Genome-wide analysis and identification of the <Emphasis Type="Italic">SMXL</Emphasis> gene family in apple (<Emphasis Type="Italic">Malus</Emphasis> × <Emphasis Type="Italic">domestica</Emphasis>)

Strigolactones (SLs) are a recently discovered type of plant hormone that controls various developmental processes. The DWARF53 (D53) protein in rice and the SMAX1-LIKE (SMXL) family in Arabidopsis repress SL signaling. In this study, bioinformatics analyses were performed, and 236 SMXL proteins were identified in 28 sequenced plants. A phylogenetic analysis indicated that all potential SMXL proteins could be divided into three groups and that the SMXL proteins may have originated in Bryophytes. An analysis of the SMXL chromosomal locations suggested that gene duplication events at different times led to expansion of the SMXL family members in Angiospermae. Subsequently, the gene structure and protein modeling of MdSMXLs showed that they are highly conserved. The expression patterns of MdSMXLs indicated that they were expressed in different organs of apple (stems, roots, leaves, flowers, and fruits) at varying levels and that MdSMXLs may participate in the SL signaling pathway and the response to abiotic stress. This study provides a valuable foundation for additional investigations into the function of the SMXL gene family in plants.     

The role of strigolactones during plant interactions with the pathogenic fungus <Emphasis Type="Italic">Fusarium</Emphasis><Emphasis Type="Italic">oxysporum</Emphasis>

Main conclusion

Strigolactones (SLs) do not influence spore germination or hyphal growth of Fusarium oxysporum. Mutant studies revealed no role for SLs but a role for ethylene signalling in defence against this pathogen in pea. Strigolactones (SLs) play important roles both inside the plant as a hormone and outside the plant as a rhizosphere signal in interactions with mycorrhizal fungi and parasitic weeds. What is less well understood is any potential role SLs may play in interactions with disease causing microbes such as pathogenic fungi. In this paper we investigate the influence of SLs on the hemibiotrophic pathogen Fusarium oxysporum f.sp. pisi both directly via their effects on fungal growth and inside the plant through the use of a mutant deficient in SL. Given that various stereoisomers of synthetic and naturally occuring SLs can display different biological activities, we used (+)-GR24, (?)-GR24 and the naturally occurring SL, (+)-strigol, as well as a racemic mixture of 5-deoxystrigol. As a positive control, we examined the influence of a plant mutant with altered ethylene signalling, ein2, on disease development. We found no evidence that SLs influence spore germination or hyphal growth of Fusarium oxysporum and that, while ethylene signalling influences pea susceptibility to this pathogen, SLs do not.

     

Expression of <Emphasis Type="Italic">MAX2</Emphasis> under <Emphasis Type="Italic">SCARECROW</Emphasis> promoter enhances the strigolactone/MAX2 dependent response of <Emphasis Type="Italic">Arabidopsis</Emphasis> roots to low-phosphate conditions

Main conclusion

MAX2/strigolactone signaling in the endodermis and/or quiescent center of the root is partiallysufficient to exert changes in F-actin density and cellular trafficking in the root epidermis, and alter gene expression during plant response to low Pi conditions.Strigolactones (SLs) are a new group of plant hormones that regulate different developmental processes in the plant via MAX2, an F-box protein that interacts with their receptor. SLs and MAX2 are necessary for the marked increase in root-hair (RH) density in seedlings under conditions of phosphate (Pi) deprivation. This marked elevation was associated with an active reduction in actin-filament density and endosomal movement in root epidermal cells. Also, expression of MAX2 under the SCARECROW (SCR) promoter was sufficient to confer SL sensitivity in roots, suggesting that SL signaling pathways act through a root-specific, yet non-cell-autonomous regulatory mode of action. Here we show evidence for a non-cell autonomous signaling of SL/MAX2, originating from the root endodermis, and necessary for seedling response to conditions of Pi deprivation. SCR-derived expression of MAX2 in max2-1 mutant background promoted the root low Pi response, whereas supplementation of the synthetic SL GR24 to these SCR:MAX2 expressing lines further enhanced this response. Moreover, the SCR:MAX2 expression led to changes in actin density and endosome movement in epidermal cells and in TIR1 and PHO2 gene expression. These results demonstrate that MAX2 signaling in the endodermis and/or quiescent center is partially sufficient to exert changes in F-actin density and cellular trafficking in the epidermis, and alter gene expression under low Pi conditions.

     

A novel dominant rice male sterility mutant, <Emphasis Type="Italic">OsDMS-1</Emphasis>, simultaneously controlled by independent loci on chromosomes 1, 2, and 3

We found a rice dominant genetic male-sterile mutant OsDMS-1 from tissue culture-regenerated offspring of Zhonghua 11 (japonica rice). Compared to Zhonghua 11, OsDMS-1 mutant anthers were narrow and pale and incapable of pollen release although the glume opened normally. Approximately 81.4% of this mutant pollen was small and malformed and could not be stained by iodine treatment. A paraffin section assay showed delayed degradation of the OsDMS-1 mutant tapetum without starch accumulation in the mutant pollen, ultimately leading to pollen abortion. Classical genetic analysis indicated that only one dominant gene controlled the sterility in the OsDMS-1 mutant. However, molecular mapping suggested three loci simultaneously control male sterility in this mutant: OsDMS-1A (on chromosome 1), flanked by InDel markers C1D4 and C1D5, OsDMS-1B (on chromosome 2), flanked by InDel markers C2D3 and C2D10, and OsDMS-1C (on chromosome 3), flanked by InDel markers 0315 and C3D3. Molecular mapping disagreed with classical genetic analysis regarding the number of genes controlling the OsDMS-1 mutant, indicating a novel mechanism underlying sterility in OsDMS-1. We present two hypotheses to explain this novel inheritance behavior: one is described as Parent-Originated Loci Tying Inheritance (POLTI); while the alternate hypothesis is described as Loci Recombination Lethal (LRL).     

Characterization of the temporal and spatial expression of wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis> L.) plant height at the QTL level and their influence on yield-related traits

Key message

The temporal and spatial expression patterns of stable QTL for plant height and their influences on yield were characterized.

Abstract

Plant height (PH) is a complex trait in wheat (Triticum aestivum L.) that includes the spike length (SL) and the internode lengths from the first to the fifth internode, which are counted from the top and abbreviated as FIRITL, SECITL, THIITL, FOUITL, and FIFITL, respectively. This study identified eight putative additive quantitative trait loci (QTL) for PH. In addition, unconditional and conditional QTL mapping were used to analyze the temporal and spatial expression patterns of five stable QTL for PH. qPh-3A mainly regulated SL, FIRITL, and FIFITL to affect PH during the booting–heading stage (BS–HS); qPh-3D regulated all internode lengths to affect PH, especially during the BS–HS; before HS, qPh-4B mainly affected FIRITL, SECITL, THIITL, and FOUITL and qPh-5A.1 mainly affected SECITL, THIITL, and FOUITL to regulate PH; and qPh-6B mainly regulated FIRITL to affect the PH after the booting stage (BS). qPhdv-4B, a QTL for the response of PH to nitrogen stress, was stable and co-localized with qPh-4B. All five stable QTL, except for qPh-3A, were related to the 1000 kernel weight and yield per plant. Regions of qPh-3A, qPh-3D, qPh-4B, qPh-5A.1, and qPh-6B showed synteny to parts of rice chromosomes 1, 1, 3, 9, and 2, respectively. Based on comparative genomics analysis, Rht-B1b was cloned and mapped in the CI of qPh-4B. This report provides useful information for fine mapping of the stable QTL for PH and the genetic improvement of wheat plant type.

     

<Emphasis Type="Italic">Agrobacterium</Emphasis> T-DNA insertion in the rice <Emphasis Type="Italic">DWARF SHOOT AND DEFECTIVE PANICLE1</Emphasis> (<Emphasis Type="Italic">DSDP1</Emphasis>) gene causes a severe dwarf phenotype,reduces plant vigour,and affects seed germination

We report a new T-DNA-tagged rice plant chi7, which displays a severe dwarf phenotype, reduced plant vigour, and impaired panicle development in the homozygous state. By chromosome walking, T-DNA integration was mapped in chromosome 2, 1054-bp upstream of the translation start site of a gene (Os02g0820400), which we designate as DWARF SHOOT AND DEFECTIVE PANICLE1 (DSDP1). DSDP1 expression was unexpectedly higher in the homozygous mutant leaves than in the hemizygous mutant and control plant leaves. Mutant dsdp1 seeds, stored for 24 weeks, failed to germinate in soil. The growth vigour of the dsdp1 mutant reduced with increasing seed storage period. The dsdp1 mutant plants, grown in vitro on MS medium, formed short, stout, and ageotropic roots with lesser number of root hairs. The findings suggest that DSDP1 may function as a negative regulator of many developmental processes in rice.     

New branching inhibitors and their potential as strigolactone mimics in rice

Strigolactones (SLs) are rhizosphere communication chemicals. Recent studies of highly branched mutants revealed that SL or its metabolites work as a phytohormone to inhibit shoot branching. When SLs are exogenously applied to the rice d10-1 mutant that has a highly branched phenotype caused by a defect in the SL biosynthesis gene (CCD8), they inhibit tiller bud outgrowth (branching in rice) of the mutant. We focused our attention on the SL function as a phytohormone and tried to find new chemicals mimicking the hormonal action of SL by screening chemicals that inhibit branching of rice d10-1 mutant. Fortunately, we found 5-(4-chlorophenoxy)-3-methylfuran-2(5H)-one (3a) as a new chemical possessing SL-like activity against the rice d10-1 mutant. Then, we prepared several derivatives of 3a (3b-3k) to examine their ability to inhibit shoot branching of rice d10-1. These derivatives were synthesized by a one-pot coupling reaction between phenols and halo butenolide to give 5-phenoxy 3-methylfuran-2(5H)-one (3) derivatives, which possess a common substructure with SLs. Some of the derivatives showed SL-like activity more potently than GR24, a typical SL derivative, in a rice assay. As SLs also show activity by inducing seed germination of root parasitic plants, the induction activity of these derivatives was also evaluated. Here we report the structure-activity relationships of these compounds.     

Identification and Functional Analysis of Two Cotton Orthologs of <Emphasis Type="Italic">MAX2</Emphasis> Which Control Shoot Lateral Branching

Cotton (Gossypium spp.), as the most important fiber and oilseed crop in the world, is extremely important for the industry. However, due to its indeterminate growth habit and complex branching system, massive labor costs are needed for shoot apex removal and branch pruning during cotton production. Therefore, it is very important to explore branch-controlling genes and genetically modify the branch architecture of cotton. Strigolactones (SLs) are a novel class of plant hormone that inhibit the outgrowth of lateral branches. To elucidate the role of SLs in branch development of cotton, we cloned and characterized GhMAX2a and GhMAX2b from tetraploid upland cotton (Gossypium hirsutum), the orthologs of Arabidopsis MAX2, rice D3, and petunia RMS4. GhMAX2a/2b was ubiquitously expressed in all tested tissues of cotton, with relatively higher expression levels in leaves and lateral buds. Subcellular localization assay showed that the GhMAX2-GFP fusion protein localized to the nucleus. Both GhMAX2a and GhMAX2b can fully rescue the dwarfed and highly branched phenotypes of the Arabidopsis max2-1 mutant, indicating that GhMAX2s have conserved functions with that of AtMAX2. The cotton GhMAX2b interacted with Arabidopsis Skp1-like 1 (ASK1) proteins in vitro which was further confirmed in the Arabidopsis protoplasts using the co-immunoprecipitation assay, indicating that GhMAX2b probably functions through forming an SCF E3 complex with Skp and other proteins in the Arabidopsis. These results suggest that the cotton GhMAX2s encode functional MAX2 that can inhibit the shoot lateral branching. Further functional analysis of GhMAX2s in determining cotton branch architecture and yield is underway.     

Interaction of Phytohormones in Regulating the Axillary Bud Growth in Pea

In the present study the interactions of GR24, a synthetic analog of newly discovered plant hormones strigolactones (SLs), with cytokinin (CK), benzyladenine (BA), auxin naphthaleneacetic acid (NAA), gibberellic acid (GA₃) and abscisic acid (ABA) in the regulation of axillary bud growth in pea (Pisum sativum L.) were investigated. The hormones were applied directly to buds at node 1 and 2 and the dose-response experiments were performed on 8–10-day-old SL-deficient rms1 and rms5–1 mutants, branching SL-sensitive rms2–1 mutants and wild-type plants. In the mutant plants the treatment with 5 μM GR24 completely inhibited bud growth while BA up to 100 μM stimulated it. The combined application of GR24 and BA showed that GR24 efficiency to reduce bud outgrowth constantly declines as CK-stimulated bud growth increased, with the inhibiting effect of GR24 abolished at 100 μM BA applied. GA₃ accelerated bud outgrowth, but did not interfere with GR24 inhibitory action. NAA reduced bud growth in intact SL-sensitive rms2–1 mutant and also in SL-insensitive rms3–2 and rms4–1 mutants. The NAA effect was observed already at 0.5 μM, however, even at a response saturating concentration of 500 μM its inhibiting effect was inferior to that of 5 μM GR24. At combined treatment the effectiveness of GR24 in suppressing bud growth decreased with a decrease in NAA-inhibited bud growth, suggesting that auxin might act as a suppressor of SL action. ABA strongly inhibited the bud outgrowth and, like CK and auxin, reduced the inhibitory effectiveness of GR24. The revealed ability of CK, ABA and auxin to suppress bud response to SLs is supposed to result from phytohormone signaling crosstalks.     

A single nucleotide mutation of <Emphasis Type="Italic">IspF</Emphasis> gene involved in the MEP pathway for isoprenoid biosynthesis causes yellow-green leaf phenotype in rice

Key message

We identified IspF gene through yellow-green leaf mutant 505ys in rice. OsIspF was expressed in all tissues detected, and its encoded protein was targeted to the chloroplast. On expression levels of genes in this mutant, OsIspF itself and the genes encoding other enzymes of the MEP pathway and chlorophyll synthase were all up-regulated, however, among eight genes associated with photosynthesis, only psaA, psaN and psbA genes for three reaction center subunits of photosystem obviously changed.

Abstract

Isoprenoids are the most abundant natural compounds in all organisms, which originate from the basic five-carbon units isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP). In plants, IPP and DMAPP are synthesized through two independent pathways, the mevalonic acid pathway in cytoplasm and the 2-C-methyl-d-erythritol 4-phosphate (MEP) pathway in plastids. The MEP pathway comprises seven enzymatic steps, in which IspF is the fifth enzyme. So far, no IspF gene has been identified in monocotyledonous plants. In this study, we isolated a leaf-color mutant, 505ys, in rice (Oryza sativa). The mutant displayed yellow-green leaf phenotype, reduced level of photosynthetic pigments, and arrested development of chloroplasts. By map-based cloning of this mutant, we identified OsIspF gene (LOC_Os02g45660) showing significant similarity to IspF gene of Arabidopsis, in which a missense mutation occurred in the mutant, resulting in an amino acid change in the encoded protein. OsIspF gene was expressed in all tissues detected, and its encoded protein was targeted to the chloroplast. Further, the mutant phenotype of 505ys was complemented by transformation with the wild-type OsIspF gene. Therefore, we successfully identified an IspF gene in monocotyledonous plants. In addition, real-time quantitative RT-PCR implied that a positive regulation could exist between the OsIspF gene and the genes encoding other enzymes of the MEP pathway and chlorophyll synthase. At the same time, it also implied that the individual genes involved in the MEP pathway might differentially regulated expression levels of the genes associated with photosynthesis.

     

A rice <Emphasis Type="Italic">White-stripe leaf3</Emphasis> (<Emphasis Type="Italic">wsl3</Emphasis>) mutant lacking an HD domain-containing protein affects chlorophyll biosynthesis and chloroplast development

Leaf-color mutants are ideal genetic materials for understanding the mechanism of chloroplast development and chlorophyll (Chl) biosynthesis. Here we isolated and identified a new leaf-color mutant of rice, named white-stripe leaf3 (wsl3), from a ⁶⁰Co-irradiated mutant pool. The wsl3 mutant displayed a visible white-stripe leaf in both young seedlings and flag leaves of mature plant. Chl content in homozygous wsl3 mutant was approximately 47% of that in the wild type. Besides, chloroplast development in the mutant was severely arrested. By a map-based cloning strategy, the wsl3 gene was finely confined to a 50.8 kb region on chromosome 1. Moreover, a 9-bp deletion was identified in the genomic region of LOC_Os01g01920, which encodes an HD (histidine and aspartic acid) domaincontaining protein. Genetic complementation confirmed that LOC_Os01g01920 could recover the lesion of wsl3 mutation. Real-time PCR analyses showed that the expression levels of WSL3 were the highest in young and flag leaves among various tissues, and most of the genes associated with Chl biosynthesis were significantly down-regulated in the wsl3 mutant. Meanwhile, in contrast to many nuclear gene-encoded phage-type RNA polymerase(s) (NEP) transcribed genes were up-regulated, most of plastid-encoded bacterialtype RNA polymerase (PEP) transcribed genes were downregulated. These results demonstrated that the WSL3 gene, as an HD domain-containing protein, is involved in chl biosynthesis and chloroplast development in rice.     

RNA-Binding Domain is Necessary for PprM Function in Response to the Extreme Environmental Stress in <Emphasis Type="Italic">Deinococcus radiodurans</Emphasis>

Deinococcus radiodurans was considered as one of the most radiation-resistant organisms on Earth because of its strong resistance to the damaging factors of both DNA and protein, including ionizing radiation, ultraviolet radiation, oxidants, and desiccation. PprM, as a bacterial cold shock protein homolog, was involved in the radiation resistance and oxidative stress response of D. radiodurans, but its potential mechanisms are poorly expounded. In this study, we found that PprM was highly conserved with the RNA-binding domain in Deinococcus genus through performing phylogenic analysis. Moreover, the paper presents the analysis on the tolerance of environmental stresses both in the wild-type and the pprM/pprM RBD mutant strains, demonstrating that pprM and RNA-binding domain disruptant strain were with higher sensitivity than the wild-type strain to cold stress, mitomycin C, UV radiation, and hydrogen peroxide. In the following step, the recombinant PprM was purified, with the finding that PprM was bound to the 5’-untranslated region of its own mRNA by gel mobility shift assay in vitro. With all these findings taken into consideration, it was suggested that PprM act as a cold shock protein and its RNA-binding domain may be involved in reaction to the extreme environmental stress in D. radiodurans.     

Loss of function of <Emphasis Type="Italic">OsMADS34</Emphasis> leads to large sterile lemma and low grain yield in rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.)

Rice (Oryza sativa L.) is one of the most important food crops, especially in Asia. The spikelet is a characteristic structure of grass inflorescences that determines crop output. However, the molecular mechanism that controls spikelet development and grain yield in rice remains unclear. In this study, we isolated a new osmads34 allelic mutant (i.e., osmads34-t). The osmads34-t mutant showed more primary branch numbers, short panicles, and long sterile lemmas. The sterile lemmas were transformed into the lemmas and had the lemma identity in the osmads34-t mutant, suggesting that the sterile lemma and lemma are homologous organs. Additionally, osmads34-t displayed smaller grains on its secondary branches of panicles and a lower seed-setting rate. These results suggest that OsMADS34 plays an important role in determination of grain size and yield in rice. OsMADS34 was expressed in tested organs and tissues, and its green fluorescent protein (GFP) signal was located in the nucleus. The result of this study will be used to understand the identity of unique organs in grass spikelets and may improve grain yield in breeding practice.     

<Emphasis Type="Italic">ZmCCD7/ZpCCD7</Emphasis> encodes a carotenoid cleavage dioxygenase mediating shoot branching

Main conclusion

ZmCCD7/ZpCCD7 encodes a carotenoid cleavage dioxygenase that may mediate strigolactone biosynthesis highly responsive to phosphorus deficiency and undergoes negative selection over domestication from Zea ssp. parviglumis to Zea mays.Carotenoid cleavage dioxygenase 7 (CCD7) functions to suppress shoot branching by controlling strigolactone biosynthesis. However, little is known about CCD7 and its functions in maize and its ancestor (Zea ssp. parviglumis) with numerous shoot branches. We found that ZmCCD7 and ZpCCD7 had the same coding sequence, indicating negative selection of the CCD7 gene over domestication from Zea ssp. parviglumis to Zea mays. CCD7 expression was highly responsive to phosphorus deficiency in both species, especially in the meristematic zone and the pericycle of the elongation zone of maize roots. Notably, the crown root had the strongest ZmCCD7 expression in the meristematic zone under phosphorus limitation. Transient expression of GFP tagged ZmCCD7/ZpCCD7 in maize protoplasts indicated their localization in the plastid. Further, ZmCCD7/ZpCCD7 efficiently catalyzed metabolism of six different linear and cyclic carotenoids in E. coli, and generated β-ionone by cleaving β-carotene at the 9,10 (9′,10′) position. Together with suppression of shoot branching in the max3 mutant by transformation of ZmCCD7/ZpCCD7, our work suggested that ZmCCD7/ZpCCD7 encodes a carotenoid cleavage dioxygenase mediating strigolactone biosynthesis in maize and its ancestor.