SLG2 specifically regulates grain width through WOX11-mediated cell expansion control in rice.

Grain size is specified by three dimensions of length, width and thickness, and slender grain is a desirable quality trait in rice. Up to now, many grain size regulators have been identified. However, most of these molecules show influence on multi-dimensions of grain development, and only a few of them function specifically in grain width, a key factor determining grain yield and appearance quality. In this study, we identify the SLG2 (SLENDER GUY2) gene that specifically regulates grain width by affecting cell expansion in the spikelet hulls. SLG2 encodes a WD40 domain containing protein, and our biochemical analyses show that SLG2 acts as a transcription activator of its interacting WOX family protein WOX11. We demonstrate that the SLG2-associated WOX11 binds directly to the promoter of OsEXPB7, one of the downstream cell expansion genes. We show that knockout of WOX11 results in plants with a slender grain phenotype similar to the slg2 mutant. We also present that finer grains with different widths could be produced by combining SLG2 with the grain width regulator GW8. Collectively, we uncover the crucial role of SLG2 in grain width control, and provide a promising route to design rice plants with better grain shape and quality.

Separable regulation of POW1 in grain size and leaf angle development in rice.

Leaf angle is one of the key factors that determines rice plant architecture. However, the improvement of leaf angle erectness is often accompanied by unfavourable changes in other traits, especially grain size reduction. In this study, we identified the pow1 (put on weight 1) mutant that leads to increased grain size and leaf angle, typical brassinosteroid (BR)-related phenotypes caused by excessive cell proliferation and cell expansion. We show that modulation of the BR biosynthesis genes OsDWARF4 (D4) and D11 and the BR signalling gene D61 could rescue the phenotype of leaf angle but not grain size in the pow1 mutant. We further demonstrated that POW1 functions in grain size regulation by repressing the transactivation activity of the interacting protein TAF2, a highly conserved member of the TFIID transcription initiation complex. Down-regulation of TAF2 rescued the enlarged grain size of pow1 but had little effect on the increased leaf angle phenotype of the mutant. The separable functions of the POW1-TAF2 and POW1-BR modules in grain size and leaf angle control provide a promising strategy for designing varieties with compact plant architecture and increased grain size, thus promoting high-yield breeding in rice.

The LARGE2-APO1/APO2 regulatory module controls panicle size and grain number in rice.

Panicle size and grain number are important agronomic traits and influence grain yield in rice (Oryza sativa), but the molecular and genetic mechanisms underlying panicle size and grain number control remain largely unknown in crops. Here we report that LARGE2 encodes a HECT-domain E3 ubiquitin ligase OsUPL2 and regulates panicle size and grain number in rice. The loss of function large2 mutants produce large panicles with increased grain number, wide grains and leaves, and thick culms. LARGE2 regulates panicle size and grain number by repressing meristematic activity. LARGE2 is highly expressed in young panicles and grains. Biochemical analyses show that LARGE2 physically associates with ABERRANT PANICLE ORGANIZATION1 (APO1) and APO2, two positive regulators of panicle size and grain number, and modulates their stabilities. Genetic analyses support that LARGE2 functions with APO1 and APO2 in a common pathway to regulate panicle size and grain number. These findings reveal a novel genetic and molecular mechanism of the LARGE2-APO1/APO2 module-mediated control of panicle size and grain number in rice, suggesting that this module is a promising target for improving panicle size and grain number in crops.

Natural variations of SLG1 confer high-temperature tolerance in indica rice.

With global warming and climate change, breeding crop plants tolerant to high-temperature stress is of immense significance. tRNA 2-thiolation is a highly conserved form of tRNA modification among living organisms. Here, we report the identification of SLG1 (Slender Guy 1), which encodes the cytosolic tRNA 2-thiolation protein 2 (RCTU2) in rice. SLG1 plays a key role in the response of rice plants to high-temperature stress at both seedling and reproductive stages. Dysfunction of SLG1 results in plants with thermosensitive phenotype, while overexpression of SLG1 enhances the tolerance of plants to high temperature. SLG1 is differentiated between the two Asian cultivated rice subspecies, indica and japonica, and the variations at both promoter and coding regions lead to an increased level of thiolated tRNA and enhanced thermotolerance of indica rice varieties. Our results demonstrate that the allelic differentiation of SLG1 confers indica rice to high-temperature tolerance, and tRNA thiolation pathway might be a potential target in the next generation rice breeding for the warming globe.

A point mutation in LTT1 enhances cold tolerance at the booting stage in rice.

The cold tolerance of rice at the booting stage is a main factor determining sustainability and regional adaptability. However, relatively few cold tolerance genes have been identified that can be effectively used in breeding programmes. Here, we show that a point mutation in the low-temperature tolerance 1 (LTT1) gene improves cold tolerance by maintaining tapetum degradation and pollen development, by activation of systems that metabolize reactive oxygen species (ROS). Cold-induced ROS accumulation is therefore prevented in the anthers of the ltt1 mutants allowing correct development. In contrast, exposure to cold stress dramatically increases ROS accumulation in the wild type anthers, together with the expression of genes encoding proteins associated with programmed cell death and with the accelerated degradation of the tapetum that ultimately leads to pollen abortion. These results demonstrate that appropriate ROS management is critical for the cold tolerance of rice at the booting stage. Hence, the ltt1 mutation can significantly improve the seed setting ability of cold-sensitive rice varieties under low-temperature stress conditions, with little yield penalty under optimal temperature conditions. This study highlights the importance of a valuable genetic resource that may be applied in rice breeding programmes to enhance cold tolerance.

Scenario-based modelling for urban sustainability focusing on changes in cropland under rapid urbanization: A case study of Hangzhou from 1990 to 2035.

China is undergoing rapid urbanization, which has brought great pressure on croplands throughout the country, especially in fast developing cities, such as Hangzhou. In this study, an attempt was made to monitor and model the cropland dynamics of Hangzhou from 1990 to 2035. The spatial-temporal changes in the cropland were discussed based on the land cover maps along with urban-rural gradient analysis. After understanding the spatial-temporal patterns of cropland changes, the cellular automata-Markov model was employed using the historical land cover maps and other explanatory data to perform a scenario-based simulation. Accordingly, three scenarios, namely spontaneous scenario (SS), protected area ensuring scenario (PAES), and optimal agriculture developing scenario (OADS), were designed for simulating the cropland distribution in 2035. The monitoring results showed that during 1990-2015, the cropland area decreased 1512.46km2 under rapid urbanization. Areas at a distance of 12km from the city center experienced maximum cropland loss. Among all the spatial metrics, aggregation index of the cropland exhibited the highest correlation with the distance to the city center (r=0.77 in 2015), thereby suggesting an obvious trend in aggregation along the urban-rural gradient. The modelling results reported that under PAES and OADS, the study area could gain 81.76km2 and 255.14km2 more cropland, respectively, than that under SS in 2035. Thus, policies applied in PAES and OADS would be effective for cropland protection.

Control of Grain Size and Weight by the OsMKKK10-OsMKK4-OsMAPK6 Signaling Pathway in Rice.

Grain size is one of the key agronomic traits that determine grain yield in crops. However, the mechanisms underlying grain size control in crops remain elusive. Here we demonstrate that the OsMKKK10-OsMKK4-OsMAPK6 signaling pathway positively regulates grain size and weight in rice. In rice, loss of OsMKKK10 function results in small and light grains, short panicles, and semi-dwarf plants, while overexpression of constitutively active OsMKKK10 (CA-OsMKKK10) results in large and heavy grains, long panicles, and tall plants. OsMKKK10 interacts with and phosphorylates OsMKK4. We identified an OsMKK4 gain-of-function mutant (large11-1D) that produces large and heavy grains. OsMKK4A227T encoded by the large11-1D allele has stronger kinase activity than OsMKK4. Plants overexpressing a constitutively active form of OsMKK4 (OsMKK4-DD) also produce large grains. Further biochemical and genetic analyses revealed that OsMKKK10, OsMKK4, and OsMAPK6 function in a common pathway to control grain size. Taken together, our study establishes an important genetic and molecular framework for OsMKKK10-OsMKK4-OsMAPK6 cascade-mediated control of grain size and weight in rice.

Short and Solid Culm/RFL/APO2 for culm development in rice.

The culm development of rice is characterized by elongation and medullary cavity (MC) formation, which are determined by node formation meristem and residual meristem, respectively. Although many factors have been shown to affect culm elongation, molecules involved in MC formation remained to be identified. In this study, we show that a point mutation in SHORT and SOLID CULM (SSC), the rice homologue of Arabidopsis LFY, resulted in plants with drastically reduced culm length and completely abolished MC formation. Analysis of transgenic plants with moderately enhanced SSC expression revealed significant decreases in plant height and MC size in contrast to slight changes in heading date, indicating that the culm developmental process is much more tightly monitored by the gene. Transcriptomic analysis revealed the differential expression of knotted-1 like homeobox (KNOX) protein genes and gibberellin (GA) metabolic genes in the ssc mutant background, and most of the genes contained well-conserved LFY-binding cis-elements that could be effectively recognized by SSC. Genetic analysis found that the reduced culm length of the mutant could be largely rescued by the GA-accumulating mutation eui, whereas MC formation remained unchanged in the double mutant plants. Taken together, our results suggest that SSC affects culm elongation mainly through maintaining GA homeostasis, while functions in MC formation by mediating residual meristem activity possibly via the KNOX pathway. The present study provides a potential strategy for improving the culm morphology and plant architecture in rice by manipulating SSC and/or its downstream components.

CRR1 encoding callose synthase functions in ovary expansion by affecting vascular cell patterning in rice.

The ovary of rice undergoes rapid expansion immediately after fertilization, and this process determines the final sink strength potential of caryopses. To date, work on rice grain development has mainly focused on endosperm filling, whereas information on the essential elements for ovary expansion remains limited. We report here a functional analysis of the ovary expansion retarded mutant crr1 in rice. Map-based cloning revealed that CRR1 encodes a protein homologous to the Arabidopsis callose synthases AtGSL8 and AtGSL10. Point mutation in crr1 resulted in alternative splicing, which led to the formation of the truncated crr1 protein without the ß-glucan synthase domain. Iodine staining showed that there were few starch granules and these were unevenly distributed in the pericarp of crr1, and a 5,6-carboxyfluorescein diacetate transport assay revealed that carbohydrates were less efficiently unloaded from the lateral vasculature into the developing caryopsis. CRR1 transcripts were detected in all plant organs, with the highest level found in receptacles, which are mainly composed of vascular tissues. Analysis of pCRR1::GUS transgenic plants showed that CRR1 was specifically expressed in vascular bundle cells. Consistently, loss of function of CRR1 led to disordered patterns of vascular cells in the ovaries and receptacles of the mutant. Furthermore, a small portion of cells in the vascular bundles of crr1 showed defective cell wall formation, and callose deposition was specifically reduced at the plasmodesmata (PD) of cells with aberrant walls. Our results suggest that CRR1 performs a pivotal role in determining initial ovary expansion in rice, possibly via the PD-mediated permeability of cell fate determinants for vascular cell differentiation.

ALT1, a Snf2 family chromatin remodeling ATPase, negatively regulates alkaline tolerance through enhanced defense against oxidative stress in rice.

Alkaline salt stress adversely affects rice growth, productivity and grain quality. However, the mechanism underlying this process remains elusive. We characterized here an alkaline tolerant mutant, alt1 in rice. Map-based cloning revealed that alt1 harbors a mutation in a chromatin remodeling ATPase gene. ALT1-RNAi transgenic plants under different genetic background mimicked the alt1 phenotype, exhibiting tolerance to alkaline stress in a transcript dosage-dependent manner. The predicted ALT1 protein belonged to the Ris1 subgroup of the Snf2 family and was localized in the nucleus, and transcription of ALT1 was transiently suppressed after alkaline treatment. Although the absorption of several metal ions maintained well in the mutant under alkaline stress, expression level of the genes involved in metal ions homeostasis was not altered in the alt1 mutant. Classification of differentially expressed abiotic stress related genes, as revealed by microarray analysis, found that the majority (50/78) were involved in ROS production, ROS scavenging, and DNA repair. This finding was further confirmed by that alt1 exhibited lower levels of H2O2 under alkaline stress and tolerance to methyl viologen treatment. Taken together, these results suggest that ALT1 negatively functions in alkaline tolerance mainly through the defense against oxidative damage, and provide a potential two-step strategy for improving the tolerance of rice plants to alkaline stress.