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Root meristem activity is the most critical process influencing root development. Although several factors that regulate meristem activity have been identified in rice, studies on the enhancement of meristem activity in roots are limited. We identified a T-DNA activation tagging line of a zinc-finger homeobox gene, OsZHD2, which has longer seminal and lateral roots due to increased meristem activity. The phenotypes were confirmed in transgenic plants overexpressing OsZHD2. In addition, the overexpressing plants showed enhanced grain yield under low nutrient and paddy field conditions. OsZHD2 was preferentially expressed in the shoot apical meristem and root tips. Transcriptome analyses and quantitative real-time PCR experiments on roots from the activation tagging line and the wild type showed that genes for ethylene biosynthesis were up-regulated in the activation line. Ethylene levels were higher in the activation lines compared with the wild type. ChIP assay results suggested that OsZHD2 induces ethylene biosynthesis by controlling ACS5 directly. Treatment with ACC (1-aminocyclopropane-1-carboxylic acid), an ethylene precursor, induced the expression of the DR5 reporter at the root tip and stele, whereas treatment with an ethylene biosynthesis inhibitor, AVG (aminoethoxyvinylglycine), decreased that expression in both the wild type and the OsZHD2 overexpression line. These observations suggest that OsZHD2 enhances root meristem activity by influencing ethylene biosynthesis and, in turn, auxin.

ABSTRACT Sucrose functions as a signaling molecule in several metabolic pathways as well as in various developmental processes. However, the molecular mechanisms by which sucrose regulates these processes remain largely unknown. In the present study, we demonstrate that sucrose promotes flowering by mediating the stability of a regulatory protein that represses flowering in rice. Exogenous application of sucrose promoted flowering by inducing florigen gene expression. Reduction of sucrose levels in the phloem through genetic modifications, such as the overexpression of the vacuolar invertase OsVIN2 or the mutation of OsSUT2, a sucrose transporter, delayed flowering. Analysis of relative transcript levels of floral regulatory genes showed that sucrose activated Ehd1 upstream of the florigen, with no significant effect on the expression of other upstream genes. Examination of protein stability after sucrose treatment of major floral repressors revealed that the Ghd7 protein was specifically degraded. The Ghd7 protein interacted with the E3 ligase IPA INTERACTING PROTEIN1 (IPI1), and sucrose‐induced K48‐linked polyubiquitination of Ghd7 via IPI1, leading to protein degradation. Mutants defective in IPI1 delayed flowering, confirming its role in modulating proteins involved in flowering. We conclude that sucrose acts as a signaling molecule to induce flowering by promoting Ghd7 degradation via IPI1. [ABSTRACT FROM AUTHOR]

Understanding the molecular mechanisms underlying early seed development is important in improving the grain yield and quality of crop plants. We performed a comparative label-free quantitative proteomic analysis of developing rice seeds for the WT and osctps1-2 mutant, encoding a cytidine triphosphate synthase previously reported as the endospermless 2 (enl2) mutant in rice, harvested at 0 and 1 d after pollination (DAP) to understand the molecular mechanism of early seed development. In total, 5231 proteins were identified, of which 902 changed in abundance between 0 and 1 DAP seeds. Proteins that preferentially accumulated at 1 DAP were involved in DNA replication and pyrimidine biosynthetic pathways. Notably, an increased abundance of OsCTPS1 was observed at 1 DAP; however, no such changes were observed at the transcriptional level. We further observed that the inhibition of phosphorylation increased the stability of this protein. Furthermore, in osctps1-2, minichromosome maintenance (MCM) proteins were significantly reduced compared with those in the WT at 1 DAP, and mutations in OsMCM5 caused defects in seed development. These results highlight the molecular mechanisms underlying early seed development in rice at the post-transcriptional level. [ABSTRACT FROM AUTHOR]

Floral transition starts in the leaves when florigens respond to various environmental and developmental factors. Among several regulatory genes that are preferentially expressed in the inflorescence meristem during the floral transition, this study examines the homeobox genes OsZHD1 and OsZHD2 for their roles in regulating this transition. Although single mutations in these genes did not result in visible phenotype changes, double mutations in these genes delayed flowering. Florigen expression was not altered in the double mutants, indicating that the delay was due to a defect in florigen signaling. Morphological analysis of shoot apical meristem at the early developmental stage indicated that inflorescence meristem development was significantly delayed in the double mutants. Overexpression of ZHD2 causes early flowering because of downstream signals after the generation of florigens. Expression levels of the auxin biosynthesis genes were reduced in the mutants and the addition of indole‐3‐acetic acid recovered the defect in the mutants, suggesting that these homeobox genes play a role in auxin biosynthesis. A rice florigen, RICE FLOWERING LOCUS T 1, binds to the promoter regions of homeobox genes. These results indicate that florigens stimulate the expression of homeobox genes, enhancing inflorescence development in the shoot apex. Summary statement: The homeobox transcription factors OsZHD1 and OsZHD2 promote meristem activity during floral transition in the shoot apex. [ABSTRACT FROM AUTHOR]

Summary: Grain number is an important agronomic trait. We investigated the roles of chromatin interacting factor Oryza sativa VIN3‐LIKE 2 (OsVIL2), which controls plant biomass and yield in rice. Mutations in OsVIL2 led to shorter plants and fewer grains whereas its overexpression (OX) enhanced biomass production and grain numbers when compared with the wild type. RNA‐sequencing analyses revealed that 1958 genes were up‐regulated and 2096 genes were down‐regulated in the region of active division within the first internodes of OX plants. Chromatin immunoprecipitation analysis showed that, among the downregulated genes, OsVIL2 was directly associated with chromatins in the promoter region of CYTOKININ OXIDASE/DEHYDROGENASE2 (OsCKX2), a gene responsible for cytokinin degradation. Likewise, active cytokinin levels were increased in the OX plants. We conclude that OsVIL2 improves the production of biomass and grain by suppressing OsCKX2 chromatin. [ABSTRACT FROM AUTHOR]

In chromatin remodeling, the post-translational modification of histone proteins is mediated by multimeric protein complexes. VERNALIZATION INSENSITIVE3 (VIN3) forms a complex with Polycomb Repressive Complex 2 (PRC2), which mediates the trimethylation of H3K27 to repress target gene expression. In rice, four genes (OsVIL1-OsVIL4) encoding the VIN3-like proteins are expressed ubiquitously in various tissues. Null mutants of osvil2 display pleiotropic phenotypes such as altered flowering time, floral organ defects, and reduced tiller size. In contrast, osvil1 mutants did not show significant phenotypes except in fertilization compared with the wild type. However, transgenic plants overexpressing OsVIL1 showed phenotypes of increased biomass and grain yield. Cross-sections of the basal region of elongating stems revealed that the increased biomass was mediated by inducing cell proliferation in the meristem. Chromatin immunoprecipitation assay indicated that OsVIL1 repressed expression of cytokinin oxidase/dehydrogenase gene (OsCKX2) by binding to the promoter and genic regions of OsCKX2. We also observed that OsVIL1 modified the levels of H3K27me3 in the OsCKX2 chromatin. Because OsCKX2 encodes an enzyme that degrades active cytokinin, we conclude that OsVIL1 functions in the regulation of endogenous active cytokinin levels, thereby increasing plant height and productivity. [ABSTRACT FROM AUTHOR]

Root hairs are important for absorption of nutrients and water from the rhizosphere. The Root Hair Defective-Six Like (RSL) Class II family of transcription factors is expressed preferentially in root hairs and has a conserved role in root hair development in land plants. We functionally characterized the seven members of the RSL Class II subfamily in the rice ( Oryza sativa ) genome. In root hairs, six of these genes were preferentially expressed and four were strongly expressed. Phenotypic analysis of each mutant revealed that Os07g39940 plays a major role in root hair formation, based on observations of a short root hair phenotype in those mutants. Overexpression (OX) for each of four family members in rice resulted in an increase in the density and length of root hairs. These four members contain a transcription activation domain and are targeted to the nucleus. They interact with rice Root Hairless1 (OsRHL1), a key regulator of root hair development. When heterologously expressed in epidermal cells of Nicotiana benthamiana leaves, OsRHL1 was predominantly localized to the cytoplasm. When coexpressed with each of the four RSL Class II members, however, OsRLH1 was translocated to the nucleus. Transcriptome analysis using Os07g39940- OX plants revealed that 86 genes, including Class III peroxidases, were highly up-regulated. Furthermore, reactive oxygen species levels in the root hairs were increased in Os07g39940- OX plants but were drastically reduced in the os07g39940 and rhl1 mutants. Our results demonstrate that RSL Class II members function as essential regulators of root hair development in rice., (© 2019 American Society of Plant Biologists. All Rights Reserved.)

In much of the tropics and subtropics, rice (Oryza sativa L.) is grown under long days (LDs). Therefore, LD must play a major role in inducing flowering signal in rice. However, little is known on LD-dependent flowering signal in the species. We previously reported that OsMADS50, which is highly homologous to Arabidopsis SOC1, functions as a positive regulator for flowering. However, its detailed photoperiodic mechanism was not yet elucidated. Here, we report the functional analysis of OsMADS50 and its closely related gene OsMADS56. Knock-out of OsMADS50 caused a late-flowering phenotype only under LD conditions. Overexpression of OsMADS56 (56OX) also resulted in delayed flowering under LD. In the osmads50 mutants and 56OX transgenic plants, transcripts of Ehd1, Hd3a and RFT1 were reduced, although that of OsLFL1 increased. On the other hand, mRNA levels of OsGI, Hd1, OsId1, OsDof12, Ghd7, Hd6 and SE5 were unchanged. These observations imply that OsMADS50 and OsMADS56 function antagonistically through OsLFL1-Ehd1 in regulating LD-dependent flowering. Yeast two-hybrid and co-immunoprecipitation analyses indicated an interaction between those two proteins as well as their formation of homodimers. These results suggest that OsMADS50 and OsMADS56 may form a complex that regulates downstream target genes.