How might SLs be biosynthesized? Essay

In 2005, Matusova et al. described that SLs might be biosynthesized from a carotenoid pathway by utilizing a fluridone, and some carotenoid metabolism mutant that are carotenoid biosynthetic inhibitor. In 2008, two mutants of rice, d17 and d10, defective in Carotenoid Cleavage Dioxygenase 7 (CCD7) and CCD8, respectively, and these ccd8/rms1 mutant of pea were shown to be SLdeficient mutants (umehera et al., 2008)(Gomez et al., 2008). This outcome favors the hypothesis that SLs are produced from carotenoid. In addition to CCD7 and CCD8, a cytochrome P450, MAX1 also function in SL biosynthesis (Crawford et al., 2010). A Fecontaining protein, D27 also found as a SL biosynthetic component (lin et al., 2009) [27]. Recently, Alder et al, (2012) identified that CL is a product of sequential reactions by three biosynthetic enzymes, D27, CCD7 and CCD8 using all transbcarotene as a substrate in vitro Alder et al, (2012). In these reactions, D27 catalyze the reversible isomerization of alltransb carotene at C9 position to produce 9cisbcarotene. Initially, Scwartz et al, (2004) stated that CCD7 uses alltransb carotene as a substrate and catalyzes a 9,10 oxidative cleavage to produce alltransb100 carotenal Scwartz et al, (2004) . Alder et al.(2012) revealed that 9cisb carotene cleaved by CCD7 to produce 9cisbapo100 carotenal, which is then cleaved by CCD8 to give CL (Figure 1).
In the last step, the Dring part which is the characteristic portion of SLs, synthesized when the intramolecular rearrangement catalyzed by CCD8. Alder et al.(2012) suggested the reaction mechanism of CCD8 yet, it has not been cleared up. CL is a biosynthetic precursor for SLs because it inhibited the shoot branching of rice SL biosynthetic mutants such as d10 and d27, as well as encouraged the germination of Striga hermonthica seeds Alder et al., (2012). Before the discovery of CL, SLs were biosynthesized by coupling of separately synthesized two portions, the ABCring and Dring parts (Matusova et al., 2005) (Crawford et al., 2010) (lin et al., 2009) (alder et al., 2012) (scwartz et al., 2004) (rani et al., 2008). However, the biochemical examination of these three enzymes shown that one carotenoid molecule produces one SL molecule. Alder et al., (2012). Recently, Seto et al., (2014) successfully identified CL in plant tissues and determined its absolute stereochemistry, using liquid chromatographyquadrupole/ timeofflight tandem mass spectrometry (LCMS/MS). So far, All SLs have the similar stereochemistry at the C20 position (R configuration) natural 5deoxystrigol (5DS) is (+)5DS or ( )20 epi5DS (xie et al., 2013) (ueno et al., 2011) (Figure 1). To convert into the simplest SL (5DS), the CL should be further oxidized at least at two positions (C19 and C7). (Figure1). In these oxidation steps of CL, MAX1 might be involved (Figure 1). The stereochemistry of endogenous CL was determined to be the identical at the corresponding position (11R in CL Figure 1). Moreover, stable isotopelabeled CL ([13C]CL) was transformed into SLs, such as ( )20 epi5DS and orobanchol, in rice plants. For these conversions, only (11R)CL, but not (11S)CL could be a