Bicarbonate blocks iron translocation from cotyledons inducing iron stress responses in Citrus roots

Mechanisms of plant saline-alkaline tolerance

Saline-alkaline soil affects crop growth and development, thereby suppressing the yields. Human activities and climate changes are putting arable land under the threat of saline-alkalization. To feed a growing global population in limited arable land, it is of great urgence to breed saline-alkaline tolerant crops to cope with food security. Plant salt-tolerance mechanisms have already been explored for decades. However, to date, the molecular mechanisms underlying plants responses to saline-alkaline stress have remained largely elusive. Here, we summarize recent advances in plant response to saline-alkaline stress and propose some points deserving of further exploration.

Direct and Bicarbonate-Induced Iron Deficiency Differently Affect Iron Translocation in Kiwifruit Roots

Bicarbonate-induced iron (Fe) deficiency (+Bic) is frequently observed in kiwifruit orchards, but more research attention has been paid to direct Fe deficiency (-Fe) in plants, including kiwifruit. Here we compared the differences of kiwifruit plants between -Fe and +Bic in: (1) the traits of ⁵⁷Fe uptake and translocation within plants, (2) Fe forms in roots, and (3) some acidic ions and metabolites in roots. The concentration of ⁵⁷Fe derived from nutrient solution (⁵⁷Fedfs) in roots was less reduced in +Bic than -Fe treatment, despite similar decrease in shoots of both treatments. +Bic treatment increased ⁵⁷Fedfs distribution in fine roots but decreased it in new leaves and stem, thereby displaying the inhibition of ⁵⁷Fedfs translocation from roots to shoots and from fine roots to xylem of coarse roots. Moreover, +Bic imposition induced the accumulation of water-soluble Fe and apoplastic Fe in roots. However, the opposite was observed in -Fe-treated plants. Additionally, the cell wall Fe and hemicellulose Fe in roots were less reduced by +Bic than -Fe treatment. +Bic treatment also triggered the reduction in H⁺ extrusion and the accumulation of NH₄⁺, succinic acid, and some amino acids in roots. These results suggest that, contrary to -Fe, +Bic treatment inhibits Fe translocation to shoots by accumulating water-soluble and apoplastic Fe and slowing down the release of hemicellulose Fe in the cell wall in kiwifruit roots, which may be related to the decreased H⁺ extrusion and the imbalance between C and N metabolisms.

GsSLAH3, a Glycine soja slow type anion channel homolog, positively modulates plant bicarbonate stress tolerance

Alkaline stress is a major form of abiotic stress that severely inhibits plant growth and development, thus restricting crop productivity. However, little is known about how plants respond to alkali. In this study, a slow-type anion channel homolog 3 gene, GsSLAH3, was isolated and functionally characterized. Bioinformatics analysis showed that the GsSLAH3 protein contains 10 transmembrane helices. Consistently, GsSLAH3 was found to locate on plasma membrane by transient expression in onion epidermal cells. In wild soybeans, GsSLAH3 expression was induced by NaHCO₃ treatment, suggesting its involvement in plant response to alkaline stress. Ectopic expression of GsSLAH3 in yeast increased sensitivity to alkali treatment. Dramatically, overexpression of GsSLAH3 in Arabidopsis thaliana enhanced alkaline tolerance during the germination, seedling and adult stages. More interestingly, we found that transgenic lines also improved plant tolerance to KHCO₃ rather than high pH treatment. A nitrate content analysis of Arabidopsis shoots showed that GsSLAH3 overexpressing lines accumulated more NO₃^- than wild-type. In summary, our data suggest that GsSLAH3 is a positive alkali responsive gene that increases bicarbonate resistance specifically.

Transport and Use of Bicarbonate in Plants: Current Knowledge and Challenges Ahead

Bicarbonate plays a fundamental role in the cell pH status in all organisms. In autotrophs, HCO₃− may further contribute to carbon concentration mechanisms (CCM). This is especially relevant in the CO₂-poor habitats of cyanobacteria, aquatic microalgae, and macrophytes. Photosynthesis of terrestrial plants can also benefit from CCM as evidenced by the evolution of C₄ and Crassulacean Acid Metabolism (CAM). The presence of HCO₃− in all organisms leads to more questions regarding the mechanisms of uptake and membrane transport in these different biological systems. This review aims to provide an overview of the transport and metabolic processes related to HCO₃− in microalgae, macroalgae, seagrasses, and terrestrial plants. HCO₃− transport in cyanobacteria and human cells is much better documented and is included for comparison. We further comment on the metabolic roles of HCO₃− in plants by focusing on the diversity and functions of carbonic anhydrases and PEP carboxylases as well as on the signaling role of CO₂/HCO₃− in stomatal guard cells. Plant responses to excess soil HCO₃− is briefly addressed. In conclusion, there are still considerable gaps in our knowledge of HCO₃− uptake and transport in plants that hamper the development of breeding strategies for both more efficient CCM and better HCO₃− tolerance in crop plants.

Effects of Fe deficiency on the protein profiles and lignin composition of stem tissues from Medicago truncatula in absence or presence of calcium carbonate

Iron deficiency is a yield-limiting factor with major implications for crop production, especially in soils with high CaCO3. Because stems are essential for the delivery of nutrients to the shoots, the aim of this work was to study the effects of Fe deficiency on the stem proteome of Medicago truncatula. Two-dimensional electrophoresis separation of stem protein extracts resolved 276 consistent spots in the whole experiment. Iron deficiency in absence or presence of CaCO3 caused significant changes in relative abundance in 10 and 31 spots, respectively, and 80% of them were identified by mass spectrometry. Overall results indicate that Fe deficiency by itself has a mild effect on the stem proteome, whereas Fe deficiency in the presence of CaCO3 has a stronger impact and causes changes in a larger number of proteins, including increases in stress and protein metabolism related proteins not observed in the absence of CaCO3. Both treatments resulted in increases in cell wall related proteins, which were more intense in the presence of CaCO3. The increases induced by Fe-deficiency in the lignin per protein ratio and changes in the lignin monomer composition, assessed by pyrolysis-gas chromatography-mass spectrometry and microscopy, respectively, further support the existence of cell wall alterations.

BIOLOGICAL SIGNIFICANCE

In spite of being essential for the delivery of nutrients to the shoots, our knowledge of stem responses to nutrient deficiencies is very limited. The present work applies 2-DE techniques to unravel the response of this understudied tissue to Fe deficiency. Proteomics data, complemented with mineral, lignin and microscopy analyses, indicate that stems respond to Fe deficiency by increasing stress and defense related proteins, probably in response of mineral and osmotic unbalances, and eliciting significant changes in cell wall composition. The changes observed are likely to ultimately affect solute transport and distribution to the leaves.