Adaptive Mechanisms Make Lupin a Choice Crop for Acidic Soils Affected by Aluminum Toxicity

Almost half of the world’s agricultural soils are acidic, and most of them present significant levels of aluminum (Al) contamination, with Al3+ as the prevailing phytotoxic species. Lupin is a protein crop that is considered as an optimal alternative to soybean cultivation in cold climates. Lupins establish symbiosis with certain soil bacteria, collectively known as rhizobia, which are capable of fixing atmospheric nitrogen. Moreover, some lupin species, especially white lupin, form cluster roots, bottlebrush-like structures specialized in the mobilization and uptake of nutrients in poor soils. Cluster roots are also induced by Al toxicity. They exude phenolic compounds and organic acids that chelate Al to form non-phytotoxic complexes in the rhizosphere and inside the root cells, where Al complexes are accumulated in the vacuole. Lupins flourish in highly acidic soils where most crops, including other legumes, are unable to grow. Some lupin response mechanisms to Al toxicity are common to other plants, but lupin presents specific tolerance mechanisms, partly as a result of the formation of cluster roots. Al-induced lupin organic acid secretion differs from P-induced secretion, and organic acid transporters functions differ from those in other legumes. Additionally, symbiotic rhizobia can contribute to Al detoxification. After revising the existing knowledge on lupin distinct Al tolerance mechanisms, we conclude that further research is required to elucidate the specific organic acid secretion and Al accumulation mechanisms in this unique legume, but definitely, white lupin arises as a choice crop for cultivation in Al-rich acidic soils in temperate climate regions.

Extraction of Damba Black Soybean Gene by Means of Magnetic Nanoparticles and Investigation of Aluminum Tolerance

The aluminum tolerance of Tamba black soybean (Glycine max cv. Tamba) is closely related to organic acid secretion mechanisms. The gene responsible for this tolerance in this variety (GmFER84) is extracted from lysates of soybean root tips using silylated Fe3O4 nanomagnetic beads. GmFER84 (Glycine max XP 003540203.1) is a stable protein. Tobacco genetically transformed with GmFER84 using an Agrobacterium-mediated transformation was tested for aluminum tolerance. Citrate synthase and citric acid secretion in the roots of transgenic tobacco prove to be significantly higher than those of wild tobacco, and the antioxidant properties of transgenic tobacco are also substantially increased. Research on GmFER84 may enable further agronomic development.

Metabolic acclimation supports higher aluminium-induced secretion of citrate and malate in an aluminium-tolerant hybrid clone of Eucalyptus

Background Eucalyptus is the main timber species, most of which are hybrid clones, and usually grow in aluminized acid soil in China. The exudation of organic acids from roots may contribute to detoxification of Al and lead to the Al-tolerance in Eucalyptus genotypes. To further understand the organic acid response in Al tolerance in Eucalyptus, the Al-tolerant Eucalyptus grandis × E. urophylla clone GL-9 (marked as “G9”) and the Al-sensitive Eucalyptus urophylla clone GL-4 (marked as “W4”) were used to investigate the secretion and metabolism of citrate and malate in roots. Results Eucalyptus seedlings in hydroponics were exposed to the presence or absence of 4.4 mM Al at pH 4.0 for 24 hours. The protein synthesis inhibitor cycloheximide (CHM) and the anion channel blocker phenylglyoxal (PG) were applied to explore possible pathways involved in organic acid secretion. The Al treatments caused higher Al accumulation in roots of both clones. The secretion of malate and citrate was greater in G9 than in W4, corresponding to the relatively higher tolerance in G9 to Al. The peak Al concentration occurred after 1 h in G9 roots and declined afterward, indicating the activation of detoxification to alleviate Al accumulation. After 6 h of Al exposure, the efflux of citrate dramatically increased in G9 after a substantial lag phase, while both peak Al accumulation in roots and peak malate secretion occurred and there was no induction of citrate secretion in W4. Enhanced activity for citrate synthase and phosphoenolpyruvate carboxylase, and reduced activity for NADP-isocitrate dehydrogenase, aconitase and NADP-malic enzyme were closely associated with the greater secretion of citrate in G9. Both PG and CHM were effective inhibitors of citrate and malate secretion in both Eucalyptus clones, except the malate secretion in W4 was not affected by CHM. Conclusions In two different Al-tolerant Eucalyptus clones, both secretion and internal accumulation of citrate and malate in roots were involved in Al detoxification. An anion channel on the plasma membrane could be an important mode of organic acid secretion. Citrate and relevant metabolizing enzymes led more important role in the response to Al in E. grandis × E. urophylla.

Organic acids production by rhizosphere microorganisms isolated from a Typic Melanudands and its effects on the inorganic phosphates solubilization

It has been established that organic acid secretion by rhizosphere microorganisms is one of the mechanisms to solubilize the phosphorus (P) attached to insoluble mineral compounds in soil. This action is an important biotechnological alternative, especially in those soils where high fixation of this nutrient occurs, a very common situation in the tropics. This research evaluated the ability performed by five bacterial and five fungal isolates from Typic Melanudands soil to produce organic acids and generate available phosphorus from insoluble P sources. Given these concerns, the selected microorganisms were replicated for 7 days in liquid medium Pikovskaya (PVK) modified sources tricalcium phosphate (P-Ca), aluminum phosphate (P-Al) and iron phosphate (P-Fe). The results indicated that phosphorus availability in the media, correlates positively with the organic acids production in each of the sources used (P-Ca (0.63), P-Al (0.67) and P-Fe (0.63). In turn, the chemical processes linked to the phosphates solubilization (e.g., Ca availability) affected the development of the microorganisms tested. Both, fungi and bacteria varied in their ability production and type of metabolized organic acids, the most frequent were as follows: citric and gluconic acid.