Subcellular distribution of aluminum associated with differential cell ultra-structure, mineral uptake, and antioxidant enzymes in root of two different Al+3-resistance watermelon cultivars

Silicon attenuates aluminum toxicity in sugarcane plants by modifying growth, roots morphoanatomy, photosynthetic pigments, and gas exchange parameters

Aluminum (Al) inhibits growth and limits plant productivity in acidic soils. An important strategy to increase Al tolerance is the use of silicon (Si) nutrition. Thus, the aim of this study was to evaluate the interactive role of Si in increasing the growth, physiological and morphoanatomy responses of sugarcane plants under Al toxicity. A 4 × 2 factorial scheme in a completely randomized design was used to study the impact of Si (2 mM) on attenuating Al toxicity (0, 10, 15 and 20 mg L-1, as Al2(SO4)3·18H2O) in sugarcane seedlings. After 45 days, Al toxicity affected sugarcane growth by increasing Al uptake and accumulation, modifying root growth, thickness, and morphoanatomy, and decreasing pigment content, gas exchange parameters, and the number of adaxial and abaxial stomata. However, Si attenuated Al toxicity in the sugarcane seedlings by limiting Al uptake and transport to the shoots, causing positive changes in root morphoanatomy, higher pigment content, improving gas exchange parameters, thereby increased growth. Furthermore, cultivar 'CTC9003' showed beneficial impacts from Si supplementation than 'CTC9002', especially under Al toxicity. The findings of this study suggest that Si plays a notable role in improving anatomical and physiological aspects, particularly the growth of sugarcane seedlings under Al toxicity.

Mutant IAA21 genes from Dendrocalamus sinicus Chia et J. L. Sun inhibit stem and root growth in transgenic tobacco by interacting with ARF5

Woody bamboos are important resource of industrial fibres. Auxin signaling plays a key role in multiple plant developmental processes, as yet the role of auxin/indole acetic acid (Aux/IAA) in culm development of woody bamboos has not been previously characterized. Dendrocalamus sinicus Chia et J. L. Sun is the largest woody bamboo documented in the world. Here, we identified two alleles of DsIAA21 gene (sIAA21 and bIAA21) from the straight- and bent-culm variants of D. sinicus, respectively, and studied how the domains I, i, and II of DsIAA21 affect the gene transcriptional repression. The results showed that bIAA21 expression was rapidly induced by exogenous auxin in D. sinicus. In transgenic tobacco, sIAA21 and bIAA21 mutated in domains i, and II significantly regulated plant architecture and root development. Stem cross sections revealed that parenchyma cells were smaller in transgenic plants than that in wild type plants. Domain i mutation changed the leucine and proline at position 45 to proline and leucine (siaa21L45P and biaa21P45L) strongly repressed cell expansion and root elongation by reducing the gravitropic response. Substitution of isoleucine with valine in domain II of the full length DsIAA21 resulted in dwarf stature in transgenic tobacco plants. Furthermore, the DsIAA21 interacted with auxin response factor 5 (ARF5) in transgenic tobacco plants, suggesting that DsIAA21 might inhibit stem and root elongation via interacting with ARF5. Taken together, our data indicated that DsIAA21 was a negative regulator of plant development and suggested that amino acid differences in domain i of sIAA21 versus bIAA21 affected their response to auxin, and might play a key role in the formation of the bent culm variant in D. sinicus. Our results not only shed a light on the morphogenetic mechanism in D. sinicus, but also provided new insights into versatile function of Aux/IAAs in plants.

Aluminum in plant: Benefits, toxicity and tolerance mechanisms

Aluminum (Al) is the third most ubiquitous metal in the earth's crust. A decrease in soil pH below 5 increases its solubility and availability. However, its impact on plants depends largely on concentration, exposure time, plant species, developmental age, and growing conditions. Although Al can be beneficial to plants by stimulating growth and mitigating biotic and abiotic stresses, it remains unknown how Al mediates these effects since its biological significance in cellular systems is still unidentified. Al is considered a major limiting factor restricting plant growth and productivity in acidic soils. It instigates a series of phytotoxic symptoms in several Al-sensitive crops with inhibition of root growth and restriction of water and nutrient uptake as the obvious symptoms. This review explores advances in Al benefits, toxicity and tolerance mechanisms employed by plants on acidic soils. These insights will provide directions and future prospects for potential crop improvement.

Functional groups on wheat (Triticum aestivum) root surface affect aluminium transverse accumulation

Plant root growth is inhibited markedly by aluminium (Al) even at micromolar concentration and Al is mainly accumulated in plant roots outer layer cell walls. But the underlying reason for this asymmetric transverse distribution is unknown. In this study, two wheat (Triticum aestivum L.) genotypes ET8 and ES8 differing in Al resistance were investigated by hydroculture. The Al-tolerant ET8 expressed a higher root elongation rate (RER) than Al-sensitive ES8 under Al stress. Morphological examination showed symptoms such as root surface ruptures were observed in ET8 and ES8, with ES8 being more obvious. The cation exchange capacity (CEC) values of root tips of ES8 under different Al concentrations are higher than those of ET8. The sensitive genotype ES8 accumulated more Al than ET8 in plant apical root tips as well as cell walls. Under 48 h Al exposure, the root cell wall pectin concentration was increased with a higher magnitude in ES8 than in ET8. The functional groups on ET8 and ES8 roots outer layer and inner cells were investigated by Fourier transform infrared spectrometry (FTIR) under Al stress. The FTIR spectra of selected examined areas showed that the characteristic absorption peaks were located at 1692, 2920, and 3380 cm^-1. The outer layer cells had stronger peaks than inner cells at wavenumber 1680-1740 cm^-1, indicating root outer layer cells contain more carboxyls in both ET8 and ES8. The results demonstrate that Al transverse distribution on plants apical root cross section is likely influenced by functional groups such as negatively charged carboxylic acid.

Boron contributes to excessive aluminum tolerance in trifoliate orange (Poncirus trifoliata (L.) Raf.) by inhibiting cell wall deposition and promoting vacuole compartmentation

Boron (B) is an indispensable micronutrient for plant growth that can also alleviate aluminum (Al) toxicity. However, limited data are available on the underlying mechanisms behind this phenomenon. Here, we found that a certain range of B application could alleviate the inhibitory effects of Al toxicity on citrus. Transcriptome analysis revealed that several Al stress-responsive genes and pathways were differentially affected and enriched, such as coding for the secretion of organic acid and the distribution of Al in subcellular components after B addition. Specifically, B application enhanced rhizosphere pH and induced malate exudation by expressing PtALMT4 and PtALMT9 genes occurred in Al-treated root, which ultimately reduced the absorption of Al and coincided with down-regulated the expression of PtNrat1. Moreover, B supply suppressed the pectin methyl-esterase (PME) activity and displayed a lower level of PtPME2 expression, while enhanced the PtSTAR1 expression, which is responsible for reducing cell wall (CW) Al deposition. Boron addition enhanced the PtALS1 and PtALS3 expression, accompanied by a higher proportion of vacuolar Al compartmentation during Al exposure. Collectively, the protective effects of B on root injury induced by Al is mainly by subsiding the Al uptake in the root apoplast and compartmentalizing Al into vacuole.

Transcriptome Profiling Reveals the Effects of Nitric Oxide on the Growth and Physiological Characteristics of Watermelon under Aluminum Stress

Excessive aluminum ions (Al³⁺) in acidic soil can have a toxic effect on watermelons, restricting plant growth and reducing yield and quality. In this study, we found that exogenous application of nitric oxide (NO) could increase the photochemical efficiency of watermelon leaves under aluminum stress by promoting closure of leaf stomata, reducing malondialdehyde and superoxide anion in leaves, and increasing POD and CAT activity. These findings showed that the exogenous application of NO improved the ability of watermelon to withstand aluminum stress. To further reveal the mitigation mechanism of NO on watermelons under aluminum stress, the differences following different types of treatments—normal growth, Al, and Al + NO—were shown using de novo sequencing of transcriptomes. In total, 511 differentially expressed genes (DEGs) were identified between the Al + NO and Al treatment groups. Significantly enriched biological processes included nitrogen metabolism, phenylpropane metabolism, and photosynthesis. We selected 23 genes related to antioxidant enzymes and phenylpropane metabolism for qRT-PCR validation. The results showed that after exogenous application of NO, the expression of genes encoding POD and CAT increased, consistent with the results of the physiological indicators. The expression patterns of genes involved in phenylpropanoid metabolism were consistent with the transcriptome expression abundance. These results indicate that aluminum stress was involved in the inhibition of the photosynthetic pathway, and NO could activate the antioxidant enzyme defense system and phenylpropane metabolism to protect cells and scavenge reactive oxygen species. This study improves our current understanding by comprehensively analyzing the molecular mechanisms underlying NO-induced aluminum stress alleviation in watermelons.

Permissive action of H2O2 mediated ClUGT75 expression for auxin glycosylation and Al3+- tolerance in watermelon

Although Al³⁺-toxicity is one of the limiting factors for crop production in acidic soils, little is known about the Al³⁺-tolerance mechanism in watermelon, a fairly acid-tolerant crop. This work aimed to identify the interaction between the H₂O₂ scavenging pathway and auxin glycosylation relevant to watermelon Al³⁺-tolerance. By analyzing expressions of hormone-related ClUGTs and antioxidant enzyme genes in Al³⁺-tolerant (ZJ) and Al³⁺-sensitive (NBT) cultivars, we identified ClUGT75s (B1, B2, and D1) and ClSOD1-2-ClCAT as crucial components associated with Al³⁺-tolerance. Al³⁺-stress significantly increased H₂O₂ content by 92.7% in NBT and 42.3% in ZJ, accompanied by less Al³⁺-, auxin (IAA and IBA), and MDA contents in ZJ than NBT. These findings coincided with significant ClSOD1-2 expression and stable dismutation activity in NBT than ZJ. Hence, higher H₂O₂ content in the root apex of NBT than ZJ correlated with a significant increase in auxin content and ClSOD1-2 up-regulation. Moreover, Al³⁺-activated ClUGT75D1 and ClUGT75B2 in ZJ coincided with no considerable change in IBA content, suggesting that glycosylation-mediated changes in IBA content might be relevant to Al³⁺-tolerance in watermelon. Furthermore, exogenous H₂O₂ and IBA indicated ClUGT75D1 modulating IBA is likely dependent on H₂O₂ background. We hypothesize that a higher H₂O₂ level in NBT represses ClUGT75, resulting in increased auxin than those in ZJ roots. Thus, excess in both H₂O₂ and auxin aggravated the inhibition of root elongation under Al³⁺-stress. Our findings provide insights on the permissive action of H₂O₂ in the mediation of auxin glycosylation by ClUGT75 in root apex for Al³⁺-tolerance in watermelon.