Introduction
Iron (Fe), an indispensable microelement for plants, is essential for many physiological functions, including oxidation–reduction system functions, chlorophyll synthesis, and respiration (Broadley et al., 2012, Kobayashi et al., 2019). Although Fe is abundant in soil, the amount available for plants is limited because Fe always exists as insoluble complexes (Marschner and Rengel, 2012, Vasconcelos et al., 2017, Zhang et al., 2019). Moreover, the availability of Fe is significantly lower in soil with a high pH, such as the calcareous soil of northern China (Abadía et al., 2011, Zuo and Zhang, 2011).
Based on their different response mechanisms to Fe deficiency, plants can be divided into Strategy I and Strategy II plants (Kobayashi and Nishizawa, 2012, Riaz and Guerinot, 2021). Strategy I plants, including non-grass monocots and dicots, can increase Fe availability in the rhizosphere and reduce Fe3+ to Fe2+via ferric reductase under Fe-deficient conditions (Li and Lan, 2017, Rajniak et al., 2018). Fe2+ is then transported into roots via Fe-regulated transporter (IRT) proteins and natural resistance-associated macrophage protein (NRAMP), which is regulated by FER-like Fe deficiency-induced transcription factor (FIT) (Vert et al., 2002, Yuan et al., 2008, Dai et al., 2018). In contrast, Strategy II plants, including gramineous monocots, secrete mugineic acid family phytosiderophores (MAs) into the rhizosphere to chelate Fe (Conte and Walker, 2011, Grillet and Schmidt, 2019), and the MA–Fe complex is taken up by plants via yellow stripe (YS) transporters (Curie et al., 2009, Conte and Walker, 2012, Tripathi et al., 2018).
The Strategy I plant response to Fe deficiency is restricted to high pH conditions (Römheld and Marschner, 1986) and occurs frequently in calcareous soil (Zuo and Zhang, 2009, Zuo and Zhang, 2011). Recent studies have revealed that Strategy I plants can utilize the phytosiderophore 2′-deoxymugineic acid (DMA) secreted by Strategy II plants to improve Fe nutrition (Suzuki et al., 2016, Astolfi et al., 2020). The YS homologous transporter, related to DMA uptake, is also found in Strategy I plants (Xiong et al., 2013). However, in agricultural production, DMA is impractical for use as an Fe fertilizer to improve the Fe nutrition of Strategy I plants because of its easy degradation and high cost of synthesis (Takagi et al., 1988, Suzuki et al., 2021).
Recently, a novel Fe-chelating agent, proline-2′-deoxymugineic acid (PDMA), was synthesized based on the structure of DMA (Suzuki et al., 2021). PDMA is much more stable than DMA, and the cost of synthesis is lower (Kratena et al., 2021, Suzuki et al., 2021). Moreover, PDMA has been shown to improve the Fe nutrition of various plants in hydroponic culture and calcareous substrates (Suzuki et al., 2021, Ueno et al., 2021), thus revealing the great potential of PDMA as a novel biological Fe fertilizer. However, the effect of PDMA on improving the Fe nutrition and yield of crops remains unclear under field conditions.
Peanut (Arachis hypogaea ), an important oil crop, is planted widely in northern China, and its yield and quality are greatly restricted by Fe deficiency (Zuo and Zhang, 2009, Roriz et al., 2020). To correct Fe deficiency in crops, chemosynthetic Fe-chelator complexes, such as the Fe-ethylenediamine-N ,N , , -tetraacetic acid complex (EDTA–Fe), are commonly used in modern agriculture as an Fe fertilizer (Abadía et al., 2011, Nadal et al., 2012). Due to the high pH in calcareous soil, Fe fertilizer is easily fixed in the soil after application. Nonetheless, PDMA has a greater demonstrated ability to improve Fe nutrition in rice and cucumber (Suzuki et al., 2021, Ueno et al., 2021). However, the influence of PDMA on the Fe nutrition and yield of peanuts grown in calcareous soil requires further investigation.
In the present study, we evaluated the effect of neotype Fe fertilizer-PDMA on peanut Fe nutrition and examined its mechanism at the molecular and ecological levels. We investigated the effects of PDMA using pot and field trials and revealed that PDMA could improve Fe nutrition effectively, thus promoting the yield of peanuts under field conditions. These results suggest great application prospects for PDMA in agricultural systems.