Facilitating Phloem-Mediated Iron Transport Can Improve the Adaptation of Rice Seedlings to Iron Deficiency Stress

Iron (Fe) is essential for normal plant growth and development. In rice, Fe deficiency leads to stunted growth, leaf chlorosis, reduced photosynthetic capacity, and ultimately, yield loss. Most studies have focused on investigating the mechanisms of Fe deficiency responses in rice roots; however, the effects of shoot Fe redistribution on Fe deficiency response remain poorly understood. Phloem transport plays a vital role in distributing Fe to new tissues. To investigate the effects of enhanced phloem-mediated Fe transport on rice adaptability to iron deficiency, we subjected transgenic lines with higher phloem Fe efflux rates and wild-type (WT) plants to Fe-deficient conditions. The growth, leaf photosynthetic rate, and Fe content of transgenic and WT seedlings under different Fe concentrations were compared. The results showed that the transgenic lines exhibited elevated shoot length, root length, shoot dry weight, leaf chlorophyll content, and net photosynthetic rates under Fe-deficient conditions. Under both Fe-sufficient and Fe-deficient conditions, the transgenic lines had significantly higher Fe content, Fe accumulation, and phloem Fe efflux rates than the WT. RNA sequencing (RNA-seq) analysis revealed that enhanced Fe transport via phloem resulted in improved Fe availability through the sequestration of Fe ions and vacuolar transport pathways in the shoots. It also upregulated the EARLY LESION LEAF 1 (ELL1) expression and modulated the sucrose synthase activity, thereby promoting chlorophyll synthesis and leaf photosynthesis. Additionally, enhanced Fe transport influenced the gibberellin (GA) catabolism and plant hormone signal transduction in the roots, reducing the GA content and modulating the cytokinin (CTK), jasmonic acid (JA), and ethylene (ETH) signaling to induce Fe deficiency response and promote Fe uptake. These findings demonstrate that phloem-mediated Fe transport participated in Fe deficiency response, and enhancing this improved the adaptability of rice seedlings to low Fe conditions. In specific, rice seedlings with a high capacity for phloem-mediated Fe transport exhibited a strong iron uptake, translocation, and remobilization capacity, thereby maintaining normal growth and development and successfully adapting to the low-Fe environment.