OsPRMT6a-mediated arginine methylation of OsJAZ1 regulates jasmonate signaling and spikelet development in rice.

Although both protein arginine methylation (PRMT) and jasmonate (JA) signaling are crucial for regulating plant development, the relationship between these processes in spikelet development control remains unclear. Here, we utilized CRISPR/Cas9 technology to generate two OsPRMT6a loss-of-function mutants exhibiting various abnormal spikelet structures. Additionally, we found that OsPRMT6a could methylate arginine residues in the JA signal repressors OsJAZ1 and OsJAZ7. Arginine methylation of OsJAZ1 increased the affinity of OsJAZ1 for the JA receptors OsCOI1a and OsCOI1b in the presence of jasmonates (JAs), subsequently promoting the ubiquitination of OsJAZ1 by the SCFOsCOI1a/OsCOI1b complex and degradation via the 26S proteasome. This process ultimately released OsMYC2, a core transcriptional regulator in the JA signaling pathway, to activate or repress JA-responsive genes, thereby maintaining normal plant (spikelet) development. However, in the osprmt6a-1 mutant, reduced arginine methylation of OsJAZ1 impaired the interaction between OsJAZ1 and OsCOI1a/OsCOI1b in the presence of JAs. As a result, OsJAZ1 proteins became more stable, repressing JA responses, thus causing the formation of abnormal spikelet structures. Moreover, we discovered that JA signaling reduced the OsPRMT6a mRNA level in an OsMYC2-dependent manner, thereby establishing a negative feedback loop to balance JA signaling. Furthermore, we found that OsPRMT6a-mediated arginine methylation of OsJAZ1 likely serves as a switch to tune JA signaling to maintain normal spikelet development under harsh environmental conditions such as high temperatures. Thus, our study established a direct molecular link between arginine methylation and the JA signaling pathway.

A toxin-antidote system contributes to interspecific reproductive isolation in rice.

Breakdown of reproductive isolation facilitates flow of useful trait genes into crop plants from their wild relatives. Hybrid sterility, a major form of reproductive isolation exists between cultivated rice (Oryza sativa) and wild rice (O. meridionalis, Mer). Here, we report the cloning of qHMS1, a quantitative trait locus controlling hybrid male sterility between these two species. Like qHMS7, another locus we cloned previously, qHMS1 encodes a toxin-antidote system, but differs in the encoded proteins, their evolutionary origin, and action time point during pollen development. In plants heterozygous at qHMS1, ~ 50% of pollens carrying qHMS1-D (an allele from cultivated rice) are selectively killed. In plants heterozygous at both qHMS1 and qHMS7, ~ 75% pollens without co-presence of qHMS1-Mer and qHMS7-D are selectively killed, indicating that the antidotes function in a toxin-dependent manner. Our results indicate that different toxin-antidote systems provide stacked reproductive isolation for maintaining species identity and shed light on breakdown of hybrid male sterility.

A CYP78As-small grain4-coat protein complex â ¡ pathway promotes grain size in rice.

CYP78A, a cytochrome P450 subfamily that includes rice (Oryza sativa L.) BIG GRAIN2 (BG2, CYP78A13) and Arabidopsis thaliana KLUH (KLU, CYP78A5), generate an unknown mobile growth signal (referred to as a CYP78A-derived signal) that increases grain (seed) size. However, the mechanism by which the CYP78A pathway increases grain size remains elusive. Here, we characterized a rice small grain mutant, small grain4 (smg4), with smaller grains than its wild type due to restricted cell expansion and cell proliferation in spikelet hulls. SMG4 encodes a multidrug and toxic compound extrusion (MATE) transporter. Loss of function of SMG4 causes smaller grains while overexpressing SMG4 results in larger grains. SMG4 is mainly localized to endoplasmic reticulum (ER) exit sites (ERESs) and partially localized to the ER and Golgi. Biochemically, SMG4 interacts with coat protein complex Ⅱ (COPⅡ) components (Sar1, Sec23, and Sec24) and CYP78As (BG2, GRAIN LENGTH 3.2 [GL3.2], and BG2-LIKE 1 [BG2L1]). Genetically, SMG4 acts, at least in part, in a common pathway with Sar1 and CYP78As to regulate grain size. In summary, our findings reveal a CYP78As-SMG4-COPⅡ regulatory pathway for grain size in rice, thus providing new insights into the molecular and genetic regulatory mechanism of grain size.

Corrigendum: Induction of RIPK3/MLKL-mediated necroptosis by Erigeron breviscapus injection exhibits potent antitumor effect.

[This corrects the article DOI: 10.3389/fphar.2023.1219362.].

Induction of RIPK3/MLKL-mediated necroptosis by Erigeron breviscapus injection exhibits potent antitumor effect.

Colorectal cancer (CRC) is the second leading cause of tumor-related deaths worldwide. Resistance of tumor cells to drug-induced apoptosis highlights the need for safe and effective antitumor alternatives. Erigeron breviscapus (Dengzhanxixin in China) injection (EBI), extracted from the natural herb Erigeron breviscapus (Vant.) Hand.-Mazz (EHM), has been widely used in clinical practice for cardiovascular diseases. Recent studies have suggested that EBI's main active ingredients exhibit potential antitumor effects. This study aims to explore the anti-CRC effect of EBI and elucidate the underlying mechanism. The anti-CRC effect of EBI was evaluated in vitro using CCK-8, flow cytometry, and transwell analysis, and in vivo through a xenograft mice model. RNA sequencing was utilized to compare the differentially expressed genes, and the proposed mechanism was verified through in vitro and in vivo experiments. Our study demonstrates that EBI significantly inhibits the proliferation of three human CRC cell lines and effectively suppresses the migration and invasion of SW620 cells. Moreover, in the SW620 xenograft mice model, EBI markedly retards tumor growth and lung metastasis. RNA-seq analysis revealed that EBI might exert antitumor effects by inducing necroptosis of tumor cells. Additionally, EBI activates the RIPK3/MLKL signaling pathway, a classical pathway of necroptosis and greatly promotes the generation of intracellular ROS. Furthermore, the antitumor effect of EBI on SW620 is significantly alleviated after the pretreatment of GW806742X, the MLKL inhibitor. Our findings suggest that EBI is a safe and effective inducer of necroptosis for CRC treatment. Notably, necroptosis is a non-apoptotic programmed cell death pathway that can effectively circumvent resistance to apoptosis, which provides a novel approach for overcoming tumor drug resistance.

A natural gene drive system confers reproductive isolation in rice.

Hybrid sterility restricts the utilization of superior heterosis of indica-japonica inter-subspecific hybrids. In this study, we report the identification of RHS12, a major locus controlling male gamete sterility in indica-japonica hybrid rice. We show that RHS12 consists of two genes (iORF3/DUYAO and iORF4/JIEYAO) that confer preferential transmission of the RHS12-i type male gamete into the progeny, thereby forming a natural gene drive. DUYAO encodes a mitochondrion-targeted protein that interacts with OsCOX11 to trigger cytotoxicity and cell death, whereas JIEYAO encodes a protein that reroutes DUYAO to the autophagosome for degradation via direct physical interaction, thereby detoxifying DUYAO. Evolutionary trajectory analysis reveals that this system likely formed de novo in the AA genome Oryza clade and contributed to reproductive isolation (RI) between different lineages of rice. Our combined results provide mechanistic insights into the genetic basis of RI as well as insights for strategic designs of hybrid rice breeding.

ESCRT-III component OsSNF7.2 modulates leaf rolling by trafficking and endosomal degradation of auxin biosynthetic enzyme OsYUC8 in rice.

The endosomal sorting complex required for transport (ESCRT) is highly conserved in eukaryotic cells and plays an essential role in the biogenesis of multivesicular bodies and cargo degradation to the plant vacuole or lysosomes. Although ESCRT components affect a variety of plant growth and development processes, their impact on leaf development is rarely reported. Here, we found that OsSNF7.2, an ESCRT-III component, controls leaf rolling in rice (Oryza sativa). The Ossnf7.2 mutant rolled leaf 17 (rl17) has adaxially rolled leaves due to the decreased number and size of the bulliform cells. OsSNF7.2 is expressed ubiquitously in all tissues, and its protein is localized in the endosomal compartments. OsSNF7.2 homologs, including OsSNF7, OsSNF7.3, and OsSNF7.4, can physically interact with OsSNF7.2, but their single mutation did not result in leaf rolling. Other ESCRT complex subunits, namely OsVPS20, OsVPS24, and OsBRO1, also interact with OsSNF7.2. Further assays revealed that OsSNF7.2 interacts with OsYUC8 and aids its vacuolar degradation. Both Osyuc8 and rl17 Osyuc8 showed rolled leaves, indicating that OsYUC8 and OsSNF7.2 function in the same pathway, conferring leaf development. This study reveals a new biological function for the ESCRT-III components, and provides new insights into the molecular mechanisms underlying leaf rolling.

A regulatory loop establishes the link between the circadian clock and abscisic acid signaling in rice.

There is a close regulatory relationship between the circadian clock and the abscisic acid (ABA) signaling pathway in regulating many developmental processes and stress responses. However, the exact feedback regulation mechanism between them is still poorly understood. Here, we identified the rice (Oryza sativa) clock component PSEUDO-RESPONSE REGULATOR 95 (OsPRR95) as a transcriptional regulator that accelerates seed germination and seedling growth by inhibiting ABA signaling. We also found that OsPRR95 binds to the ABA receptor gene REGULATORY COMPONENTS OF ABA RECEPTORS10 (OsRCAR10) DNA and inhibits its expression. Genetic analysis showed OsRCAR10 acts downstream of OsPRR95 in mediating ABA responses. In addition, the induction of OsPRR95 by ABA partly required a functional OsRCAR10, and the ABA-responsive element-binding factor ABSCISIC ACID INSENSITIVE5 (OsABI5) bound directly to the promoter of OsPRR95 and activated its expression, thus establishing a regulatory feedback loop between OsPRR95, OsRCAR10, and OsABI5. Taken together, our results demonstrated that the OsRCAR10-OsABI5-OsPRR95 feedback loop modulates ABA signaling to fine-tune seed germination and seedling growth, thus establishing the molecular link between ABA signaling and the circadian clock.

Reactive Oxygen Species-Responsive Polypeptide Drug Delivery System Targeted Activated Hepatic Stellate Cells to Ameliorate Liver Fibrosis.

Hepatic fibrosis is a chronic liver disease that lacks effective pharmacotherapeutic treatments. As part of the disease's mechanism, hepatic stellate cells (HSCs) are activated by damage-related stimuli to secrete excessive extracellular matrix, leading to collagen deposition. Currently, the drug delivery system that targets HSCs in the treatment of liver fibrosis remains an urgent challenge due to the poor controllability of drug release. Since the level of reactive oxygen species (ROS) increases sharply in activated HSCs (aHSCs), we designed ROS-responsive micelles for the HSC-specific delivery of a traditional Chinese medicine, resveratrol (RES), for treatment of liver fibrosis. The micelles were prepared by the ROS-responsive amphiphilic block copolymer poly(l-methionine-block-Nε-trifluoro-acetyl-l-lysine) (PMK) and a PEG shell modified with a CRGD peptide insertion. The CRGD-targeted and ROS-responsive micelles (CRGD-PMK-MCs) could target aHSCs and control the release of RES under conditions of high intracellular ROS in aHSCs. The CRGD-PMK-MCs treatment specifically enhanced the targeted delivery of RES to aHSCs both in vitro and in vivo. In vitro experiments show that CRGD-PMK-MCs could significantly promote ROS consumption, reduce collagen accumulation, and avert activation of aHSCs. In vivo results demonstrate that CRGD-PMK-MCs could alleviate inflammatory infiltration, prevent fibrosis, and protect hepatocytes from damage in fibrotic mice. In conclusion, CRGD-PMK-MCs show great potential for targeted and ROS-responsive controlled drug release in the aHSCs of liver fibrosis.

Self-Charging Persistent Mechanoluminescence with Mechanics Storage and Visualization Activities.

Persistent mechanoluminescence (ML) with long lifetime is highly required to break the limits of the transient emitting behavior under mechanics stimuli. However, the existing materials with persistent ML are completely trap-controlled, and a pre-irradiation is required, which severely hinders the practical applications. In this work, a novel type of ML, self-charging persistent ML, is created by compositing the Sr3 Al2 O5 Cl2 :Dy3+ (SAOCD) powders into flexible polydimethylsiloxane (PDMS) matrix. With no need for any pre-irradiation, the as-fabricated SAOCD/PDMS elastomer could exhibit intense and persistent ML under mechanics stimuli directly, which greatly facilitates its applications in mechanics lighting, displaying, imaging, and visualization. By investigating the matrix effects as well as the thermoluminescence, cathodoluminescence, and triboelectricity properties, the interfacial triboelectrification-induced electron bombardment processes are demonstrated to be responsible for the self-charged energy in  SAOCD under mechanics stimuli. Based on the unique self-charging processes, the SAOCD/PDMS further exhibits mechanics storage and visualized reading activities, which brings novel ideas and approaches to deal with the mechanics-related problems in the fields of mechanical engineering, bioengineering, and artificial intelligence.

Dwarf and High Tillering1 represses rice tillering through mediating the splicing of D14 pre-mRNA.

Strigolactones (SLs) constitute a class of plant hormones that regulate many aspects of plant development, including repressing tillering in rice (Oryza sativa). However, how SL pathways are regulated is still poorly understood. Here, we describe a rice mutant dwarf and high tillering1 (dht1), which exhibits pleiotropic phenotypes (such as dwarfism and increased tiller numbers) similar to those of mutants defective in SL signaling. We show that DHT1 encodes a monocotyledon-specific hnRNP-like protein that acts as a previously unrecognized intron splicing factor for many precursor mRNAs (pre-mRNAs), including for the SL receptor gene D14. We find that the dht1 (DHT1I232F) mutant protein is impaired in its stability and RNA binding activity, causing defective splicing of D14 pre-mRNA and reduced D14 expression, and consequently leading to the SL signaling-defective phenotypes. Overall, our findings deepen our understanding of the functional diversification of hnRNP-like proteins and establish a connection between posttranscriptional splicing and SL signaling in the regulation of plant development.

Rice STOMATAL CYTOKINESIS DEFECTIVE2 regulates cell expansion by affecting vesicular trafficking in rice.

Vesicular trafficking plays critical roles in cell expansion in yeast and mammals, but information linking vesicular trafficking and cell expansion in plants is limited. Here, we isolated and characterized a rice (Oryza sativa) mutant, decreased plant height 1-1 (dph1-1), which exhibited a wide spectrum of developmental phenotypes, including reduced plant height and smaller panicles and grains. Cytological analysis revealed that limited cell expansion was responsible for the dph1-1 mutant phenotype compared to the wild-type. Map-based cloning revealed that DPH1 encodes a plant-specific protein, OsSCD2, which is homologous to Arabidopsis (Arabidopsis thaliana) STOMATAL CYTOKINESIS DEFECTIVE2 (SCD2). Subcellular localization revealed that OsSCD2 is associated with clathrin. Confocal microscopy showed that the dph1-1 mutant has defective endocytosis and post-Golgi trafficking. Biochemical and confocal data indicated that OsSCD2 physically interacts with OsSCD1 and that they are associated with intracellular structures that colocalize with microtubules. Furthermore, we found that cellulose synthesis was affected in the dph1-1 mutant, evidenced by reduced cellulose synthase gene accumulation at the transcript and protein levels, most likely resulting from an impaired localization pattern. Our results suggest that OsSCD2 is involved in clathrin-related vesicular trafficking with an important role in maintaining plant growth in rice.

Genetic characterization and fine mapping of qHMS4 responsible for pollen sterility in hybrids between Oryza sativa L. and Oryza glaberrima Steud.

African cultivated rice (Oryza glaberrima Steud) contains many favorable genes for tolerance to biotic and abiotic stresses and F1 hybrids between Asian cultivated rice (Oryza sativa L.) show strong heterosis. However, the hybrids of two species often exhibit hybrid sterility. Here, we identified a male sterility locus qHMS4 on chromosome 4 (Chr.4), which induces pollen semi-sterility in F1 hybrids of japonica rice variety Dianjingyou1 (DJY1) and a near-isogenic line (NIL) carrying a Chr.4 segment from Oryza glaberrima accession IRGC101854. Cytological observations indicated that non-functional pollen grains produced by the hybrids and lacking starch accumulation abort at the late bicellular stage. Molecular genetic analysis revealed distorted segregation in male gametogenesis carrying qHMS4 allele from DJY1. Fine-mapping of qHMS4 using an F2 population of 22,500 plants delimited qHMS4 to a region of 110-kb on the short arm of Chr.4. Sequence analysis showed that the corresponding sequence region in DJY1 and Oryza glaberrima were 114-kb and 323-kb, respectively, and that the sequence homology was very poor. Gene prediction analysis identified 16 and 46 open reading frames (ORFs) based on the sequences of DJY1 and O. glaberrima, respectively, among which 3 ORFs were shared by both. Future map-based cloning of qHMS4 will help to understand the underlying molecular mechanism of hybrid sterility between the two cultivated rice species. Supplementary Information: The online version contains supplementary material available at 10.1007/s11032-022-01306-8.

Determinant Factors and Regulatory Systems for Anthocyanin Biosynthesis in Rice Apiculi and Stigmas.

Anthocyanins cause purple, brown or red colors in various tissues of rice plants, but the specific determinant factors and regulatory systems for anthocyanin biosynthesis in almost all tissues remain largely unknown. In the present study, we mapped and isolated two complementary genes, OsC1 encoding a R2R3-MYB transcriptional factor and OsDFR encoding a dihydroflavonol 4-reductase, which are responsible for the purple coloration of apiculi and stigmas in indica cultivar Xieqingzao by the map-based cloning strategy. We also identified two tissue-specific pigmentation genes, OsPa for apiculi and OsPs for stigmas, by phylogenetic analysis of all anthocyanin biosynthesis-associated bHLH transcriptional factors in maize and rice, CRISPR/Cas9 knockout and transcriptional expression analysis. The OsC1, OsPa and OsPs proteins are all localized in the nucleus while the OsDFR protein is localized in the nucleus and cytoplasm, and the OsC1 and OsDFR genes are preferentially strongly expressed in both purple-colored tissues while the OsPa and OsPs genes are preferentially strongly expressed in apiculi and stigmas, respectively. OsC1 specifically interacts with OsPa or OsPs to activate OsDFR and other anthocyanin biosynthesis genes, resulting in purple-colored apiculi or stigmas. OsC1 itself does not produce color but can produce brown apiculi when functioning together with OsPa. Loss of function of OsDFR alone leads to brown apiculi and straw-white stigmas. Genotyping and phenotyping of a panel of 176 rice accessions revealed diverse genotypic combinations of OsC1, OsDFR, OsPa and OsPs that enable accurate prediction of their apiculus and stigma pigmentation phenotypes, thus validating the general applicability of the OsC1-OsDFR-OsPa and OsC1-OsDFR-OsPs models to natural populations. Our findings disclosed the biological functions of OsC1, OsPa and OsPs, and shed light on the specific regulatory systems of anthocyanin biosynthesis in apiculi and stigmas, a further step in understanding the regulatory network of anthocyanin biosynthesis in rice.

ENLARGED STARCH GRAIN1 affects amyloplast development and starch biosynthesis in rice endosperm.

Cereal crops accumulate large amounts of starch which is synthesized and stored in amyloplasts in the form of starch grains (SGs). Despite significant progress in deciphering starch biosynthesis, our understanding of amyloplast development in rice (Oryza sativa) endosperm remains largely unknown. Here, we report a novel rice floury mutant named enlarged starch grain1 (esg1). The mutant has decreased starch content, altered starch physicochemical properties, slower grain-filling rate and reduced 1000-grain weight. A distinctive feature in esg1 endosperm is that SGs are much larger, mainly due to an increased number of starch granules per SG. Spherical and loosely assembled granules, together with those weakly stained SGs may account for decreased starch content in esg1. Map-based cloning revealed that ESG1 encodes a putative permease subunit of a bacterial-type ABC (ATP-binding cassette) lipid transporter. ESG1 is constitutively expressed in various tissues. It encodes a protein localized to the chloroplast and amyloplast membranes. Mutation of ESG1 causes defective galactolipid synthesis. The overall study indicates that ESG1 is a newly identified protein affecting SG development and subsequent starch biosynthesis, which provides novel insights into amyloplast development in rice.

OsRE1 interacts with OsRIP1 to regulate rice heading date by finely modulating Ehd1 expression.

Heading date is a key agronomic trait affecting crop yield. In rice, Early heading date 1 (Ehd1) is an important B-type response regulator in determination of heading date. Although many regulatory factors of Ehd1 expression have been functionally characterized, the direct regulators of Ehd1 largely remain to be identified. Here, we identified a new regulator of Ehd1, OsRE1, that directly binds to the A-box motif in the Ehd1 promoter. Osre1 confers an early heading phenotype due to elevated expression levels of Ehd1. OsRE1 is a nucleus-localized bZIP transcription factor with a diurnal rhythmic expression pattern. Furthermore, we identified an OsRE1-interacting protein, OsRIP1, and demonstrated that OsRIP1 can repress the transcript expression of Ehd1 in an OsRE1-dependent manner. Our genetic data showed that OsRE1 and OsRIP1 may function upstream of Ehd1 in regulating heading date. Together, our results suggest that OsRE1 functions cooperatively with OsRIP1 to regulate heading date through finely modulating the expression of Ehd1. In addition, OsRE1 and OsRIP1 are two minor heading date regulators, which are more desirable for fine-tuning heading date to improve rice regional adaptability.

white panicle2 encoding thioredoxin z, regulates plastid RNA editing by interacting with multiple organellar RNA editing factors in rice.

Thioredoxins (TRXs) occur in plant chloroplasts as complex disulphide oxidoreductases. Although many biological processes are regulated by thioredoxins, the regulatory mechanism of chloroplast TRXs are largely unknown. Here we report a rice white panicle2 mutant caused by a mutation in the thioredoxin z gene, an orthologue of AtTRX z in Arabidopsis. white panicle2 (wp2) seedlings exhibited a high-temperature-sensitive albinic phenotype. We found that plastid multiple organellar RNA editing factors (MORFs) were the regulatory targets of thioredoxin z. We showed that OsTRX z protein physically interacts with OsMORFs in a redox-dependent manner and that the redox state of a conserved cysteine in the MORF box is essential for MORF-MORF interactions. wp2 and OsTRX z knockout lines show reduced editing efficiencies in many plastidial-encoded genes especially under high-temperature conditions. An Arabidopsis trx z mutant also exhibited significantly reduced chloroplast RNA editing. Our combined results suggest that thioredoxin z regulates chloroplast RNA editing in plants by controlling the redox state of MORFs.

Transcriptional activation and phosphorylation of OsCNGC9 confer enhanced chilling tolerance in rice.

Low temperature is a major environmental factor that limits plant growth and productivity. Although transient elevation of cytoplasmic calcium has long been recognized as a critical signal for plant cold tolerance, the calcium channels responsible for this process have remained largely elusive. Here we report that OsCNGC9, a cyclic nucleotide-gated channel, positively regulates chilling tolerance by mediating cytoplasmic calcium elevation in rice (Oryza sativa). We showed that the loss-of-function mutant of OsCNGC9 is defective in cold-induced calcium influx and more sensitive to prolonged cold treatment, whereas OsCNGC9 overexpression confers enhanced cold tolerance. Mechanistically, we demonstrated that in response to chilling stress, OsSAPK8, a homolog of Arabidopsis thaliana OST1, phosphorylates and activates OsCNGC9 to trigger Ca2+ influx. Moreover, we found that the transcription of OsCNGC9 is activated by a rice dehydration-responsive element-binding transcription factor, OsDREB1A. Taken together, our results suggest that OsCNGC9 enhances chilling tolerance in rice through regulating cold-induced calcium influx and cytoplasmic calcium elevation.

Rice FLOURY ENDOSPERM 18 encodes a pentatricopeptide repeat protein required for 5' processing of mitochondrial nad5 messenger RNA and endosperm development.

Pentatricopeptide repeat (PPR) proteins, composing one of the largest protein families in plants, are involved in RNA binding and regulation of organelle RNA metabolism at the post-transcriptional level. Although several PPR proteins have been implicated in endosperm development in rice (Oryza sativa), the molecular functions of many PPRs remain obscure. Here, we identified a rice endosperm mutant named floury endosperm 18 (flo18) with pleiotropic defects in both reproductive and vegetative development. Map-based cloning and complementation tests showed that FLO18 encodes a mitochondrion-targeted P-type PPR protein with 15 PPR motifs. Mitochondrial function was disrupted in the flo18 mutant, as evidenced by decreased assembly of Complex I in the mitochondrial electron transport chain and altered mitochondrial morphology. Loss of FLO18 function resulted in defective 5'-end processing of mitochondrial nad5 transcripts encoding subunit 5 of nicotinamide adenine dinucleotide hydrogenase. These results suggested that FLO18 is involved in 5'-end processing of nad5 messenger RNA and plays an important role in mitochondrial function and endosperm development.