Contribution of strigolactones to the inhibition of tiller bud outgrowth under phosphate deficiency in rice

Strigolactones (SLs) or SL-derived metabolite(s) have recently been shown to act as endogenous inhibitors of axillary bud outgrowth. SLs released from roots induce hyphal branching of arbuscular mycorrhizal (AM) fungi that facilitate the uptake of inorganic nutrients, such as phosphate (Pi) and nitrate, by the host plants. Previous studies have shown that SL levels in root exudates are highly elevated by Pi starvation, which might contribute to successful symbiosis with AM fungi in the rhizosphere. However, how endogenous SL levels elevated by Pi starvation contribute to its hormonal action has been unknown. Here, we show that tiller bud outgrowth in wild-type rice seedlings is inhibited, while root 2'-epi-5-deoxystrigol (epi-5DS) levels are elevated, in response to decreasing Pi concentrations in the media. However, the suppression of tiller bud outgrowth under Pi deficiency does not occur in the SL-deficient and -insensitive mutants. We also show that the responsiveness to exogenous SL is slightly increased by Pi deficiency. When Pi-starved seedlings are transferred to Pi-sufficient media, tiller bud outgrowth is induced following a decrease in root epi-5DS levels. Taken together, these results suggest that elevated SL levels by Pi starvation contribute to the inhibition of tiller bud outgrowth in rice seedlings. We speculate that SL plays a dual role in the adaptation to Pi deficiency; one as a rhizosphere signal to maximize AM fungi symbiosis for improved Pi acquisition and the other as an endogenous hormone or its biosynthetic precursor to optimize shoot branching for efficient Pi utilization.

Strigolactones and Brassinosteroids Antagonistically Regulate the Stability of the D53-OsBZR1 Complex to Determine FC1 Expression in Rice Tillering

Rice tillering, a key architecture trait determining grain yield, is highly regulated by a class of newly identified phytohormones, strigolactones (SLs). However, the whole SL signaling pathway from the receptor to downstream transcription factors to finally inhibit tillering remains unrevealed. In this study, we first found that brassinosteroids (BRs) strongly enhance tillering by promoting bud outgrowth in rice, which is largely different from the function of BRs in Arabidopsis. Genetic and biochemical analyses indicated that both the SL and BR signaling pathways control rice tillering by regulating the stability of D53 and/or the OsBZR1-RLA1-DLT module, a transcriptional complex in the rice BR signaling pathway. We further found that D53 interacts with OsBZR1 to inhibit the expression of FC1, a local inhibitor of tillering, and that this inhibition depends on direct DNA binding by OsBZR1, which recruits D53 to the FC1 promoter in rice buds. Taken together, these findings uncover a mechanism illustrating how SLs and BRs coordinately regulate rice tillering via the early responsive gene FC1.

How does nitrogen shape plant architecture?

Plant nitrogen (N), acquired mainly in the form of nitrate and ammonium from soil, dominates growth and development, and high-yield crop production relies heavily on N fertilization. The mechanisms of root adaptation to altered supply of N forms and concentrations have been well characterized and reviewed, while reports concerning the effects of N on the architecture of vegetative and reproductive organs are limited and are widely dispersed in the literature. In this review, we summarize the nitrate and amino acid regulation of shoot branching, flowering, and panicle development, as well as the N regulation of cell division and expansion in shaping plant architecture, mainly in cereal crops. The basic regulatory steps involving the control of plant architecture by the N supply are auxin-, cytokinin-, and strigolactone-controlled cell division in shoot apical meristem and gibberellin-controlled inverse regulation of shoot height and tillering. In addition, transport of amino acids has been shown to be involved in the control of shoot branching. The N supply may alter the timing and duration of the transition from the vegetative to the reproductive growth phase, which in turn may affect cereal crop architecture, particularly the structure of panicles for grain yield. Thus, proper manipulation of N-regulated architecture can increase crop yield and N use efficiency.

ζ-Carotene Isomerase Suppresses Tillering in Rice through the Coordinated Biosynthesis of Strigolactone and Abscisic Acid

Rice tillering is an important agronomic trait affecting grain yield. Here, we identified a high-tillering mutant tillering20 (t20), which could be restored to the wild type by treatment with the strigolactone (SL) analog rac-GR24. T20 encodes a chloroplast ζ-carotene isomerase (Z-ISO), which is involved in the biosynthesis of carotenoids and their metabolites, SL and abscisic acid (ABA). The t20 mutant has reduced SL and ABA, raising the question of how SL and ABA biosynthesis is coordinated, and whether they have overlapping functions in tillering. We discovered that rac-GR24 stimulated T20 expression and enhanced all-trans-β-carotene biosynthesis. Importantly, rac-GR24 also stimulated expression of Oryza sativa 9-CIS-EPOXYCAROTENOID DIOXYGENASE 1 (OsNCED1) through induction of Oryza sativa HOMEOBOX12 (OsHOX12), promoting ABA biosynthesis in shoot base. On the other hand, ABA treatment significantly repressed SL biosynthesis and the ABA biosynthetic mutants displayed elevated SL biosynthesis. ABA treatment reduced the number of basal tillers in both t20 and wild-type plants. Furthermore, while ABA-deficient mutants aba1 and aba2 had the same number of basal tillers as wild type, they had more unproductive upper tillers at maturity. This work demonstrates complex interactions in the biosynthesis of carotenoid, SLs and ABA, and reveals a role for ABA in the regulation of rice tillering.

The amino acid transporter AAP1 mediates growth and grain yield by regulating neutral amino acid uptake and reallocation in Oryza sativa

Nitrogen (N) is a major element necessary for crop yield. In most plants, organic N is primarily transported in the form of amino acids. Here, we show that amino acid permease 1 (AAP1) functions as a positive regulator of growth and grain yield in rice. We found that the OsAAP1 gene is highly expressed in rice axillary buds, leaves, and young panicles, and that the OsAAP1 protein is localized to both the plasma membrane and the nuclear membrane. Compared with the wild-type ZH11, OsAAP1 overexpression (OE) lines exhibited increased filled grain numbers as a result of enhanced tillering, while RNAi and CRISPR (clustered regularly interspaced short palindromic repeat; Osaap1) knockout lines showed the opposite phenotype. In addition, OsAAP1-OE lines had higher concentrations of neutral and acidic amino acids, but lower concentrations of basic amino acids in the straw. An exogenous treatment with neutral amino acids promoted axillary bud outgrowth more strongly in the OE lines than in the WT, RNAi, or Osaap1 lines. Transcriptome analysis of Osaap1 further demonstrated that OsAAP1 may affect N transport and metabolism, and auxin, cytokinin, and strigolactone signaling in regulating rice tillering. Taken together, these results support that increasing neutral amino acid uptake and reallocation via OsAAP1 could improve growth and grain yield in rice.

Abscisic acid and its role in the modulation of plant growth, development, and yield stability

Abscisic acid (ABA) is known to confer stress tolerance; however, at elevated levels it impairs plant growth under prolonged stress. Paradoxically, at its basal level, ABA plays many vital roles in promoting plant growth and development, including modulation of tillering, flowering, and seed development, as well as seed maturation. In this review, we provide insight into novel discoveries of ABA fluxes, ABA signaling responses, and their impact on yield stability. We discuss ABA homeostasis implicated under pre- and postanthesis drought and its impact on productive tillers, grain number determination, and seed development to address yield stability in cereal crops while considering the new knowledge that emerged from the model plant systems.

Photosynthetic leaf area modulates tiller bud outgrowth in sorghum

Shoot branches or tillers develop from axillary buds. The dormancy versus outgrowth fates of buds depends on genetic, environmental and hormonal signals. Defoliation inhibits bud outgrowth indicating the role of leaf-derived metabolic factors such as sucrose in bud outgrowth. In this study, the sensitivity of bud outgrowth to selective defoliation was investigated. At 6 d after planting (6 DAP), the first two leaves of sorghum were fully expanded and the third was partially emerged. Therefore, the leaves were selectively defoliated at 6 DAP and the length of the bud in the first leaf axil was measured at 8 DAP. Bud outgrowth was inhibited by defoliation of only 2 cm from the tip of the second leaf blade. The expression of dormancy and sucrose-starvation marker genes was up-regulated and cell cycle and sucrose-inducible genes was down-regulated during the first 24 h post-defoliation of the second leaf. At 48 h, the expression of these genes was similar to controls as the defoliated plant recovers. Our results demonstrate that small changes in photosynthetic leaf area affect the propensity of tiller buds for outgrowth. Therefore, variation in leaf area and photosynthetic activity should be included when integrating sucrose into models of shoot branching.

The Genetic Architecture of Barley Plant Stature

Plant stature in temperate cereals is predominantly controlled by tillering and plant height as complex agronomic traits, representing important determinants of grain yield. This study was designed to reveal the genetic basis of tillering at five developmental stages and plant height at harvest in 218 worldwide spring barley (Hordeum vulgare L.) accessions under greenhouse conditions. The accessions were structured based on row-type classes [two- vs. six-rowed] and photoperiod response [photoperiod-sensitive (Ppd-H1) vs. reduced photoperiod sensitivity (ppd-H1)]. Phenotypic analyses of both factors revealed profound between group effects on tiller development. To further verify the row-type effect on the studied traits, Six-rowed spike 1 (vrs1) mutants and their two-rowed progenitors were examined for tiller number per plant and plant height. Here, wild-type (Vrs1) plants were significantly taller and had more tillers than mutants suggesting a negative pleiotropic effect of this row-type locus on both traits. Our genome-wide association scans further revealed highly significant associations, thereby establishing a link between the genetic control of row-type, heading time, tillering, and plant height. We further show that associations for tillering and plant height are co-localized with chromosomal segments harboring known plant stature-related phytohormone and sugar-related genes. This work demonstrates the feasibility of the GWAS approach for identifying putative candidate genes for improving plant architecture.

Genetic variation in strigolactone production and tillering in rice and its effect on Striga hermonthica infection

Tillering in cereals is a complex process in the regulation of which also signals from the roots in the form of strigolactones play an important role. The strigolactones are signalling molecules that are secreted into the rhizosphere where they act as germination stimulants for root parasitic plants and hyphal branching factors for arbuscular mycorrhizal fungi. On the other hand, they are also transported from the roots to the shoot where they inhibit tillering or branching. In the present study, the genetic variation in strigolactone production and tillering phenotype was studied in twenty rice varieties collected from all over the world and correlated with S. hermonthica infection. Rice cultivars like IAC 165, IAC 1246, Gangweondo and Kinko produced high amounts of the strigolactones orobanchol, 2'-epi-5-deoxystrigol and three methoxy-5-deoxystrigol isomers and displayed low amounts of tillers. These varieties induced high S. hermonthica germination, attachment, emergence as well as dry biomass. In contrast, rice cultivars such as Super Basmati, TN 1, Anakila and Agee displayed high tillering in combination with low production of the aforementioned strigolactones. These varieties induced only low S. hermonthica germination, attachment, emergence and dry biomass. Statistical analysis across all the varieties confirmed a positive correlation between strigolactone production and S. hermonthica infection and a negative relationship with tillering. These results show that genetic variation in tillering capacity is the result of genetic variation in strigolactone production and hence could be a helpful tool in selecting rice cultivars that are less susceptible to S. hermonthica infection.

Targeted mutagenesis in tetraploid switchgrass (Panicum virgatum L.) using CRISPR/Cas9

The CRISPR/Cas9 system has become a powerful tool for targeted mutagenesis. Switchgrass (Panicum virgatum L.) is a high yielding perennial grass species that has been designated as a model biomass crop by the U.S. Department of Energy. The self-infertility and high ploidy level make it difficult to study gene function or improve germplasm. To overcome these constraints, we explored the feasibility of using CRISPR/Cas9 for targeted mutagenesis in a tetraploid cultivar 'Alamo' switchgrass. We first developed a transient assay by which a non-functional green-fluorescent protein gene containing a 1-bp frameshift insertion in its 5' coding region was successfully mutated by a Cas9/sgRNA complex resulting in its restored function. Agrobacterium-mediated stable transformation of embryogenic calli derived from mature caryopses averaged a 3.0% transformation efficiency targeting the genes of teosinte branched 1(tb1)a and b and phosphoglycerate mutase (PGM). With a single construct containing two sgRNAs targeting different regions of tb1a and tb1b genes, primary transformants (T0) containing CRISPR/Cas9-induced mutations were obtained at frequencies of 95.5% (tb1a) and 11% (tb1b), respectively, with T0 mutants exhibiting increased tiller production. Meanwhile, a mutation frequency of 13.7% was obtained for the PGM gene with a CRISPR/Cas9 construct containing a single sgRNA. Among the PGM T0 mutants, six are heterozygous and one is homozygous for a 1-bp deletion in the target region with no apparent phenotypical alterations. We show that CRISPR/Cas9 system can generate targeted mutagenesis effectively and obtain targeted homozygous mutants in T0 generation in switchgrass, circumventing the need of inbreeding.

Methyl phenlactonoates are efficient strigolactone analogs with simple structure

Strigolactones (SLs) are a new class of phytohormones that also act as germination stimulants for root parasitic plants, such as Striga spp., and as branching factors for symbiotic arbuscular mycorrhizal fungi. Sources for natural SLs are very limited. Hence, efficient and simple SL analogs are needed for elucidating SL-related biological processes as well as for agricultural applications. Based on the structure of the non-canonical SL methyl carlactonoate, we developed a new, easy to synthesize series of analogs, termed methyl phenlactonoates (MPs), evaluated their efficacy in exerting different SL functions, and determined their affinity for SL receptors from rice and Striga hermonthica. Most of the MPs showed considerable activity in regulating plant architecture, triggering leaf senescence, and inducing parasitic seed germination. Moreover, some MPs outperformed GR24, a widely used SL analog with a complex structure, in exerting particular SL functions, such as modulating Arabidopsis roots architecture and inhibiting rice tillering. Thus, MPs will help in elucidating the functions of SLs and are promising candidates for agricultural applications. Moreover, MPs demonstrate that slight structural modifications clearly impact the efficiency in exerting particular SL functions, indicating that structural diversity of natural SLs may mirror a functional specificity.

Low red/far-red ratios delay spike and stem growth in wheat

The responses to low red light/far-red light (R/FR) ratios simulating dense stands were evaluated in wheat (Triticum aestivum L) cultivars released at different times in the 20th century and consequently resulting from an increasingly prolonged breeding and selection history. While tillering responses to the R/FR ratio were unaffected by the cultivars, low R/FR ratios reduced grain yield per plant (primarily grain number and secondarily grain weight per plant) particularly in modern cultivars. Low R/FR ratios delayed spike growth and development, reduced the expression of spike marker genes, accelerated the development of florets already initiated, and reduced the number of fertile florets at anthesis. It is noteworthy that low R/FR ratios did not promote stem or leaf sheath growth and therefore the observed reduction of yield cannot be accounted for as a consequence of divergence of resources towards increased plant stature. It is proposed that the regulation of yield components by the R/FR ratio could help plants to adjust to the limited availability of resources under crop conditions.

Repeat-length variation in a wheat cellulose synthase-like gene is associated with altered tiller number and stem cell wall composition

The tiller inhibition gene (tin) that reduces tillering in wheat (Triticum aestivum) is also associated with large spikes, increased grain weight, and thick leaves and stems. In this study, comparison of near-isogenic lines (NILs) revealed changes in stem morphology, cell wall composition, and stem strength. Microscopic analysis of stem cross-sections and chemical analysis of stem tissue indicated that cell walls in tin lines were thicker and more lignified than in free-tillering NILs. Increased lignification was associated with stronger stems in tin plants. A candidate gene for tin was identified through map-based cloning and was predicted to encode a cellulose synthase-like (Csl) protein with homology to members of the CslA clade. Dinucleotide repeat-length polymorphism in the 5'UTR region of the Csl gene was associated with tiller number in diverse wheat germplasm and linked to expression differences of Csl transcripts between NILs. We propose that regulation of Csl transcript and/or protein levels affects carbon partitioning throughout the plant, which plays a key role in the tin phenotype.

Abscisic acid influences tillering by modulation of strigolactones in barley

Strigolactones (SLs) represent a class of plant hormones that are involved in inhibiting shoot branching and in promoting abiotic stress responses. There is evidence that the biosynthetic pathways of SLs and abscisic acid (ABA) are functionally connected. However, little is known about the mechanisms underlying the interaction of SLs and ABA, and the relevance of this interaction for shoot architecture. Based on sequence homology, four genes (HvD27, HvMAX1, HvCCD7, and HvCCD8) involved in SL biosynthesis were identified in barley and functionally verified by complementation of Arabidopsis mutants or by virus-induced gene silencing. To investigate the influence of ABA on SLs, two transgenic lines accumulating ABA as a result of RNAi-mediated down-regulation of HvABA 8'-hydroxylase 1 and 3 were employed. LC-MS/MS analysis confirmed higher ABA levels in root and stem base tissues in these transgenic lines. Both lines showed enhanced tiller formation and lower concentrations of 5-deoxystrigol in root exudates, which was detected for the first time as a naturally occurring SL in barley. Lower expression levels of HvD27, HvMAX1, HvCCD7, and HvCCD8 indicated that ABA suppresses SL biosynthesis, leading to enhanced tiller formation in barley.

MiR319-targeted OsTCP21 and OsGAmyb regulate tillering and grain yield in rice

Multiple genes and microRNAs (miRNAs) improve grain yield by promoting tillering. MiR319s are known to regulate several aspects of plant development; however, whether miR319s are essential for tillering regulation remains unclear. Here, we report that miR319 is highly expressed in the basal part of rice plant at different development stages. The miR319 knockdown line Short Tandem Target Mimic 319 (STTM319) showed higher tiller bud length in seedlings under low nitrogen (N) condition and higher tiller bud number under high N condition compared with the miR319a-overexpression line. Through targets prediction, we identified OsTCP21 and OsGAmyb as downstream targets of miR319. Moreover, OsTCP21 and OsGAmyb overexpression lines and STTM319 had increased tiller bud length and biomass, whereas both were decreased in OsTCP21 and OsGAmyb knockout lines and OE319a. These data suggest that miR319 regulates rice tiller bud development and tillering through targeting OsTCP21 and OsGAmyb. Notably, the tiller number and grain yield increased in STTM319 and overexpression lines of OsTCP21 and OsGAmyb but decreased in OE319a and knockout lines of OsTCP21 and OsGAmyb. Taken together, our findings indicate that miR319s negatively affect tiller number and grain yield by targeting OsTCP21 and OsGAmyb, revealing a novel function for miR319 in rice.

The tillering phenotype of the rice plastid terminal oxidase (PTOX) loss-of-function mutant is associated with strigolactone deficiency

The significance of plastid terminal oxidase (PTOX) in phytoene desaturation and chloroplast function has been demonstrated using PTOX-deficient mutants, particularly in Arabidopsis. However, studies on its role in monocots are lacking. Here, we report cloning and characterization of the rice (Oryza sativa) PTOX1 gene. Using Ecotype Targeting Induced Local Lesions IN Genomes (EcoTILLING) and TILLING as forward genetic tools, we identified the causative mutation of an EMS mutant characterized by excessive tillering, semi-dwarfism and leaf variegation that corresponded to the PTOX1 gene. The tillering and semi-dwarf phenotypes of the ptox1 mutant are similar to phenotypes of known strigolactone (SL)-related rice mutants, and both phenotypic traits could be rescued by application of the synthetic SL GR24. The ptox1 mutant accumulated phytoene in white leaf sectors with a corresponding deficiency in β-carotene, consistent with the expected function of PTOX1 in promoting phytoene desaturase activity. There was also no accumulation of the carotenoid-derived SL ent-2'-epi-5-deoxystrigol in root exudates. Elevated concentrations of auxin were detected in the mutant, supporting previous observations that SL interaction with auxin is important in shoot branching control. Our results demonstrate that PTOX1 is required for both carotenoid and SL synthesis resulting in SL-deficient phenotypes in rice.

A physiological framework to explain genetic and environmental regulation of tillering in sorghum

Tillering determines the plant size of sorghum (Sorghum bicolor) and an understanding of its regulation is important to match genotypes to prevalent growing conditions in target production environments. The aim of this study was to determine the physiological and environmental regulation of variability in tillering among sorghum genotypes, and to develop a framework for this regulation. Diverse sorghum genotypes were grown in three experiments with contrasting temperature, radiation and plant density to create variation in tillering. Data on phenology, tillering, and leaf and plant size were collected. A carbohydrate supply/demand (S/D) index that incorporated environmental and genotypic parameters was developed to represent the effects of assimilate availability on tillering. Genotypic differences in tillering not explained by this index were defined as propensity to tiller (PTT) and probably represented hormonal effects. Genotypic variation in tillering was associated with differences in leaf width, stem diameter and PTT. The S/D index captured most of the environmental effects on tillering and PTT most of the genotypic effects. A framework that captures genetic and environmental regulation of tillering through assimilate availability and PTT was developed, and provides a basis for the development of a model that connects genetic control of tillering to its phenotypic consequences.

Optimizing tiller production and survival for grain yield improvement in a bread wheat × spelt mapping population

BACKGROUND AND AIMS

Tiller production and survival determine final spike number, and play key roles in grain yield formation in wheat (Triticum aestivum). This study aimed to understand the genetic and physiological basis of the tillering process, and its trade-offs with other yield components, by introducing genetic variation in tillering patterns via a mapping population of wheat × spelt (Triticum spelta).

METHODS

The dynamics of tillering and red/far-red ratio (R:FR) at the base of a canopy arising from neighbouring plants in a bread wheat (Triticum aestivum 'Forno') × spelt (Triticum spelta 'Oberkulmer') mapping population were measured in the field in two growing seasons. Additional thinning and shading experiments were conducted in the field and glasshouse, respectively. Yield components were analysed for all experiments, followed by identification of quantitative trait loci (QTL) associated with each trait.

KEY RESULTS

Large genetic variation in tillering was observed, and more fertile shoots per plant were associated with more total shoots initiated, faster tillering rate, delayed tillering onset and cessation, and higher shoot survival. A total of 34 QTL for tillering traits were identified, and analysis of allelic effects confirmed the above associations. Low R:FR was associated with early tillering cessation, few total shoots, high infertile shoot number and shoot abortion, and these results concurred with the thinning and shading experiments. These effects probably resulted from an assimilate shortage for tiller buds or developing tillers, due to early stem elongation and enhanced stem growth induced by low R:FR. More fertile tillers normally contributed to plant yield and grain number without reducing yield and grain set of individual shoots. However, there was a decrease in grain weight, partly because of smaller carpels and fewer stem water-soluble carbohydrates at anthesis caused by pleiotropy or tight gene linkages.

CONCLUSIONS

Tillering is under the control of both genetic factors and R:FR. Genetic variation in tillering and tolerance to low R:FR can be used to optimize tillering patterns for yield improvement in wheat.

Mutation of ONAC096 Enhances Grain Yield by Increasing Panicle Number and Delaying Leaf Senescence during Grain Filling in Rice

Exploring genetic methods to improve yield in grain crops such as rice (Oryza sativa) is essential to help meet the needs of the increasing population. Here, we report that rice ONAC096 affects grain yield by regulating leaf senescence and panicle number. ONAC096 expression increased rapidly in rice leaves upon the initiation of aging- and dark-induced senescence. Two independent T-DNA insertion mutants (onac096-1 and onac096-2) with downregulated ONAC096 expression retained their green leaf color during natural senescence in the field, thus extending their photosynthetic capacity. Reverse-transcription quantitative PCR analysis showed that ONAC096 upregulated genes controlling chlorophyll degradation and leaf senescence. Repressed OsCKX2 (encoding cytokinin oxidase/dehydrogenase) expression in the onac096 mutants led to a 15% increase in panicle number without affecting grain weight or fertility. ONAC096 mediates abscisic acid (ABA)-induced leaf senescence by upregulating the ABA signaling genes ABA INSENSITIVE5 and ENHANCED EM LEVEL. The onac096 mutants showed a 16% increase in grain yield, highlighting the potential for using this gene to increase grain production.

PIL transcription factors directly interact with SPLs and repress tillering/branching in plants

Tillering is an important parameter of plant architecture in cereal crops. In this study, we identified the PHYTOCHROME-INTERACTING FACTOR-LIKE (PIL) family transcription factors as new repressors of tillering in cereal crops. Using biochemical and genetic approaches, we explore the roles of TaPIL1 in regulating wheat plant architecture. We found that the PIL protein TaPIL1 controls tiller number in wheat. Overexpression of TaPIL1 reduces wheat tiller number; additionally, overexpression of TaPIL1-SUPERMAN repression domain increases wheat tiller number. Furthermore, we show that TaPIL1 activates the transcriptional expression of wheat TEOSINTE BRANCHED1 (TaTB1); moreover, TaPIL1 physically interacts with wheat SQUAMOSA PROMOTER BINDING PROTEIN-LIKE (TaSPL)3/17, which are activators of TaTB1 transcription. In rice, overexpression and loss-of-function mutations of OsPIL11 reduce or increase tiller number by regulating the expression of OsTB1. In Arabidopsis, we demonstrate that PHYTOCHROME-INTERACTING FACTOR 4 interacts with SPL9 to inhibit shoot branching. This study reveals that PIL family transcription factors directly interact with SPLs and play an important role in repressing tillering/branching in plants.

A New Series of Carlactonoic Acid Based Strigolactone Analogs for Fundamental and Applied Research

Strigolactones (SLs) are a group of carotenoid derived plant hormones that play a key role in establishing plant architecture and adapting it to environmental changes, and are involved in plants response to biotic and abiotic stress. SLs are also released into the soil to serve as a chemical signal attracting beneficial mycorrhizal fungi. However, this signal also induces seed germination in root parasitic weeds that represent a major global threat for agriculture. This wide spectrum of biological functions has made SL research one of the most important current topics in fundamental and applied plant science. The availability of SLs is crucial for investigating SL biology as well as for agricultural application. However, natural SLs are produced in very low amounts, and their organic synthesis is quite difficult, which creates a need for efficient and easy-to-synthesize analogs and mimics. Recently, we have generated a set of SL analogs, Methyl Phenlactonoates (MPs), which resemble the non-canonical SL carlactonoic acid. In this paper, we describe the development and characterization of a new series of easy-to-synthesize MPs. The new analogs were assessed with respect to regulation of shoot branching, impact on leaf senescence, and induction of seed germination in different root parasitic plants species. Some of the new analogs showed higher efficiency in inhibiting shoot branching as well as in triggering parasitic seed germination, compared to the commonly used GR24. MP16 was the most outstanding analog showing high activity in different SL biological functions. In summary, our new analogs series contains very promising candidates for different applications, which include the usage in studies for understanding different aspects of SL biology as well as large scale field application for combating root parasitic weeds, such as Striga hermonthica that devastates cereal yields in sub-Saharan Africa.

Homeobox Is Pivotal for OsWUS Controlling Tiller Development and Female Fertility in Rice

OsWUS has recently been shown to be a transcription factor gene critical for tiller development and fertility in rice. The OsWUS protein consists of three conserved structural domains, but their biological functions are still unclear. We discovered a new rice mutant resulting from tissue culture, which hardly produced tillers and exhibited complete female sterility. The male and female floral organs of the mutant were morphologically indistinguishable from those of the wild type. We named the mutant srt1 for completely sterile and reduced tillering 1. Map-based cloning revealed that the mutant phenotypes were caused by a mutation in OsWUS. Compared with the two previously reported null allelic mutants of OsWUS (tab1-1 and moc3-1), which could produce partial N-terminal peptides of OsWUS, the srt1 protein contained a deletion of only seven amino acids within the conserved homeobox domain of OsWUS. However, the mutant phenotypes (monoculm and female sterility) displayed in srt1 were as typical and severe as those in tab1-1 and moc3-1. This indicates that the homeobox domain of SRT1 is essential for the regulation of tillering and sterility in rice. In addition, srt1 showed an opposite effect on panicle development to that of the two null allelic mutants, implying that the srt1 protein might still have partial or even new functions on panicle development. The results of this study suggest that the homeobox domain is pivotal for OsWUS function.

Relationship between gene responses and symptoms induced by Rice grassy stunt virus

Rice grassy stunt virus (RGSV) is a serious threat to rice production in Southeast Asia. RGSV is a member of the genus Tenuivirus, and it induces leaf yellowing, stunting, and excess tillering on rice plants. Here we examined gene responses of rice to RGSV infection to gain insight into the gene responses which might be associated with the disease symptoms. The results indicated that (1) many genes related to cell wall synthesis and chlorophyll synthesis were predominantly suppressed by RGSV infection; (2) RGSV infection induced genes associated with tillering process; (3) RGSV activated genes involved in inactivation of gibberellic acid and indole-3-acetic acid; and (4) the genes for strigolactone signaling were suppressed by RGSV. These results suggest that these gene responses to RGSV infection account for the excess tillering specific to RGSV infection as well as other symptoms by RGSV, such as stunting and leaf chlorosis.

Genetic control of duration of pre-anthesis phases in wheat (Triticum aestivum L.) and relationships to leaf appearance, tillering, and dry matter accumulation

The duration of pre-anthesis developmental phases is of interest in breeding for improved adaptation and yield potential in temperate cereals. Yet despite numerous studies on the genetic control of anthesis (flowering) time and floral initiation, little is known about the genetic control of other pre-anthesis phases. Furthermore, little is known about the effect that changes in the duration of pre-anthesis phases could have on traits related to leaf appearance and tillering, or dry matter accumulation before terminal spikelet initiation (TS). The genetic control of the leaf and spikelet initiation phase (LS; from sowing to TS), the stem elongation phase (SE; from TS to anthesis), and, within the latter, from TS to flag leaf appearance and from then to anthesis, was studied in two doubled-haploid, mapping bread wheat populations, Cranbrook × Halberd and CD87 × Katepwa, in two field experiments (ACT and NSW, Australia). The lengths of phases were estimated from measurements of both TS and the onset of stem elongation. Dry weight per plant before TS, rate of leaf appearance, tillering rate, maximum number of tillers and number of leaves, and dry weight per plant at TS were also estimated in the Cranbrook × Halberd population. More genomic regions were identified for the length of the different pre-anthesis phases than for total time to anthesis. Although overall genetic correlations between LS and SE were significant and positive, independent genetic variability between LS and SE, and several quantitative trait loci (QTLs) with different effects on both phases were found in the two populations. Several of these QTLs (which did not seem to coincide with reported major genes) could be of interest for breeding purposes since they were only significant for either LS or SE. There was no relationship between LS and the rate of leaf appearance. LS was strongly and positively correlated with dry weight at TS but only slightly negatively correlated with early vigour (dry weight before TS). Despite significant genetic correlations between LS and some tillering traits, shortening LS so as to lengthen SE without modifying total time to anthesis would not necessarily reduce tillering capacity, as QTLs for tillering traits did not coincide with those QTLs significant only for LS or SE. Therefore, the study of different pre-anthesis phases is relevant for a better understanding of genetic factors regulating developmental time and may offer new tools for fine-tuning it in breeding for both adaptability and yield potential.

Quantitative Changes in the Transcription of Phytohormone-Related Genes: Some Transcription Factors Are Major Causes of the Wheat Mutant dmc Not Tillering

Tiller number is an important agronomic trait for grain yield of wheat (Triticum aestivum L.). A dwarf-monoculm wheat mutant (dmc) was obtained from cultivar Guomai 301 (wild type, WT). Here, we explored the molecular basis for the restrained tiller development of the mutant dmc. Two bulked samples of the mutant dmc (T1, T2 and T3) and WT (T4, T5 and T6) with three biological replicates were comparatively analyzed at the transcriptional level by bulked RNA sequencing (RNA-Seq). In total, 68.8 Gb data and 463 million reads were generated, 80% of which were mapped to the wheat reference genome of Chinese Spring. A total of 4904 differentially expressed genes (DEGs) were identified between the mutant dmc and WT. DEGs and their related major biological functions were characterized based on GO (Gene Ontology) and KEGG (Kyoto Encyclopedia of Genes and Genomes) categories. These results were confirmed by quantitatively analyzing the expression profiles of twelve selected DEGs via real-time qRT-PCR. The down-regulated gene expressions related to phytohormone syntheses of auxin, zeatin, cytokinin and some transcription factor (TF) families of TALE, and WOX might be the major causes of the mutant dmc, not tillering. Our work provides a foundation for subsequent tiller development research in the future.