Specialisation within the DWARF14 protein family confers distinct responses to karrikins and strigolactones in Arabidopsis Bioactive fractions from the pasture legume Biserrula pelecinus L. have an anti-methanogenic effect against key rumen methanogens View project Pure Culture on Biserrula fractions View project

  • Scaffidi A
  • Flematti G
  • Dixon K
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INTRODUCTION Seed germination is a critical event in the plant life cycle. Many species exhibit physiological seed dormancy, which limits germination under favourable but transient environmental conditions that may not support long-term survival (Finkelstein et al., 2008). Wildfires present a brief opportunity for plants to exploit reduced competition for light, water and nutrients, and the dormant seed of a wide taxonomic range of species exhibit enhanced germination following smoke exposure (Roche et al., 1997; Chiwocha et al., 2009). The butenolide 3-methyl-2H-furo[2,3-c]pyran-2-one, or KAR 1 , was identified in smoke as a bioactive compound that defines a family of related molecules known as karrikins (Flematti et al., 2004). Karrikins are potent germination stimulants, which are active in some species at concentrations as low as 1 nM (Flematti et al., 2004; Long et al., 2010). They also increase the sensitivity of seedlings to light, potentially enhancing seedling establishment and survival in the post-fire environment (Nelson et al., 2010). Elucidating the molecular genetic basis for responses to karrikins is key to exploiting their potential application in restoration ecology and agriculture (Daws et al., 2007; Stevens et al., 2007). Germination of dormant Arabidopsis thaliana seed is generally promoted by karrikins, although karrikins cannot overcome the germination requirement for light and de novo synthesis of the phytohormone gibberellin (Nelson et al., 2009). To discover the molecular components of karrikin perception in Arabidopsis, we initiated a genetic screen for karrikin insensitive (kai) mutants. Two such mutants, exhibiting increased seed dormancy that could not be alleviated by karrikins, carried mutations in the MAX2 gene. Additional max2 alleles also conferred increased dormancy and insensitivity to karrikins, confirming that MAX2 is required for karrikin responses (Nelson et al., 2011). MAX2 is most renowned for its role in mediating responses to strigolactones, a class of plant-synthesized compounds that are exuded from roots, triggering the germination of parasitic weeds and promoting hyphal branching in arbuscular mycorrhizal fungi (Yoneyama et al., 2007; Dor et al., 2011a; Dor et al., 2011b). Recently, strigolactones were shown to be endogenous plant hormones that inhibit the outgrowth of axillary buds (Sorefan et al., 2003; Gomez-Roldan et al., 2008; Umehara et al., 2008) and influence root architecture (Kapulnik et al., 2011; Koltai, 2011; Ruyter-Spira et al., 2011). Two carotenoid-cleavage dioxygenases, CCD7/MAX3 and CCD8/MAX4, as well as a cytochrome P450, MAX1, are involved in strigolactone biosynthesis in Arabidopsis. Mutations in orthologous genes in rice, petunia and pea indicate that strigolactone control of shoot branching is conserved in angiosperms (Sorefan et al., 2003; Snowden et al., 2005; Arite et al., 2007). All of the max mutants share an increased shoot branching phenotype, but only max2 is strigolactone insensitive, implying that MAX2 is involved in the strigolactone response. Both karrikins and strigolactones induce similar effects at the germination and seedling stages in a

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  • Adrian Scaffidi

  • Gavin Flematti

  • Kingsley Dixon

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