The LRR receptor-like kinase ALR1 is a plant aluminum ion sensor.

Plant survival requires an ability to adapt to differing concentrations of nutrient and toxic soil ions, yet ion sensors and associated signaling pathways are mostly unknown. Aluminum (Al) ions are highly phytotoxic, and cause severe crop yield loss and forest decline on acidic soils which represent ∼30% of land areas worldwide. Here we found an Arabidopsis mutant hypersensitive to Al. The gene encoding a leucine-rich-repeat receptor-like kinase, was named Al Resistance1 (ALR1). Al ions binding to ALR1 cytoplasmic domain recruits BAK1 co-receptor kinase and promotes ALR1-dependent phosphorylation of the NADPH oxidase RbohD, thereby enhancing reactive oxygen species (ROS) generation. ROS in turn oxidatively modify the RAE1 F-box protein to inhibit RAE1-dependent proteolysis of the central regulator STOP1, thus activating organic acid anion secretion to detoxify Al. These findings establish ALR1 as an Al ion receptor that confers resistance through an integrated Al-triggered signaling pathway, providing novel insights into ion-sensing mechanisms in living organisms, and enabling future molecular breeding of acid-soil-tolerant crops and trees, with huge potential for enhancing both global food security and forest restoration.

[CH4 Fluxes and Their Comprehensive Greenhouse Effects with CO2 Fluxes in Direct-seeded Rice in Poyang Lake Plain].

Paddy fields are complex ecosystems that both emit CH4 and absorb CO2, which plays an important role in the global water-carbon cycle and carbon budget. In this study, the CH4 fluxes and CO2 fluxes of double-cropping direct-seeded rice fields in 2020 in the Poyang Lake Plain were obtained using the eddy covariance method, and the variation characteristics, accumulation in the whole growth period, and comprehensive greenhouse effects of two greenhouse gases were quantitatively revealed. The results showed that, the double-cropping direct-seeded rice field in Poyang Lake Plain was the source of CH4 emission, and the emission during the whole growth period was 52.6 g·m-2, with an average daily emission of 0.208 g·(m2·d)-1. CH4 emission and daily average emission in the early rice season were 20.7 g·m-2 and 0.188 g·(m2·d)-1, respectively, which were lower than the emissions of 31.9 g·m-2 and 0.255 g·(m2·d)-1 in the late rice season. CH4 flux had significant seasonal variation characteristics. The strong emission period (emission peak) of CH4 was concentrated in the middle growth stage of early rice and the early growth stage of late rice. A total of 85.5% of CH4 in the early rice season and 92.1% of CH4 in the late rice season were released during the strong emission periods, and seasonal peak values were 0.638 g·(m2·d)-1 and 1.282 g·(m2·d)-1, respectively. The diurnal variation characteristics of CH4 flux showed three types:obvious unimodal type, non-obvious unimodal type, and irregular type. The strong emission period was mainly the unimodal type, and the peak values of 0.453 µmol·(m2·s)-1 in the early rice season and 0.977 µmol·(m2·s)-1 in the late rice season appeared at 14:00-15:00 and maintained a high emission rate at 12:30-16:00. The CO2 accumulation in the whole growth period of early rice and late rice was -990.4 g·m-2 and -1156.6 g·m-2, respectively, and the total was -2147.0 g·m-2. The comprehensive greenhouse effect of CH4 emission and CO2 exchange in the double-cropping paddy field was -673.6 g·m-2 (calculated using the CO2 equivalent), which showed a cooling effect. Excluding CH4 emissions when evaluating the greenhouse effect of the paddy field, the CO2 equivalent emission of 1473.4 g·m-2 would be underestimated, accounting for 68.6% of the net CO2 absorption. Considering CH4 emissions, CO2 exchanges, and carbon emissions caused by rice harvest, the two-season direct seeding paddy field in Poyang Lake Plain was the source of greenhouse gas emissions.

[Status of Heavy Metal in Organic Fertilizers in Main Tea Growing Regions of China].

To provide guidance for the safe use of organic fertilizers and improve soil quality and tea safety, it is necessary to conduct systematic analyses of the heavy metal content of organic fertilizers applied in the main tea producing areas of China. In this study, we analyzed the heavy metal contents in organic fertilizer samples collected from 2017 to 2019. The risks of collected organic fertilizers from different areas and sources were calculated. The results showed that the average concentrations of ω(As), ω(Hg), ω(Pb), ω(Cd), ω(Cr), ω(Cu), ω(Zn), and ω(Ni) in the collected organic fertilizers were 4.60, 0.22, 27.1, 0.78, 27.9, 58.3, 250.1, and 16.3 mg·kg-1, respectively. According to the assessment standard in NY/T 525- 2021, the over-limit rates of As, Hg, Pb, Cd, and Cr were 6.19%, 1.33%, 4.42%, 4.42%, and 1.33%, respectively. With respect to the area, the qualified rates were 100% in Shaanxi, Jiangsu, Anhui, Fujian, and Guangxi; 80%-90% in Shandong, Zhejiang, Hubei, Sichuan, Yunnan, and Guangdong; and only 54.5% in Jiangxi. The qualified rates of sources were 100% in rapeseed cake, soybean cake, and pig manure; 95.8% in sheep manure; 91.7% in cow manure; 90.7% in chicken manure; 87.2% in manure of other animals; 82.4% in the mixture of plant and animal sources; 65.2% in other plant sources; and 63.6% in other sources. According to the recommended application rate, the accumulation rate of heavy metals in soil with pig manure, cow manure, chicken manure, and sheep manure would be much higher than that with rapeseed cake and soybean cake. The average accumulation rate of organic fertilizer from animal sources was 7-30 times higher than that from plant sources. Therefore, it is recommended to use rapeseed cake or soybean cake fertilizer in tea plantation and to increase the supervision of heavy metal accumulation in soil and tea in those high-risk areas.

Tease out the future: How tea research might enable crop breeding for acid soil tolerance.

Unlike most crops, in which soil acidity severely limits productivity, tea (Camellia sinensis) actually prefers acid soils (pH 4.0-5.5). Specifically, tea is very tolerant of acidity-promoted aluminum (Al) toxicity, a major factor that limits the yield of most other crops, and it even requires Al for optimum growth. Understanding tea Al tolerance and Al-stimulatory mechanisms could therefore be fundamental for the future development of crops adapted to acid soils. Here, we summarize the Al-tolerance mechanisms of tea plants, propose possible mechanistic explanations for the stimulation of tea growth by Al based on recent research, and put forward ideas for future crop breeding for acid soils.

Distinct catalytic capacities of two aluminium-repressed Arabidopsis thaliana xyloglucan endotransglucosylase/hydrolases, XTH15 and XTH31, heterologously produced in Pichia.

Xyloglucan plays an important structural role in primary cell walls, possibly tethering adjacent microfibrils and restraining cell expansion. There is therefore considerable interest in understanding the role of xyloglucan endotransglucosylase/hydrolases (XTHs), which are encoded in Arabidopsis by a 33-member gene family. We compared the key catalytic properties of two very different Arabidopsis XTHs (heterologously produced in Pichia), both of which are aluminium-repressed. Reductively tritiated oligosaccharides of xyloglucan were used as model acceptor substrates. Untransformed Pichia produced no xyloglucan-acting enzymes; therefore purification of the XTHs was unnecessary. XTH15, a classical group-I/II XTH, had high XET and undetectable XEH activity in vitro; its XET Km values were 31 µM XXXGol (acceptor substrate) and 2.9 mg/ml xyloglucan (donor substrate). In contrast, XTH31, a group-III-A XTH, showed predominant XEH activity and only slight XET activity in vitro; its XET Km was 86µM XXXGol (acceptor), indicating a low affinity of this predominantly hydrolytic protein for a transglycosylation acceptor substrate. The Km of XTH31's XEH activity was 1.6 mg/ml xyloglucan. For both proteins, the preferred XET acceptor substrate, among five cellotetraitol-based oligosaccharides tested, was XXXGol. XTH31's XET activity was strongly compromised when the second Xyl residue was galactosylated. XTH15's XET activity, in contrast, tolerated substitution at the second Xyl residue. The two enzymes also showed different pH preferences, XTH31 exhibiting an unusually low pH optimum and XTH15 an unusually broad optimum. XTH31's hydrolase activity increased almost linearly with decreasing pH in the apoplastic range, 6.2-4.5, consistent with a possible role in 'acid growth'. In conclusion, these two Al(3+)-repressed XTHs differ, in several important enzymic features, from other members of the Arabidopsis XTH family.

Glucose alleviates cadmium toxicity by increasing cadmium fixation in root cell wall and sequestration into vacuole in Arabidopsis.

Glucose (Glu) is involved in not only plant physiological and developmental events but also plant responses to abiotic stresses. Here, we found that the exogenous Glu improved root and shoot growth, reduced shoot cadmium (Cd) concentration, and rescued Cd-induced chlorosis in Arabidopsis thaliana (Columbia ecotype, Col-0) under Cd stressed conditions. Glucose increased Cd retained in the roots, thus reducing its translocation from root to shoot significantly. The most Cd retained in the roots was found in the hemicellulose 1. Glucose combined with Cd (Glu + Cd) treatment did not affect the content of pectin and its binding capacity of Cd while it increased the content of hemicelluloses 1 and the amount of Cd retained in it significantly. Furthermore, Leadmium Green staining indicated that more Cd was compartmented into vacuoles in Glu + Cd treatment compared with Cd treatment alone, which was in accordance with the significant upregulation of the expression of tonoplast-localized metal transporter genes, suggesting that compartmentation of Cd into vacuoles also contributes to the Glu-alleviated Cd toxicity. Taken together, we demonstrated that Glu-alleviated Cd toxicity is mediated through increasing Cd fixation in the root cell wall and sequestration into the vacuoles.

TRICHOME BIREFRINGENCE-LIKE27 affects aluminum sensitivity by modulating the O-acetylation of xyloglucan and aluminum-binding capacity in Arabidopsis.

Xyloglucan (XyG) has been reported to contribute to the aluminum (Al)-binding capacity of the cell wall in Arabidopsis (Arabidopsis thaliana). However, the influence of O-acetylation of XyG, accomplished by the putative O-acetyltransferase TRICHOME BIREFRINGENCE-LIKE27 (TBL27 [AXY4]), on its Al-binding capacity is not known. In this study, we found that the two corresponding TBL27 mutants, axy4-1 and axy4-3, were more Al sensitive than wild-type Columbia-0 plants. TBL27 was expressed in roots as well as in leaves, stems, flowers, and siliques. Upon Al treatment, even within 30 min, TBL27 transcript accumulation was strongly down-regulated. The mutants axy4-1 and axy4-3 accumulated significantly more Al in the root and wall, which could not be correlated with pectin content or pectin methylesterase activity, as no difference in the mutants was observed compared with the wild type when exposed to Al stress. The increased Al accumulation in the wall of the mutants was found to be in the hemicellulose fraction. While the total sugar content of the hemicellulose fraction did not change, the O-acetylation level of XyG was reduced by Al treatment. Taken together, we conclude that modulation of the O-acetylation level of XyG influences the Al sensitivity in Arabidopsis by affecting the Al-binding capacity in the hemicellulose.

Xyloglucan Endotransglucosylase-Hydrolase17 Interacts with Xyloglucan Endotransglucosylase-Hydrolase31 to Confer Xyloglucan Endotransglucosylase Action and Affect Aluminum Sensitivity in Arabidopsis.

Previously, we reported that although the Arabidopsis (Arabidopsis thaliana) Xyloglucan Endotransglucosylase-Hydrolase31 (XTH31) has predominately xyloglucan endohydrolase activity in vitro, loss of XTH31 results in remarkably reduced in vivo xyloglucan endotransglucosylase (XET) action and enhanced Al resistance. Here, we report that XTH17, predicted to have XET activity, binds XTH31 in yeast (Saccharomyces cerevisiae) two-hybrid and coimmunoprecipitations assays and that this interaction may be required for XTH17 XET activity in planta. XTH17 and XTH31 may be colocalized in plant cells because tagged XTH17 fusion proteins, like XTH31 fusion proteins, appear to target to the plasma membrane. XTH17 expression, like that of XTH31, was substantially reduced in the presence of aluminum (Al), even at concentrations as low as 10 µm for 24 h or 25 µm for just 30 min. Agrobacterium tumefaciens-mediated transfer DNA insertion mutant of XTH17, xth17, showed low XET action and had moderately shorter roots than the wild type but was more Al resistant than the wild type. Similar to xth31, xth17 had low hemicellulose content and retained less Al in the cell wall. These data suggest a model whereby XTH17 and XTH31 may exist as a dimer at the plasma membrane to confer in vivo XET action, which modulates cell wall Al-binding capacity and thereby affects Al sensitivity in Arabidopsis.

Exogenous auxin alleviates cadmium toxicity in Arabidopsis thaliana by stimulating synthesis of hemicellulose 1 and increasing the cadmium fixation capacity of root cell walls.

Auxin is involved in not only plant physiological and developmental processes but also plant responses to abiotic stresses. In this study, cadmium (Cd(2+)) stress decreased the endogenous auxin level, whereas exogenous auxin (α-naphthaleneacetic acid, NAA, a permeable auxin analog) reduced shoot Cd(2+) concentration and rescued Cd(2+)-induced chlorosis in Arabidopsis thaliana. Under Cd(2+) stress conditions, NAA increased Cd(2+) retention in the roots and most Cd(2+) in the roots was fixed in hemicellulose 1 of the cell wall. NAA treatment did not affect pectin content and its binding capacity for Cd(2+), whereas it significantly increased the content of hemicellulose 1 and the amount of Cd(2+) retained in it. There were highly significant correlations between Cd(2+) concentrations in the root, cell wall and hemicellulose 1 when the plants were subjected to Cd(2+) or NAA+Cd(2+) treatment for 1 to 7d, suggesting that the increase in hemicellulose 1 contributes greatly to the fixation of Cd(2+) in the cell wall. Taken together, these results demonstrate that auxin-induced alleviation of Cd(2+) toxicity in Arabidopsis is mediated through increasing hemicellulose 1 content and Cd(2+) fixation in the root, thus reducing the translocation of Cd(2+) from roots to shoots.

Coordination between apoplastic and symplastic detoxification confers plant aluminum resistance.

Whether aluminum toxicity is an apoplastic or symplastic phenomenon is still a matter of debate. Here, we found that three auxin overproducing mutants, yucca, the recessive mutant superroot2, and superroot1 had increased aluminum sensitivity, while a transfer DNA insertion mutant, xyloglucan endotransglucosylase/hydrolases15 (xth15), showed enhanced aluminum resistance, accompanied by low endogenous indole-3-acetic acid levels, implying that auxin may be involved in plant responses to aluminum stress. We used yucca and xth15 mutants for further study. The two mutants accumulated similar total aluminum in roots and had significantly reduced cell wall aluminum and increased symplastic aluminum content relative to the wild-type ecotype Columbia, indicating that altered aluminum levels in the symplast or cell wall cannot fully explain the differential aluminum resistance of these two mutants. The expression of Al sensitive1 (ALS1), a gene that functions in aluminum redistribution between the cytoplasm and vacuole and contributes to symplastic aluminum detoxification, was less abundant in yucca and more abundant in xth15 than the wild type, consistent with possible ALS1 function conferring altered aluminum sensitivity in the two mutants. Consistent with the idea that xth15 can tolerate more symplastic aluminum because of possible ALS1 targeting to the vacuole, morin staining of yucca root tip sections showed more aluminum accumulation in the cytosol than in the wild type, and xth15 showed reduced morin staining of cytosolic aluminum, even though yucca and xth15 had similar overall symplastic aluminum content. Exogenous application of an active auxin analog, naphthylacetic acid, to the wild type mimicked the aluminum sensitivity and distribution phenotypes of yucca, verifying that auxin may regulate aluminum distribution in cells. Together, these data demonstrate that auxin negatively regulates aluminum tolerance through altering ALS1 expression and aluminum distribution within plant cells, and plants must coordinate exclusion and internal detoxification to reduce aluminum toxicity effectively.

The 14-3-3 protein GENERAL REGULATORY FACTOR11 (GRF11) acts downstream of nitric oxide to regulate iron acquisition in Arabidopsis thaliana.

Here we report the function of a general regulatory factor, GENERAL REGULATORY FACTOR11 (GRF11), in terms of the iron (Fe) deficiency response. Physiological and molecular responses of the loss-of-function Arabidopsis thaliana grf11 mutant to Fe supply were investigated. Genes involved in posttranscriptional regulation of FER-LIKE FE DEFICIENCY-INDUCED TRANSCRIPTION FACTOR (FIT) were also analyzed. In addition, the molecular link between the signaling molecule nitric oxide (NO) and Fe deficiency responses was further dissected. Our results suggest that GRF11 is necessary for induction of Fe-deficiency-tolerance mechanisms. The FIT protein can bind to the promoter of GRF11, which contains an E-box motif. GRF11 also positively affects FIT transcription but has no influence on the genes involved in posttranscriptional regulation of FIT. Furthermore, NO positively regulates GRF11 induction upon the onset of Fe deficiency. We propose that, upon the onset of Fe deficiency, induction of FIT expression is dependent on GRF11, which acts downstream of NO to mediate Fe deficiency responses.

XTH31, encoding an in vitro XEH/XET-active enzyme, regulates aluminum sensitivity by modulating in vivo XET action, cell wall xyloglucan content, and aluminum binding capacity in Arabidopsis.

Xyloglucan endohydrolase (XEH) and xyloglucan endotransglucosylase (XET) activities, encoded by xyloglucan endotransglucosylase-hydrolase (XTH) genes, are involved in cell wall extension by cutting or cutting and rejoining xyloglucan chains, respectively. However, the physiological significance of this biochemical activity remains incompletely understood. Here, we find that an XTH31 T-DNA insertion mutant, xth31, is more Al resistant than the wild type. XTH31 is bound to the plasma membrane and the encoding gene is expressed in the root elongation zone and in nascent leaves, suggesting a role in cell expansion. XTH31 transcript accumulation is strongly downregulated by Al treatment. XTH31 expression in yeast yields a protein with an in vitro XEH:XET activity ratio of >5000:1. xth31 accumulates significantly less Al in the root apex and cell wall, shows remarkably lower in vivo XET action and extractable XET activity, has a lower xyloglucan content, and exhibits slower elongation. An exogenous supply of xyloglucan significantly ameliorates Al toxicity by reducing Al accumulation in the roots, owing to the formation of an Al-xyloglucan complex in the medium, as verified by an obvious change in chemical shift of (27)Al-NMR. Taken together, the data indicate that XTH31 affects Al sensitivity by modulating cell wall xyloglucan content and Al binding capacity.

Gibberellic acid alleviates cadmium toxicity by reducing nitric oxide accumulation and expression of IRT1 in Arabidopsis thaliana.

Gibberellic acid (GA) is involved in not only plant growth and development but also plant responses to abiotic stresses. Here it was found that treating the plants with GA concentrations from 0.1 to 5 µM for 24 h had no obvious effect on root elongation in the absence of cadmium (Cd), whereas in the presence of Cd2+, GA at 5 µM improved root growth, reduced Cd content and lipid peroxidation in the roots, indicating that GA can partially alleviate Cd toxicity. Cd2+ increased nitric oxide (NO) accumulation in the roots, but GA remarkably reduced it, and suppressed the up-regulation of the expression of IRT1. In contrary, the beneficial effect of GA on alleviating Cd toxicity was not observed in an IRT1 knock-out mutant irt1, suggesting the involvement of IRT1 in Cd2+ absorption. Furthermore, the GA-induced reduction of NO and Cd content can also be partially reversed by the application of a NO donor (S-nitrosoglutathione [GSNO]). Taken all these together, the results showed that GA-alleviated Cd toxicity is mediated through the reduction of the Cd-dependent NO accumulation and expression of Cd2+ uptake related gene-IRT1 in Arabidopsis.

Cell wall hemicellulose contributes significantly to aluminum adsorption and root growth in Arabidopsis.

The cell wall (CW) has been recognized as the major target of aluminum (Al) toxicity. However, the components responsible for Al accumulation and the mechanisms of Al-induced CW function disruption are still elusive. The contribution of different CW components (pectin, hemicellulose 1 [HC1], and HC2) to adsorb Al and the effect of Al on xyloglucan endotransglucosylase/hydrolyase activity were investigated in Arabidopsis (Arabidopsis thaliana) in this study. A fractionation procedure was optimized to effectively extract different CW components, especially to prevent the HC fraction from pectin contamination. When CW materials extracted from Al-treated roots (50 µm Al for 24 h) were fractionated, about 75% of CW Al accumulated in the HC1 fraction. A time-dependent kinetic study showed that only when the HC1 fraction was removed was the amount of Al adsorbed decreased sharply. In vivo localization of xyloglucan endotransglucosylase (XET) activity showed that Al greatly inhibited this enzyme activity within 30 min of exposure, which was concomitant with Al-induced callose deposition in roots. Results from real-time reverse transcription-polymerase chain reaction indicated that three genes may constitute the major contributors to XET activity and that the inhibition of XET activity by Al is caused by transcriptional regulation. These results, to our knowledge for the first time, demonstrate that HC is the major pool for Al accumulation. Furthermore, Al-induced reduction in XET activity could play an important role in Al-induced root growth inhibition.

Accumulation and distribution of arsenic and cadmium by tea plants.

It is important to research the rules about accumulation and distribution of arsenic and cadmium by tea plants, which will give us some scientific ideas about how to control the contents of arsenic and cadmium in tea. In this study, by field investigation and pot trial, we found that mobility of arsenic and cadmium in tea plants was low. Most arsenic and cadmium absorbed were fixed in feeding roots and only small amount was transported to the above-ground parts. Distribution of arsenic and cadmium, based on their concentrations of unit dry matter, in tea plants grown on un-contaminated soil was in the order: feeding roots>stems approximately main roots>old leaves>young leaves. When tea plants were grown on polluted soils simulated by adding salts of these two metals, feeding roots possibly acted as a buffer and defense, and arsenic and cadmium were transported less to the above-ground parts. The concentration of cadmium in soil significantly and negatively correlated with chlorophyll content, photosynthetic rate, transpiration rate and biomass production of tea plants.

Effect of liming and seasonal variation on lead concentration of tea plant (Camellia sinensis (L.) O. Kuntze).

Tea is a widely consumed beverage. However, recent studies revealed that there were an increasing number of cases of tea products exceeding the former maximum permissible concentration (MPC) in China for Pb (2 mg kg(-1)). Tea Pb contamination is an issue affecting trade and consumer confidence. Root uptake of Pb could contribute significantly to Pb accumulation in tea leaves due to the strong acidity of many tea garden soils. We conducted pot and field experiments to evaluate the effect of liming on Pb uptake by tea plants on two highly acidic soils (pH3.6). Additions of CaCO(3) significantly increased soil pH by up to 1 unit and decreased soil extractable Pb by up to 32%. Liming resulted in a decrease in the proportion of Pb in the exchangeable and carbonate-bound fractions, with a concurrent increase in the fractions bound to Fe/Mn oxides and residues. Liming significantly decreased Pb concentrations of fine roots, stems and new shoots of tea plants in the pot experiment. In the field experiments, the effect of liming was not significant during the first year following CaCO(3) application, but became significant during the second and third years and Pb concentration in the new shoots was decreased by approximately 20-50%, indicating that liming of acidic tea garden soils is an effective way to reduce Pb contamination of tea. The study also reveals a distinct seasonal variation, with Pb concentration in the new shoots following the order of spring>autumn>summer.

Scale and causes of lead contamination in Chinese tea.

We investigated the scale and causes of Pb contamination in Chinese tea. Lead concentrations in 1,225 tea samples collected nationally between 1999 and 2001 varied from <0.2 to 97.9 mg kg(-1) dry weight (DW), with 32% of the samples exceeding the national maximum permissible concentration (MPC) of 2.0 mg kg(-1) DW and a significant difference between tea types. There was an increasing trend in tea Pb concentration from 1989 to 2000. Proximity to highway and surface dust contamination were found to cause elevated Pb concentrations in tea leaves. Furthermore, Pb concentration in tea leaves correlated significantly and positively with soil extractable Pb, and negatively with soil pH, suggesting that root uptake of Pb from soils also contributed to Pb accumulation in tea. Potential contributions to human Pb intake from drinking tea were small at the median or national MPC Pb values, but considerable at the highest concentration found in the study.