抗壞⾎酸與蘋果酸在鐵氧化還原循環中的協同作⽤調控阿拉伯芥缺磷根系反應

Abstract

磷 (P) 缺乏會限制植物⽣⾧，因為⼟壤中的有效磷含量降低。為了應對這⼀挑戰，植物會分泌有機酸，如蘋果酸，以提⾼磷的溶解度並促進養分吸收。 本研究探討了在缺磷條件下， 鐵 (Fe)、 蘋果酸 (malate) 和抗壞⾎酸 (ascorbate) 在阿拉伯芥中的相互作⽤。我們發現，蘋果酸能促進 Fe³⁺ 還原為 Fe²⁺，提⾼鐵的吸收能⼒。有趣的是，磷酸鹽會抑制鐵的還原，但在蘋果酸存在時，這種抑制效應會被削弱，顯⽰蘋果酸可能能夠緩解磷酸鹽對鐵吸收的⼲擾。此外，雖然體外實驗顯⽰光照可以在培養基中⾼效還原 Fe³⁺ ⾄ Fe²⁺，但在⼟壤環境中，光的穿透⼒極低，因此在⾃然條件下幾乎不可能透過光還原鐵。這表明，植物可能會分泌還原性分⼦，以維持根圈中鐵的可溶性。我們的實驗發現，當植物在磷充⾜條件下供應 Fe-Malate，植株會出現嚴重的葉⽚⿈化 (chlorosis)，這表明 Fe-Malate 在磷充⾜的情況下無法作為有效的鐵來源。此外，在缺磷植物中，鐵的還原並不依賴於 FRO2或 F6’H1 這些已知的鐵還原系統，暗⽰植物可能存在另⼀條獨⽴的鐵還原途徑。這種還原活性僅針對 Fe-Malate，⽽不適⽤於 Fe-EDTA，本研究顯⽰，鐵的配位型態在決定其可利⽤性⽅⾯扮演關鍵⾓⾊ 。我們進⼀步發現， 蘋果酸在維持鐵穩態中具有多層次的調控作⽤， 並運作出⼀種於磷匱乏環境下啟動的潛在鐵還原機制 。對這些機制的深⼊理解， 將有助於發展具低磷適應性的作物， 減少磷肥施⽤並促進永續農業。

Phosphate (Pi) deficiency limits plant growth due to reduced Pi availability in the soil. In response, plants secrete organic acids like malate to increase Pi solubility and enhance nutrient uptake. This study explores the interaction between iron (Fe), malate, and ascorbate in Arabidopsis under Pi-deficient conditions. We found that malate facilitates Fe reduction from Fe³⁺ to Fe²⁺, promoting Fe accumulation in roots. Interestingly, Fe reduction is inhibited by phosphate but is restored in the presence of malate, suggesting that malate mitigates phosphate inhibition of Fe reduction. Although in vitro experiments show that light can efficiently reduce Fe³⁺ to Fe²⁺ in agar-based media, light penetration in soil is minimal, making photoreduction unlikely under natural conditions. This suggests that plants may secrete reductive molecules to maintain Fe solubility in the rhizosphere. Our experiments revealed that plants supplied with Fe-malate under Pi-sufficient conditions exhibit severe chlorosis, indicating that Fe-malate cannot be used as an Fe source when phosphate is abundant. Additionally, Fe reduction in Pi-deficient plants is independent of known Fe reduction systems like the plasma membrane ferric reductase and the coumarin-mediated reduction, suggesting the existence of an alternative pathway. This reduction activity is specific to Fe-malate but not Fe-EDTA, indicating that the Fe-ligand form is a critical factor in Fe availability under different environmental conditions. Our results demonstrate that malate plays a multifaceted role in the regulation of Fe homeostasis and uncover a distinct Fe reduction pathway that becomes active under Pi deficiency. Understanding these processes could pave the way for developing crops capable of maintaining growth in low-P environments, reducing the need for phosphate fertilizers and promoting sustainable agricultural practices.