Specialisation within the DWARF14 protein family confers distinct responses to karrikins and strigolactones in Arabidopsis

Karrikins are butenolides derived from burnt vegetation that stimulate seed germination and enhance seedling responses to light. Strigolactones are endogenous butenolide hormones that regulate shoot and root architecture, and stimulate the branching of arbuscular mycorrhizal fungi. Thus, karrikins and strigolactones are structurally similar but physiologically distinct plant growth regulators. In Arabidopsis thaliana, responses to both classes of butenolides require the F-box protein MAX2, but it remains unclear how discrete responses to karrikins and strigolactones are achieved. In rice, the DWARF14 protein is required for strigolactone-dependent inhibition of shoot branching. Here, we show that the Arabidopsis DWARF14 orthologue, AtD14, is also necessary for normal strigolactone responses in seedlings and adult plants. However, the AtD14 paralogue KARRIKIN INSENSITIVE 2 (KAI2) is specifically required for responses to karrikins, and not to strigolactones. Phylogenetic analysis indicates that KAI2 is ancestral and that AtD14 functional specialisation has evolved subsequently. Atd14 and kai2 mutants exhibit distinct subsets of max2 phenotypes, and expression patterns of AtD14 and KAI2 are consistent with the capacity to respond to either strigolactones or karrikins at different stages of plant development. We propose that AtD14 and KAI2 define a class of proteins that permit the separate regulation of karrikin and strigolactone signalling by MAX2. Our results support the existence of an endogenous, butenolide-based signalling mechanism that is distinct from the strigolactone pathway, providing a molecular basis for the adaptive response of plants to smoke.     

Characterization of MORE AXILLARY GROWTH Genes in Populus

Background

Strigolactones are a new class of plant hormones that play a key role in regulating shoot branching. Studies of branching mutants in Arabidopsis, pea, rice and petunia have identified several key genes involved in strigolactone biosynthesis or signaling pathway. In the model plant Arabidopsis, MORE AXILLARY GROWTH1 (MAX1), MAX2, MAX3 and MAX4 are four founding members of strigolactone pathway genes. However, little is known about the strigolactone pathway genes in the woody perennial plants.

Methodology/Principal Finding

Here we report the identification of MAX homologues in the woody model plant Populus trichocarpa. We identified the sequence homologues for each MAX protein in P. trichocarpa. Gene expression analysis revealed that Populus MAX paralogous genes are differentially expressed across various tissues and organs. Furthermore, we showed that Populus MAX genes could complement or partially complement the shoot branching phenotypes of the corresponding Arabidopsis max mutants.

Conclusion/Significance

This study provides genetic evidence that strigolactone pathway genes are likely conserved in the woody perennial plants and lays a foundation for further characterization of strigolactone pathway and its functions in the woody perennial plants.     

MAX1 encodes a cytochrome P450 family member that acts downstream of MAX3/4 to produce a carotenoid-derived branch-inhibiting hormone

The plant shoot body plan is highly variable, depending on the degree of branching. Characterization of the max1-max4 mutants of Arabidopsis demonstrates that branching is regulated by at least one carotenoid-derived hormone. Here we show that all four MAX genes act in a single pathway, with MAX1, MAX3, and MAX4 acting in hormone synthesis, and MAX2 acting in perception. We propose that MAX1 acts on a mobile substrate, downstream of MAX3 and MAX4, which have immobile substrates. These roles for MAX3, MAX4, and MAX2 are consistent with their known molecular identities. We identify MAX1 as a member of the cytochrome P450 family with high similarity to mammalian Thromboxane A2 synthase. This, with its expression pattern, supports its suggested role in the MAX pathway. Moreover, the proposed enzymatic series for MAX hormone synthesis resembles that of two already characterized signal biosynthetic pathways: prostaglandins in animals and oxilipins in plants.     

Chemical screening of novel strigolactone agonists that specifically interact with DWARF14 protein

Strigolactones (SLs) are a class of plant hormones that regulate shoot branching as well as being known as root-derived signals for parasitic and symbiotic interactions. The physical interaction between SLs and the DWARF14 (D14) receptor family can be examined by differential scanning fluorimetry (DSF) that monitors the changes in protein melting temperature (Tm). The Tm of D14 is lowered by bioactive SLs in DSF analysis. In this report, we screened the compounds that lower the Tm of Arabidopsis D14 (AtD14) as potential candidates for SL agonists using DSF analysis. Subsequent physiological analyzes revealed that 113D10 acts as a novel SL agonist in a D14-dependent manner. Intriguingly, 113D10 has a chemical structure different from natural SLs in that it does not possess an enol ether bond that connects to a methylbutenolide moiety. Moreover, 113D10 does not stimulate seed germination of root parasitic plants. Accordingly, 113D10 can be a useful tool for SL studies and agricultural applications.     

Modulation of hepatocyte nuclear factor-4alpha function by the peroxisome-proliferator-activated receptor-gamma co-activator-1alpha in the acute-phase response

Recent studies with the high-tillering mutants in rice (Oryza sativa), the max (more axillary growth) mutants in Arabidopsis thaliana and the rms (ramosus) mutants in pea (Pisum sativum) have indicated the presence of a novel plant hormone that inhibits branching in an auxin-dependent manner. The synthesis of this inhibitor is initiated by the two CCDs [carotenoid-cleaving (di)oxygenases] OsCCD7/OsCCD8b, MAX3/MAX4 and RMS5/RMS1 in rice, Arabidopsis and pea respectively. MAX3 and MAX4 are thought to catalyse the successive cleavage of a carotenoid substrate yielding an apocarotenoid that, possibly after further modification, inhibits the outgrowth of axillary buds. To elucidate the substrate specificity of OsCCD8b, MAX4 and RMS1, we investigated their activities in vitro using naturally accumulated carotenoids and synthetic apocarotenoid substrates, and in vivo using carotenoid-accumulating Escherichia coli strains. The results obtained suggest that these enzymes are highly specific, converting the C27 compounds beta-apo-10'-carotenal and its alcohol into beta-apo-13-carotenone in vitro. Our data suggest that the second cleavage step in the biosynthesis of the plant branching inhibitor is conserved in monocotyledonous and dicotyledonous species.     

The Arabidopsis MAX pathway controls shoot branching by regulating auxin transport

BACKGROUND: Plants achieve remarkable plasticity in shoot system architecture by regulating the activity of secondary shoot meristems, laid down in the axil of each leaf. Axillary meristem activity, and hence shoot branching, is regulated by a network of interacting hormonal signals that move through the plant. Among these, auxin, moving down the plant in the main stem, indirectly inhibits axillary bud outgrowth, and an as yet undefined hormone, the synthesis of which in Arabidopsis requires MAX1, MAX3, and MAX4, moves up the plant and also inhibits shoot branching. Since the axillary buds of max4 mutants are resistant to the inhibitory effects of apically supplied auxin, auxin and the MAX-dependent hormone must interact to inhibit branching. RESULTS: Here we show that the resistance of max mutant buds to apically supplied auxin is largely independent of the known, AXR1-mediated, auxin signal transduction pathway. Instead, it is caused by increased capacity for auxin transport in max primary stems, which show increased expression of PIN auxin efflux facilitators. The max phenotype is dependent on PIN1 activity, but it is independent of flavonoids, which are known regulators of PIN-dependent auxin transport. CONCLUSIONS: The MAX-dependent hormone is a novel regulator of auxin transport. Modulation of auxin transport in the stem is sufficient to regulate bud outgrowth, independent of AXR1-mediated auxin signaling. We therefore propose an additional mechanism for long-range signaling by auxin in which bud growth is regulated by competition between auxin sources for auxin transport capacity in the primary stem.     

MAX3/CCD7 is a carotenoid cleavage dioxygenase required for the synthesis of a novel plant signaling molecule

BACKGROUND: Plant development is exquisitely environmentally sensitive, with plant hormones acting as long-range signals that integrate developmental, genetic, and environmental inputs to regulate development. A good example of this is in the control of shoot branching, where wide variation in plant form can be generated in a single genotype in response to environmental and developmental cues. RESULTS: Here we present evidence for a novel plant signaling molecule involved in the regulation of shoot branching. We show that the MAX3 gene of Arabidopsis is required for the production of a graft-transmissible, highly active branch inhibitor that is distinct from any of the previously characterized branch-inhibiting hormones. Consistent with its proposed function in the synthesis of a novel signaling molecule, we show that MAX3 encodes a plastidic dioxygenase that can cleave multiple carotenoids. CONCLUSIONS: We conclude that MAX3 is required for the synthesis of a novel carotenoid-derived long-range signal that regulates shoot branching.     

独脚金内酯调控水稻分蘖的研究进展

水稻(Oryza sativa)作为世界上最主要的粮食作物之一, 对其主要农艺性状调控机理的研究具有重要意义。分蘖是水稻生长发育过程中一种特殊的分枝, 它不仅是与水稻产量密切相关的重要农艺性状, 也是揭示高等植物侧枝生长发育机制的理想模型。独脚金内酯(strigolactone, SL)是一类新型植物激素, 能够抑制植物分枝的生长发育。近年来, 关于SL合成与信号在调控水稻分蘖方面的研究取得了重要进展, 但对其信号转导的下游组分的研究还相对匮乏。该文综述了SL合成途径、信号途径及下游靶基因调控水稻分蘖的研究进展, 并与在拟南芥(Arabidopsis thaliana)、豌豆(Pisum sativum)和矮牵牛(Petunia hybrida)中的研究进行了比较, 同时还对如何挖掘SL途径的新组分进行了讨论。     

The genetic architecture of shoot branching in Arabidopsis thaliana: a comparative assessment of candidate gene associations vs. quantitative trait locus mapping

Association mapping focused on 36 genes involved in branch development was used to identify candidate genes for variation in shoot branching in Arabidopsis thaliana. The associations between four branching traits and moderate-frequency haplogroups at the studied genes were tested in a panel of 96 accessions from a restricted geographic range in Central Europe. Using a mixed-model association-mapping method, we identified three loci--MORE AXILLARY GROWTH 2 (MAX2), MORE AXILLARY GROWTH 3 (MAX3), and SUPERSHOOT 1 (SPS1)--that were significantly associated with branching variation. On the basis of a more extensive examination of the MAX2 and MAX3 genomic regions, we find that linkage disequilibrium in these regions decays within approximately 10 kb and trait associations localize to the candidate genes in these regions. When the significant associations are compared to relevant quantitative trait loci (QTL) from previous Ler x Col and Cvi x Ler recombinant inbred line (RIL) mapping studies, no additive QTL overlapping these candidate genes are observed, although epistatic QTL for branching, including one that spans the SPS1, are found. These results suggest that epistasis is prevalent in determining branching variation in A. thaliana and may need to be considered in linkage disequilibrium mapping studies of genetically diverse accessions.     

The Interactions among DWARF10, Auxin and Cytokinin Underlie Lateral Bud Outgrowth in Rice

Previous studies have shown that DWARF10 (D10) is a rice ortholog of MAX4/RMS1/DAD1, encoding a carotenoid cleavage dioxygenase and functioning in strigolactones/strigolactone-derivatives (SL)biosynthesis. Here we use D10- RNA interference (RNAi) transgenic plants similar to d10 mutant in phenotypes to investigate the interactions among D10, auxin and cytokinin in regulating rice shoot branching. Auxin levels in node 1 of both decapitated D10-RNAi and wild type plants decreased significantly, showing that decapitation does reduce endogenous auxin concentration, but decapitation has no clear effects on auxin levels in node 2 of the same plants. This implies that node 1 may be the location where a possible interaction between auxin and D10 gene would be detected. D10 expression in node 1 is inhibited by decapitation, and this inhibition can be restored by exogenous auxin application,indicating that D10 may play an important role in auxin regulation of SL. The decreased expression of most OsPINs in shoot nodes of D10- RNAi plants may cause a reduced auxin transport capacity.Furthermore, effects of auxin treatment of decapitated plants on the expression of cytokinin biosynthetic genes suggest that D10 promotes cytokinin biosynthesis by reducing auxin levels. Besides, in D10- RNAi plants, decreased storage cytokinin levels in the shoot node may partly account for the increased active cytokinin contents, resulting in more tillering phenotypes.