Figures (3)  Tables (1)

    • Figure 1. 

      The Yang cycle in plants. S-adenosyl-methionine (SAM) is first derived from methionine by SAM synthetase (SAMS), SAM then is converted to 5'-S-methyl-5'-thioadenosine (MTA) and 1-aminocyclopropane-1-carboxylic acid (ACC) by ACC synthase (ACS). MTA is then depurinated to 5-methylthioribose (MTR) by MTA nucleosidase (MTN). MTR is subsequently phosphorylated to 5-methylthioribose-1-phosphate (MTR-P), catalyzed by MTR kinase (MTK) in the presence of adenosine triphosphate (ATP). MTR-P gets isomerization to yield 5-methylthioribulose-1-phosphate (MTRu-P) by MTR-P isomerase (MTI). MTRu-P then is metabolized to 1,2-dihidroxy-3-keto-5-methylthiopentene (DHKMP) by dehydratase-enolase-phosphatase (DEP). DHKMP is converted to 2-keto-4-methylthiobutyrate (KMTB) by acireductone dioxygenase (ARD) in the penultimate step. At last, KMTB turns to methionine by an unknown aminotransferase (AT).

    • Figure 2. 

      The metabolism and homeostasis of methionine and S-adenosyl-methionine (SAM) in plants. Methionine can be de novo synthesized from O-phosphohomoserine (OPH), in which cystathionine γ-synthase (CGS) competes with threonine synthase (TS) for OPH. CGS metabolizes OPH and cysteine to cystathionine which is then converted to homocysteine by cystathionine β-lyase (CBL). Methionine is produced from homocysteine by methionine synthase (MS). Methionine can be metabolized to methanethiol, α-ketobutyrate and ammonia by methionine γ-lyase (MGL). It also can be converted to S-methylmethionine (SMM) by methionine S-methyltransferase (MMT). The conversion of SAM to S-adenosylhomocysteine (SAH) by diverse methyltransferases (MTs) provides methyl moiety for acceptors including histone, DNA, and RNA.

    • Figure 3. 

      The enzyme proteins of the Yang cycle in several major plant lineages. The Yang cycle proteins in each species were identified using BLAST with an E-value threshold of 1e−5, employing the default settings of TBtools-II. Searches were conducted with the amino acid sequences of each enzyme from Arabidopsis thaliana as references. The numbers indicate enzyme paralogs. Gray boxes indicate an absence of enzyme paralog.

    • Enzyme Plant species Main content Ref.
      SAMS Arabidopsis thaliana Excessive methionine accumulates in the mto3-1 and mto3-2 mutants [61,62]
      Overexpressing SAMS is morphologically indistinguishable from wild-type plants, or leads to abnormal floral organ development [62,65]
      SAMS is phosphorylated by CDPK28 [74]
      Nicotiana tabacum Suppressing SAMS renders accumulation of methanethiol and methionine [63]
      Rice Suppressing OsSAMS1, 2 and 3 causes pleiotropic phenotypes [64]
      OsSAMS1 functions as a regulator for grain size and yield [67]
      OsWAK112 interacts with OsSAMS1, 2 and 3 [72]
      OsSAMS1 interacts with OsLCD3 [76]
      OsSAMS1 is targeted by F-box protein OsFBK12 [77]
      Pumpkin SAMS interact with a long non-coding RNA with promoted stability [76]
      Tomato SlSAMS1 influences fruit ripening and it interacts with FERONIA-like [79]
      ACS Arabidopsis thaliana The expression of ACS genes are not vascular-specific [6,83]
      ACS has an additional Cβ-S lyase activity [84]
      MTN Lupinus luteus Enzyme is purified from seed [86]
      Arabidopsis thaliana MTN1 and MTN2 exhibit diverse expression patterns, preferentially in vascular tissues [6]
      The cystal structures of MTN1 and MTN2 are determined [88,89]
      mtn1 and mtn2 single and double mutants are characterized [42,43]
      Rice OsMTN recombinant enzyme is characterized [90]
      Maize ZmMTN1 mutant is linked with Fe and NA hemeostasis [91]
      Apple MTN activity is first detected in fruit extract [85]
      Tomato SlMTN expression and activity are characterized in fruit [28,48]
      MTK Lupinus luteus MTK activity is partially purified from seed [93]
      Rice OsMTK1 and OsMTK2 are cloned and evaluated under sulfur deficiency [94]
      Arabidopsis thaliana MTK is preferentially expressed in phloem [6]
      MTK is cloned and T-DNA insertional mutants are characterized [94]
      An eto3mtk double mutant is evaluated [95]
      Tomato SlMTK expression and activity are characterized in fruit [28,48,96]
      MTI Arabidopsis thaliana MTI is cloned and shows phloem-specific in expression [6]
      The functions of MTI in sulfur metabolism during flowering and seed development is evaluated [99]
      DEP Arabidopsis thaliana DEP is cloned and its expression also shows phloem-specific [6]
      The functions of DEP in sulfur metabolism during flowering and seed development is evaluated [99]
      Apple MdDEP1 is ectopically expressed in Arabidopsis, enhancing stress tolerance and flowering [100]
      The expression of MdDEP1 is regulated by MdBHLH3, and the activity is affected by MdY3IP1 [101,102]
      ARD Rice ARD has dual enzymatic activity with different binding metals, OsARD1 is induced by submergence and ethylene [107]
      OsARD1 promotes stress tolerance [110]
      Potato StARD1 is wounding responsive [109]
      Arabidopsis thaliana ARD1 function in hypocotyl growth is evaluated, ARD1 is an effector for AGB1 [111]
      Tomato The expression of SlARD1 and SlARD2 is characterized in fruit [28]
      Apple The physiological roles of an apple ARD gene are investigated by ectopically expressing in tomato plant [113]
      AT Tomato and maize GTK is proposed to convert KMTB to methionine [115]

      Table 1. 

      Studies on the enzymes involved in the Yang cycle in plants.