The receptor for activated C kinase 1 (RACK1) is one member of the most important WD repeat–containing family of proteins found in all eukaryotes and is involved in multiple signaling pathways. However, compared with the progress in the area of mammalian RACK1, our understanding of the functions and molecular mechanisms of RACK1 in the regulation of plant growth and development is still in its infancy. In the present study, we investigated the roles of rice RACK1A gene (OsRACK1A) in controlling seed germination and its molecular mechanisms by generating a series of transgenic rice lines, of which OsRACK1A was either over-expressed or under-expressed. Our results showed that OsRACK1A positively regulated seed germination and negatively regulated the responses of seed germination to both exogenous ABA and H2O2. Inhibition of ABA biosynthesis had no enhancing effect on germination, whereas inhibition of ABA catabolism significantly suppressed germination. ABA inhibition on seed germination was almost fully recovered by exogenous H2O2 treatment. Quantitative analyses showed that endogenous ABA levels were significantly higher and H2O2 levels significantly lower in OsRACK1A-down regulated transgenic lines as compared with those in wildtype or OsRACK1A-up regulated lines. Quantitative real-time PCR analyses showed that the transcript levels of OsRbohs and amylase genes, RAmy1A and RAmy3D, were significantly lower in OsRACK1A-down regulated transgenic lines. It is concluded that OsRACK1A positively regulates seed germination by controlling endogenous levels of ABA and H2O2 and their interaction.

SummaryPhytochrome‐interacting factors (PIFs) play major roles in regulating plant growth and development, but their roles in drought stress remain elusive. Here, we cloned and characterized a maize (Zea mays) PIF transcription factor, ZmPIF1. The expression level of ZmPIF1 was significantly induced by independent drought and abscisic acid (ABA) treatments. The ZmPIF1 transgenic rice and Arabidopsis displayed water saving and drought resistance, which were associated with reduced a stomatal aperture and transpiration rate. Moreover, the ZmPIF1 transgenic rice were hypersensitive to exogenous ABA, while the endogenous ABA level was not significantly changed, suggesting that ZmPIF1 was a positive regulator of the ABA signalling pathway. Digital gene expression (DGE) results further indicated that ZmPIF1 participated in ABA signalling pathway and regulated the stomatal aperture in rice. In addition, grain yield and agronomic traits analysis over 4 years showed that ZmPIF1 was able to increase the grain yield through an increase in tiller and panicle numbers in transgenic rice. Overall, ZmPIF1 plays an important role in the ABA‐mediated regulation of stomatal closure to control water loss. ZmPIF1 can enhance water saving and drought resistance and improve the crop yield in rice, illustrating the capacity of ZmPIF1 for crop improvement.

Heterotrimeric G proteins, which consist of G , G and G subunits, function as molecular switches to regulate a wide range of developmental processes in plants. In this study, we characterize the function of rice RGG2, which encodes a type B G subunit, in grain size and yield production. The expression levels of RGG2 are significantly higher than those of other rice G -encoding genes in all tissues tested, suggesting that RGG2 plays essential roles in rice growth and development. By regulating cell expansion, RGG2 overexpression in Nipponbare (NIP) leads to reduced plant height and decreased grain size. By contrast, two mutants generated by the clustered, regularly interspaced, short palindromic repeat (CRISPR)/CRISPR-associated 9 (Cas9) system in the Zhenshan 97 (ZS97) background, zrgg2-1 and zrgg2-2, exhibit enhanced growth, including elongated internodes, increased 1000-grain weight and plant biomass, and enhanced grain yield per plant (+11.8% and 16.0%, respectively). These results demonstrate that RGG2 acts as a negative regulator of plant growth and organ size in rice. By measuring the length of the second leaf sheath after gibberellin (GA ) treatment and the GA-induced α-amylase activity of seeds, we found that RGG2 is also involved in GA signaling. In summary, we propose that RGG2 may regulate grain and organ size via the GA pathway and that manipulation of RGG2 provides a novel strategy for rice grain yield enhancement. This article is protected by copyright. All rights reserved.

Summary Plants maintain a dynamic balance between plant growth and stress tolerance to optimise their fitness and ensure survival. Here, we investigated the roles of a clade A type 2C protein phosphatase (PP2C)‐encoding gene, OsPP2C09, in regulating the trade‐off between plant growth and drought tolerance in rice (Oryza sativa L.). The OsPP2C09 protein interacted with the core components of abscisic acid (ABA) signalling and showed PP2C phosphatase activity in vitro. OsPP2C09 positively affected plant growth but acted as a negative regulator of drought tolerance through ABA signalling. Transcript and protein levels of OsPP2C09 were rapidly induced by exogenous ABA treatments, which suppressed excessive ABA signalling and plant growth arrest. OsPP2C09 transcript levels in roots were much higher than those in shoots under normal conditions. After ABA, polyethylene glycol and dehydration treatments, the accumulation rate of OsPP2C09 transcripts in roots was more rapid and greater than that in shoots. This differential expression between the roots and shoots may increase the plant’s root‐to‐shoot ratio under drought‐stress conditions. This study sheds new light on the roles of OsPP2C09 in coordinating plant growth and drought tolerance. In particular, we propose that OsPP2C09‐mediated ABA desensitisation contributes to root elongation under drought‐stress conditions in rice.

Auxin is a phytohormone essential for plant development. However, our understanding of auxin-regulated endosperm development remains limited. Here, we described rice YUCCA (YUC) flavin-containing monooxygenase encoding gene OsYUC11 as a key contributor to auxin biosynthesis in rice (Oryza sativa) endosperm. Grain filling or storage product accumulation was halted by mutation of OsYUC11, but the deficiencies could be recovered by exogenous application of auxin. A rice transcription factor (TF) yeast library was screened, and 41 TFs that potentially bind to the OsYUC11 promoter were identified, of which OsNF-YB1, a member of the nuclear factor Y family, is predominantly expressed in the endosperm. Both osyuc11 and osnf-yb1 mutants exhibited reduced seed size and increased chalkiness, accompanied by a reduction in indole-3-acetic acid biosynthesis. OsNF-YB1 can bind the OsYUC11 promoter to induce gene expression in vivo. We also found that OsYUC11 was a dynamically imprinted gene that predominantly expressed the paternal allele in the endosperm up to 10 days after fertilization (DAF) but then became a non-imprinted gene at 15 DAF. A functional maternal allele of OsYUC11 was able to recover the paternal defects of this gene. Overall, the findings indicate that OsYUC11-mediated auxin biosynthesis is essential for endosperm development in rice.

Polyamines, including putrescine (Put), spermidine (Spd), and spermine (Spm), play essential roles in a wide variety of prokaryotic and eukaryotic organisms. Rice (Oryza sativa) contains four putative spermidine/spermine synthase (SPMS)-encoding genes (OsSPMS1, OsSPMS2, OsSPMS3, and OsACAULIS5), but none have been functionally characterized. In this study, we used a reverse genetic strategy to investigate the biological function of OsSPMS1. We generated several homozygous RNA interference (RNAi) and overexpression (OE) lines of OsSPMS1. Phenotypic analysis indicated that OsSPMS1 negatively regulates seed germination, grain size, and grain yield per plant. The ratio of Spm to Spd was significantly lower in the RNAi lines and considerably higher in the OE lines than in the wild-type (WT), suggesting that OsSPMS1 may function as a spermine synthase. S-adenosyl-L-methionine is a common precursor of polyamines and ethylene biosynthesis. The 1-aminocyclopropane-1-carboxylic acid (ACC) and ethylene contents in seeds significantly increased in RNAi lines and decreased in OE lines, respectively, compared with wild-type. Additionally, the reduced germination rates and growth defects of OE lines could be rescued with ACC treatment. These data suggest that OsSPMS1 affects ethylene synthesis and may regulate seed germination and plant growth by affecting the ACC and ethylene pathways. Most importantly, an OsSPMS1 knockout mutant showed an increase in grain yield per plant in a high-yield variety, Suken118, suggesting that OsSPMS1 is an important target for yield enhancement in rice.

The receptor for activated C kinase 1 (RACK1) is a WD40 type protein that is involved in multiple signaling pathways and is conserved from prokaryotes to eukaryotes. Here we report that rice RACK1A (OsRACK1A) is regulated by circadian clocks and plays an important role in the salt stress response. OsRACK1A was found to follow a rhythmic expression profile under circadian conditions at both the transcription and the translation levels, although the expression was arrhythmic under salt stress. Analysis of plant survival rates, fresh weight, proline content, malondialdehyde, and chlorophyll showed that suppression of OsRACK1A enhanced tolerance to salt stress. The ion concentration in both roots and leaves revealed that OsRACK1A-suppressed transgenic rice could maintain low Na+ and high K+ concentrations. Furthermore, OsRACK1A-suppressed transgenic rice accumulated significantly more abscisic acid (ABA) and more transcripts of ABA- and stress-inducible genes compared with the wild-type plants. Real-time quantitative polymerase chain reaction analysis revealed that many stress-related genes, including APETALA 2/Ethylene Responsive Factor (AP2/ERF) transcription factors, were upregulated in the OsRACK1A-suppressed transgenic rice line. We identified putative interactors of OsRACK1A, and found that OsRACK1A interacted with many salt stress-responsive proteins directly. These results suggest that OsRACK1A is regulated by circadian rhythm, and involved in the regulation of salt stress responses.Electronic supplementary materialThe online version of this article (10.1186/s12284-018-0232-3) contains supplementary material, which is available to authorized users.

Heterotrimeric G protein-mediated signal transduction is one of the most important and highly conserved signaling pathways in eukaryotes, which involves in the regulation of many important biological processes. As compared with those in mammals and Arabidopsis thaliana, the functions of rice heterotrimeric G protein and their molecular mechanisms are largely unknown. The rice genome contains a single Gα (RGA1) and Gβ (RGB1), and five Gγ (RGG1, RGG2, GS3, DEP1/qPE9–1, and GGC2) subunits. Recent genetic studies have shown that DEP1/qPE9–1, an atypical putative Gγ protein, is responsible for the grain size as well as the dense and erect panicles, but the biochemical and molecular mechanisms underlying the control of grain size are not well understood. Here, we report that rice plants carrying DEP1/qPE9–1 have more endosperm cells per grain than plants contain the dep1/qpe9–1 allele. The DEP1/qPE9–1 line has a higher rate and more prolonged period of starch accumulation than the dep1/qpe9–1 line. Additionally, the expression of several essential genes encoding enzymes catalyzing sucrose metabolism and starch biosynthesis is higher in the DEP1/qPE9–1 line than in the dep1/qpe9–1 line, especially from the mid to late grain-filling stage. Grains of the DEP1/qPE9–1 line also have higher contents of three phytohormones, ABA, auxin and cytokinin. Exogenous application of auxin or cytokinin enhanced the starch accumulation and the expression of genes encoding grain-filling-related enzymes in the grains of dep1/qpe9–1, whereas ABA produced no effects. Based on these results, we conclude that DEP1/qPE9–1 positively regulates starch accumulation primarily through auxin and cytokinin, which enhance the expression of genes encoding starch biosynthesis during the mid to late grain-filling stage, resulting in increased duration of the grain-filling process.