Steel slag and biochar amendments decreased CO2 emissions by altering soil chemical properties and bacterial community structure over two-year in a subtropical paddy field.

Waste amendments, such as steel slag and biochar, have been reported as a strategy for improving soil fertility, crop productivity, and carbon (C) sequestration in agricultural lands. However, information regarding the subsequent effects of steel slag and biochar on C cycling and the underlying microbial mechanisms in paddy soils remains limited. Hence, this study aimed to examine the effect of these waste amendments (applied in 2015-2017) on total soil CO2 emissions, total and active soil organic C (SOC) contents, and microbial communities in the early and late seasons in a subtropical paddy field. The results showed that despite the exogenous C input from these waste amendments (steel slag, biochar and slag + biochar), they significantly (P < 0.05) decreased total CO2 emissions (e.g., by 41.9-59.6% at the early season), compared to the control soil. These amendments also significantly (P < 0.001) increased soil salinity and pH. The increased soil pH had a negative effect (r = -0.37, P < 0.05) on microbial biomass C (MBC). The biochar and slag + biochar treatments (cf. control) significantly (P < 0.001) increased SOC contents in the both seasons. The amendments altered the soil microbial community structure that associated with soil C cycling: (1) all three amendments increased the relative abundance of Agromyces and Streptomyces, which was associated with higher soil pH (cf. control); and (2) biochar and slag + biochar treatments caused a higher relative abundance of Sphingomonas, which was supported by high SOC contents under those amendments. Overall, this study demonstrated that the steel slag and biochar amendments altered microbial community composition due to changes in key soil properties, such as salinity, pH and SOC contents, with implications for increasing soil C stocks while mitigating CO2 emissions in the paddy field.

Coupled steel slag and biochar amendment correlated with higher methanotrophic abundance and lower CH4 emission in subtropical paddies.

Aerobic methanotrophs in paddies serve as methane (CH4) filters and thereby reduce CH4 emissions. Amending soil with waste products can mitigate CH4 emissions in crops, but little is known about the impacts of amendments with steel slag and biochar on the populations and activities of aerobic methanotrophs in rice cropland. We used real-time quantitative PCR detecting system and high-throughput sequencing to determine the effects of slag and biochar amendments on CH4 emission, abundance, and community structure of methanotrophs, and the relationships between soil properties and the abundance and community composition of methanotrophs during the rice growing season in both early and late paddies. Soil salinity and pH were significantly higher for an amendment with both slag and biochar than the control in both the early and late paddies, and pH was significantly higher for a slag amendment in the late paddy. Cumulative CH4 emission was lower for the slag and slag + biochar amendments than the control in early paddy by-34.1%. Methanotrophic abundance was three- and sixfold higher for the slag + biochar amendment than the control in the early and late paddies (p < 0.05), respectively. The abundance of different groups of methanotrophs varied among the treatments. The relative abundance of Methylosarcina was higher for the slag amendment than the control, and the relative abundance of Methylomonas was lower for biochar, and slag + biochar amendments than the control. The relative abundance of Methylocystis was higher for the slag and slag + biochar amendments than the control in the early paddy, and the relative abundance of Methylocystis was higher for the slag, biochar, and slag + biochar amendments in the late paddy. Univariate and multivariate analyses indicated that the higher abundance of methanotrophic bacteria for the slag and slag + biochar amendments was correlated with soil pH, salinity, soil organic carbon, and C/N ratio, and the relative abundances of Methylocystis, Methylomonas, and Methylosarcina were associated with the effective mitigation of CH4 emission in the paddies. A discriminant general analysis indicated that the total population of methanotrophs was larger for the slag + biochar amendment than the control, and that this effect was only weakly correlated with changes in the soil properties, demonstrating that this effect on the size and species composition of methanotrophic soil populations was mostly associated with a direct effect of the slag + biochar amendment.

The cotton transcription factor TCP14 functions in auxin-mediated epidermal cell differentiation and elongation.

Plant-specific TEOSINTE-BRANCHED1/CYCLOIDEA/PCF (TCP) transcription factors play crucial roles in development, but their functional mechanisms remain largely unknown. Here, we characterized the cellular functions of the class I TCP transcription factor GhTCP14 from upland cotton (Gossypium hirsutum). GhTCP14 is expressed predominantly in fiber cells, especially at the initiation and elongation stages of development, and its expression increased in response to exogenous auxin. Induced heterologous overexpression of GhTCP14 in Arabidopsis (Arabidopsis thaliana) enhanced initiation and elongation of trichomes and root hairs. In addition, root gravitropism was severely affected, similar to mutant of the auxin efflux carrier PIN-FORMED2 (PIN2) gene. Examination of auxin distribution in GhTCP14-expressing Arabidopsis by observation of auxin-responsive reporters revealed substantial alterations in auxin distribution in sepal trichomes and root cortical regions. Consistent with these changes, expression of the auxin uptake carrier AUXIN1 (AUX1) was up-regulated and PIN2 expression was down-regulated in the GhTCP14-expressing plants. The association of GhTCP14 with auxin responses was also evidenced by the enhanced expression of auxin response gene IAA3, a gene in the AUXIN/INDOLE-3-ACETIC ACID (Aux/IAA) family. Electrophoretic mobility shift assays showed that GhTCP14 bound the promoters of PIN2, IAA3, and AUX1, and transactivation assays indicated that GhTCP14 had transcription activation activity. Taken together, these results demonstrate that GhTCP14 is a dual-function transcription factor able to positively or negatively regulate expression of auxin response and transporter genes, thus potentially acting as a crucial regulator in auxin-mediated differentiation and elongation of cotton fiber cells.

MAP18 regulates the direction of pollen tube growth in Arabidopsis by modulating F-actin organization.

For fertilization to occur in plants, the pollen tube must be guided to enter the ovule via the micropyle. Previous reports have implicated actin filaments, actin binding proteins, and the tip-focused calcium gradient as key contributors to polar growth of pollen tubes; however, the regulation of directional pollen tube growth is largely unknown. We reported previously that Arabidopsis thaliana MICROTUBULE-ASSOCIATED PROTEIN18 (MAP18) contributes to directional cell growth and cortical microtubule organization. The preferential expression of MAP18 in pollen and in pollen tubes suggests that MAP18 also may function in pollen tube growth. In this study, we demonstrate that MAP18 functions in pollen tubes by influencing actin organization, rather than microtubule assembly. In vitro biochemical results indicate that MAP18 exhibits Ca(2+)-dependent filamentous (F)-actin-severing activity. Abnormal expression of MAP18 in map18 and MAP18 OX plants was associated with disorganization of the actin cytoskeleton in the tube apex, resulting in aberrant pollen tube growth patterns and morphologies, inaccurate micropyle targeting, and fewer fertilization events. Experiments with MAP18 mutants created by site-directed mutagenesis suggest that F-actin-severing activity is essential to the effects of MAP18 on pollen tube growth direction. Our study demonstrates that in Arabidopsis, MAP18 guides the direction of pollen tube growth by modulating actin filaments.

The unfolded protein response induced by salt stress in Arabidopsis.

Salt stress has a major impact on plant growth and crop production, pointing to the importance of understanding the mechanism of salt tolerance in plants. Disruption of the protein-folding capacity in the endoplasmic reticulum (ER) induces the accumulation of unfolded protein and ER stress, which activates an "unfolded protein response" (UPR). Although reports show that salt stress leads to UPR in various organisms, including plants, it remains to be determined how salt stress induces UPR. Zinc deficiency also induces UPR in a wide range of organisms. Here we provide a detailed description of the role of zinc in initiating UPR in the plant response to salt stress along with details of the methodology required for its investigation.

Zinc homeostasis is involved in unfolded protein response under salt stress.

Accumulation of unfolded protein or misfolded protein causes endoplasmic reticulum (ER) stress. Increased salt concentration activates a stress response pathway in the ER in Arabidopsis thaliana to induce the expression of several salt stress response genes, leading to a more optimal protein folding environment in the ER. In addition, some salt stress-regulated proteins require zinc for their activity, including some zinc-dependent DNA binding proteins and zinc-finger proteins. In a recent study, we reported that ZTP29, a putative zinc transporter at the ER membrane, is involved in the response to salt stress through regulation of zinc level in the ER to induce the UPR pathway. In this addendum, we propose a testable hypothesis for the role of ZTP29 in the response to salt stress via the regulation of zinc levels in the ER.

The putative Arabidopsis zinc transporter ZTP29 is involved in the response to salt stress.

Salt stress leads to a stress response, called the unfolded protein response (UPR), in the endoplasmic reticulum (ER). UPR is also induced in a wide range of organisms by zinc deficiency. However, it is not clear whether regulation of zinc levels is involved in the initiation of the UPR in plant response to salt stress. In this study, a putative zinc transporter, ZTP29, was identified in Arabidopsis thaliana. ZTP29 localizes to the ER membrane and is expressed primarily in hypocotyl and cotyledon tissues, but its expression can be induced in root tissue by salt stress. T-DNA insertion into the ZTP29 gene led to NaCl hypersensitivity in seed germination and seedling growth, leaf etiolation, and widening of cells in the root elongation zone. In addition, in ztp29 mutant plants, salt stress-induced upregulation of the UPR pathway genes BiP2 and bZIP60 was inhibited. Furthermore, under conditions of salt stress, upregulation of BiP2 and bZIP60 was inhibited by treatment with high concentrations of zinc in both control and ztp29 plants. However, zinc chelation restored salt stress-induced BiP2 and bZIP60 upregulation in ztp29 mutant plants. These experimental results suggest that ZTP29 is involved in the response to salt stress, perhaps through regulation of zinc levels required to induce the UPR pathway.