Mechanisms associated with differential tolerance to Fe deficiency in okra (Abelmoschus esculentus Moench)

• Morpho-physiological traits revealed that BARI-1 was tolerant to Fe deficiency.
• Fe chelate reductase and proton extrusion in roots were greatly increased in BARI-1.
• ZIP1 and IRT1 were greatly induced in roots under Fe deficiency in tolerant line.
• BARI-1 was less affected by oxidative stress but showed enhanced scavenging activity.
• Glutathione, cysteine, and citrate significantly increased in BARI-1 under Fe stress.

The study aimed at characterizing the physiological, biochemical and molecular mechanisms conferring differential tolerance to Fe deficiency in two contrasting Okra genotypes (BARI-1 and Orca Onamica). Fe deficiency caused greater decline in morpho-physiological traits in Orca Onamica compared to BARI-1, suggesting greater Fe-efficiency in BARI-1. Fe reductase, proton extrusion and phenol content in roots increased greatly in BARI-1 suggesting these responses contribute immensely to Fe-efficiency in this genotype. Semi-quantitative RT-PCR revealed the upregulation of ZIP1 (metal transporter) and IRT1 (iron-regulated transporter) genes in Fe-deficient roots of BARI-1 and Orca Onamica, albeit to a lesser extent. Also, BARI-1 was less affected by oxidative stress but showed enhanced scavenging activity that may stabilize antioxidant capacities and protect against oxidative damage under Fe deficiency. In addition, increased SOD and GR activities in BARI-1 suggest better capacity to detoxify ROS and to facilitate generating antioxidant metabolites, respectively. Furthermore, HPLC (high-performance liquid chromatography) data showed increased glutathione, cysteine, citrate and ascorbic acid in roots of BARI-1 and to a lesser extent in Orca Onamica under Fe deficiency. Elevated glutathione, ascorbic acid and cysteine suggest better protection of Okra from Fe-deficiency induced oxidative stress. In addition, a significant increase in citrate and Fe-citrate was observed only in xylem sap of BARI-1 under Fe deficiency, indicating a role for citrate in long-distance transport of Fe. These findings provide first evidence on the mechanisms of differential Fe-efficiency in Okra. This will allow the improvement of Fe biofortification in Okra and other plants.