Response to gallium (Ga) exposure and its distribution in rice plants

Exogenous proline mediates OsNPR1 to regulate the innate pool of IAA in response to Cr exposure in rice plants

Indole acetic acid (IAA) orchestrates a myriad of physiological and biochemical responses in plants under stressful conditions, highlighting its indispensable role in plant resilience. The widespread contamination of chromium (Cr) poses a significant threat to rice cultivation, as its accumulation in plants disrupts various metabolic processes, consequently hindering growth. Of course, the utilization of exogenous growth regulators, including proline (Pro), has notably surged as a strategy to mitigate stress in plants. Pro can trigger the activation of other growth-regulating molecules, including IAA, to coordinate stress responses. To explore the complex interaction between exogenous Pro and the endogenous pool of IAA under Cr(VI) toxicity, a hydroponic system was established. The rice plants treated with exogenous Pro in coupled with Cr(VI) [Cr(VI)+Pro] showed significantly greater content of IAA than the plants not treated with exogenous Pro [Cr(VI)-Pro]. The expression analysis of genes involved in the speciation of IAA reactions reveals that the downregulation of OsNPR1 under "Cr(VI)+Pro" treatments might be the crucial player in increasing the IAA content in rice plants. The increase in IAA by Pro treatment under Cr toxicity might lead to an improvement in root activity and root architecture elements. Importantly, no significant difference was observed in the accumulation of Cr in [Cr(VI)-Pro]- and [Cr(VI)+Pro]-treated rice plants. These results reveal that exogenous Pro can improve plant growth by inducing IAA accumulation in plant tissues exposed to Cr(VI) toxicity, without increasing Cr toxicity in plants.

The Role of Metal Tolerance Proteins (MTPs) Associated with the Homeostasis of Divalent Mineral Elements in Ga-Treated Rice Plants

Mineral elements typically act as transported substrates for metal tolerance proteins (MTPs). The chelation of MTPs with heavy metal ions is a suggestive detoxification pathway in plants; therefore, the trade-off between transporting mineral elements and chelating excess toxic metal ions is inevitable. Gallium (Ga) is an emerging pollutant associated with high-tech industries. This study investigated the impact of Ga stress on MTPs, subsequently altering the transport and distribution of mineral elements. Gallium exposure reduced rice seedling biomass, with roots accumulating more Ga than shoots. Ga stress also changed the rice plants' subcellular mineral element distribution. PCR assays showed that Ga stress negatively affected all genes belonging to the Mn group, except OsMTP9. While Mn accumulation in the rice cellular compartments did not respond positively to Ga stress, OsMTP8, OsMTP8.1, OsMTP11, and OsMTP11.1 were found to be intimately connected to Mn transport and repressed by increased Ga accumulation in roots. Mg and Cu accumulated in the cytosol and organelles of Ga-treated rice plants, while OsMTP9 expression increased, demonstrating its importance in transporting Mg and Cu. A positive link between Ga stress and Zn accumulation in the cytosol and organelles was found, and OsMTP7 and OsMTP12 expression was positive, suggesting that Ga stress did not impair their Zn transport. Notably, Ga exposure down-regulated Fe-transporting OsMTP1 and OsMTP6, wherein the subcellular concentrations of Fe showed negative responses to Ga accumulation. These findings provide valuable insights into elucidating the roles of OsMTPs in Ga tolerance and the transport of these mineral elements.

Boron deficiency decreased the root activity of Ga-exposed rice seedlings by reducing iron accumulation and increasing Ga in iron plaque

Gallium (Ga) is an emerging chemical pollutant chiefly associated with high-tech industries. Boron (B) alleviates the negative effects of toxic elements on plant growth. Thereby, the effects of B fertilization on Ga toxicity in rice seedlings was studied to clarify the role of iron plaque in the distribution of Ga, Fe, and B in Ga-treated rice seedlings in the presence or absence of B. Gallium exposure significantly reduced the biomass of rice seedlings. Boron deficiency induced a significant change in the distribution of B in Ga-treated rice seedlings compared with "Ga+B" treatments. Accumulation of Ga in roots, dithionite-citrate-bicarbonate (DCB) extracts, and shoots showed a dose-dependent manner from both +B and -B rice seedlings. Boron nutrition levels affect the distribution of Fe in roots, DCB extracts, and shoots, in which DCB-extractable Fe was significantly decreased from "Ga-B" treatments compared with "Ga+B" treatments. Root activity was significantly decreased in both Ga-exposed rice seedlings; however, B-deficient seedlings showed a severe reduction than +B rice seedlings. These results reveal that Fe plaque might be a temporary sink for B accumulation when plants are grown with proper B, wherein the re-utilization of DCB-extractable B stored in Fe plaque is mandatory for plant growth under B deficiency. Correlation analysis revealed that B deficiency decreased the root activity of Ga-exposed rice seedlings by reducing DCB-extractable Fe and increasing DCB-extractable Ga in Fe plaque. This study enhances our understanding of how B nutritional levels affect Ga toxicity in rice plants.