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 ,Nʹ ,Nʹ -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.