Discussion
Recently, PDMA, an effective Fe chelator, was synthesized based on the
structure of the phytosiderophore DMA (Suzuki et al., 2021). PDMA, with
low synthesis costs and high stability, was proven to improve Fe
nutrition in rice and cucumber under conditions of hydroponic culture
and calcareous substrates (Suzuki et al., 2021, Ueno et al., 2021). In
this study, positive effects of PDMA on Fe nutrition and peanut yields
were demonstrated in calcareous soil under field conditions.
The present study revealed that the application of PDMA significantly
corrected Fe deficiency in pot and field trials (Fig. 2, Fig. 5).
Consistent with the accumulation of Fe, because Fe is a vital element
involved in chlorophyll synthesis (Kobayashi et al., 2019), the SPAD
values were enhanced substantially by PDMA application (Fig. 1, Fig. 5).
The growth of the PDMA-treated peanut plants was therefore promoted
(Fig. 1). Similar increases were found in rice and cucumber (Suzuki et
al., 2021, Ueno et al., 2021). Importantly, our work produced the novel
finding that PDMA can increase the yield and kernel micronutrition of
peanuts (Table 1, Fig. 6), implying the great ability of PDMA to improve
the production and quality of plants under Fe-limited field conditions.
PDMA resulted in higher levels of dissolved Fe in the rhizosphere than
did EDTA–Fe, but the improvement in peanut Fe nutrition between these
treatments was not significantly different (Fig. 2, Fig. 5). These
results suggest that PDMA can chelate insoluble Fe in soil to improve Fe
nutrition in comparison to traditional Fe fertilizers without any
exogenous Fe supplementation, which realizes sustainable utilization of
the Fe in soil (Colombo et al., 2014). This type of metal-free
fertilizer effectively utilizes the residual nutrients in native soil
and is conducive to the development of sustainable agriculture (Bouis
and Welch, 2010).
Another mechanism by which PDMA may increase the Fe nutrition in peanut
is that PDMA–Fe complexes may be taken up directly by roots via AhYSL1.
A previous study indicated that PDMA can trigger YS family gene
expression in Strategy Ⅱ plants (Suzuki et al., 2021). In the present
study, expression of the YS homolog AhYSL1 in peanut root was
up-regulated by PDMA (Fig. 4), suggesting that PDMA–Fe complexes might
be directly assimilated by roots. Similar characteristics were found for
DMA–Fe, which was proven to be taken up via the YSL transporter in both
Strategy I and Strategy Ⅱ plants (Xiong et al., 2013, Schenkeveld et
al., 2014). Therefore, the absorption of PDMA–Fe, instead of regulation
via AhNRAMP1 and AhFIT , might be an important pathway for
improving the Fe nutrition of peanut (Fig. 4). Additionally, higher Fe
levels in Strategy I plants down-regulate the capacity of ferric
reductase and the expression of AhIRT1 (Martín-Barranco et al.,
2020, Riaz and Guerinot, 2021). Similarly, the Fe uptake process in
peanut root was decreased significantly (Fig. 3). Overall, PDMA improved
peanut Fe nutrition by increasing Fe availability and up-regulating the
absorption of PDMA–Fe. As an effective biostimulant, PDMA has great
potential for the biofortification of plant growth, yields, and kernel
micronutrition.
Chemosynthetic chelate–Fe, as represented by EDTA–Fe, is commonly used
to improve the Fe nutrition of plants in the field (Abadía et al., 2011,
He et al., 2013). However, chemosynthetic chelate–Fe is expensive and
thus generally applied only to cash crops (Briat et al., 2015).
Moreover, the residual chemosynthetic chelator does not degrade easily
in the soil. Biological Fe fertilizers, such as humic acid–Fe and amino
acid–Fe, have therefore been applied widely in the field in recent
years to protect the soil environment (Saini et al., 2016).
Nevertheless, because of the instability of biological Fe fertilizers in
soil, the effect on plant Fe nutrition is limited, especially in
calcareous soil (Briat et al., 2015). The neotype Fe chelator PDMA has
been proven more stable than biological Fe fertilizers. Additionally,
the biodegradability of PDMA can prevent the problem of environmental
pollution caused by chemosynthetic chelate–Fe (Suzuki et al., 2021).
In conclusion, the novel Fe fertilizer PDMA can dissolve insoluble Fe in
the rhizosphere and up-regulate AhYSL1 expression to improve Fe
nutrition in peanut and thus plant growth. Importantly, under field
conditions, PDMA enhanced the yield and kernel micronutrition of peanuts
significantly (Fig. 7). Our results demonstrate the excellent potential
of PDMA for wide use as an Fe fertilizer and biostimulant in
agriculture.