grown in high-pH soils. Despite numerous agronomic attempts to alleviate IDC, genetic tolerance remains the most effective preventative measure against symptoms. In this study, two association mapping populations and a biparental mapping population were used for genetic mapping of IDC tolerance. Quantitative trait loci (QTLs) were identified on chromosomes Gm03, Gm05, and Gm06. Heterogenous inbred families were developed to fine-map the Gm05 QTL, which was uniquely supported in all three mapping populations. Fine-mapping resulted in a QTL with an interval size of 137 kb on the end of the short arm of Gm05, which produced up to a 1.5-point reduction in IDC severity on a 1 to 9 scale in near isogenic lines.

During winter evergreens maintain a sustained form of thermal energy dissipation that results in reduced photochemical efficiency measured using the chlorophyll fluorescence parameter Fv/Fm. Eastern white pine (Pinus strobus L.) and white spruce [Picea glauca (Moench) Voss] have been shown to differ in their rate of recovery of Fv/Fm from winter stress. The goal of this study was to monitor changes in photosynthetic protein abundance and phosphorylation status during winter recovery that accompany these functional changes. An additional goal was to determine whether light-dependent changes in light harvesting complex II (LHCII) phosphorylation occur during winter conditions. We used a combination of field measurements and recovery experiments to monitor chlorophyll fluorescence and photosynthetic protein content and phosphorylation status. We found that pine recovered three times more slowly than spruce, and that the kinetics of recovery in spruce included a rapid and slow component, while in pine there was only a rapid component to recovery. Both species retained relatively high amounts of the light harvesting protein Lhcb5 (CP26) and the PsbS protein during winter, suggesting a role for these proteins in sustained thermal dissipation. Both species maintained high phosphorylation of LHCII and the D1 protein in darkness during winter. Pine and spruce differed in the kinetics of the dephosphorylation of LHCII and D1 upon warming, suggesting the rate of dephosphorylation of LHCII and D1 may be important in the rapid component of recovery from winter stress. Finally, we demonstrated that light-dependent changes in LHII phosphorylation do not continue to occur on subzero winter days and that needles are maintained in a phosphorylation pattern consistent with the high light conditions to which those needles are exposed. Our results suggest a role for retained phosphorylation of both LHCII and D1 in maintenance of the photosynthetic machinery in a winter conformation that maximizes thermal energy dissipation.

Iron deficiency is an important soybean [Glycine max (L.) Merr.] nutrient deficiency that is easily identified by interveinal chlorosis of the leaves and reduced plant growth, both of which lead to yield reductions. Research in soybean iron deficiency is often segmented into studies on soil characteristics, microbe interactions, specific phenotypes, or genetics of iron efficiency. Joining these areas of research into a comprehensive literature review will advance our understanding of iron deficiency physiology and help to bridge known iron deficiency chlorosis (IDC) resistance loci with plant responses to iron stress. This review investigates what has been accomplished in the areas of phenotyping and genetics of iron deficiency. Furthermore, this work traces iron deficiency physiology research through the plant, beginning with the role of soil, the transport of iron into and through plant tissues, and the eventual deposition in the seed. While IDC is the most phenotyped and genetically mapped trait relating to iron deficiency in soybean, the whole plant is truly affected by and involved in recovery to the stress. While often neglected in iron deficiency research, the soybeanrhizobia relationship is discussed as an area of opportunity for future advancements. Citrate and nicotianamine were identified as important compounds for iron efficiency in several studies and warrant more in-depth investigation. The aim of this review is to analyze research in soybean iron deficiency phenotyping, genetics, and physiology to reveal connections between these areas and facilitate further discoveries. INTRODUCTIONSoybean [Glycine max (L.) Merr.] is a highly valued oil crop worldwide as well as an important source of protein in both livestock feed and human diets (Masuda & Goldsmith, 2009). Consumed directly, soybean is an important dietary source of both protein and iron in developing countries (Messina, 1999). In 2020, soybean was planted on 33.6 million ha in

The soybean (Glycine max L. merr) genotype Fiskeby III is highly resistant to a multitude of abiotic stresses, including iron deficiency, incurring only mild yield loss during stress conditions. Conversely, Mandarin (Ottawa) is highly susceptible to disease and suffers severe phenotypic damage and yield loss when exposed to abiotic stresses such as iron deficiency, a major challenge to soybean production in the northern Midwestern United States. Using RNA-seq, we characterize the transcriptional response to iron deficiency in both Fiskeby III and Mandarin (Ottawa) to better understand abiotic stress tolerance. Previous work by our group identified a quantitative trait locus (QTL) on chromosome 5 associated with Fiskeby III iron efficiency, indicating Fiskeby III utilizes iron deficiency stress mechanisms not previously characterized in soybean. We targeted 10 of the potential candidate genes in the Williams 82 genome sequence associated with the QTL using virus-induced gene silencing. Coupling virus-induced gene silencing with RNA-seq, we identified a single high priority candidate gene with a significant impact on iron deficiency response pathways. Characterization of the Fiskeby III responses to iron stress and the genes underlying the chromosome 5 QTL provides novel targets for improved abiotic stress tolerance in soybean.