Pollen germination on the surface of compatible stigmatic tissues is an essential step for plant fertilization. Here we report that the Arabidopsis mutant bcl1 is male sterile as a result of the failure of pollen germination. We show that the bcl1 mutant allele cannot be transmitted by male gametophytes and no homozygous bcl1 mutants were obtained. Analysis of pollen developmental stages indicates that the bcl1 mutation affects pollen germination but not pollen maturation. Molecular analysis demonstrates that the failure of pollen germination was caused by the disruption of AtBECLIN 1. AtBECLIN 1 is expressed predominantly in mature pollen and encodes a protein with significant homology to Beclin1/Atg6/Vps30 required for the processes of autophagy and vacuolar protein sorting (VPS) in yeast. We also show that AtBECLIN 1 is required for normal plant development, and that genes related to autophagy, VPS and the glycosylphosphatidylinositol anchor system, were affected by the deficiency of AtBECLIN 1.

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1-Deoxy-d-xylulose-5-phosphate reductoisomerase (DXR) is an important enzyme involved in the 2-C-methyl-d-erythritol-4-phosphate (MEP) pathway which provides the basic five-carbon units for isoprenoid biosynthesis. To investigate the role of the MEP pathway in plant development and metabolism, we carried out detailed analyses on a dxr mutant (GK_215C01) and two DXR transgenic co-suppression lines, OX-DXR-L2 and OX-DXR-L7. We found that the dxr mutant was albino and dwarf. It never bolted, had significantly reduced number of trichomes and most of the stomata could not close normally in the leaves. The two co-suppression lines produced more yellow inflorescences and albino sepals with no trichomes. The transcription levels of genes involved in trichome initiation were found to be strongly affected, including GLABRA1, TRANSPARENT TESTA GLABROUS 1, TRIPTYCHON and SPINDLY, expression of which is regulated by gibberellic acids (GAs). Exogenous application of GA 3 could partially rescue the dwarf phenotype and the trichome initiation of dxr, whereas exogenous application of abscisic acid (ABA) could rescue the stomata closure defect, suggesting that lower levels of both GA and ABA contribute to the phenotype in the dxr mutants. We further found that genes involved in the biosynthetic pathways of GA and ABA were coordinately regulated. These results indicate that disruption of the plastidial MEP pathway leads to biosynthetic deficiency of photosynthetic pigments, GAs and ABA, and thus the developmental abnormalities, and that the flux from the cytoplasmic mevalonate pathway is not sufficient to rescue the deficiency caused by the blockage of the plastidial MEP pathway. These results reveal a critical role for the MEP biosynthetic pathway in controlling the biosynthesis of isoprenoids.

SUMMARYCytosolic acetyl-CoA is involved in the synthesis of a variety of compounds, including waxes, sterols and rubber, and is generated by the ATP citrate lyase (ACL). Plants over-expressing ACL were generated in an effort to understand the contribution of ACL activity to the carbon flux of acetyl-CoA to metabolic pathways occurring in the cytosol. Transgenic Arabidopsis plants synthesizing the polyester polyhydroxybutyrate (PHB) from cytosolic acetyl-CoA have reduced growth and wax content, consistent with a reduction in the availability of cytosolic acetyl-CoA to endogenous pathways. Increasing the ACL activity via the over-expression of the ACLA and ACLB subunits reversed the phenotypes associated with PHB synthesis while maintaining polymer synthesis. PHB production by itself was associated with an increase in ACL activity that occurred in the absence of changes in steady-state mRNA or protein level, indicating a post-translational regulation of ACL activity in response to sink strength. Over-expression of ACL in Arabidopsis was associated with a 30% increase in wax on stems, while over-expression of a chimeric homomeric ACL in the laticifer of roots of dandelion led to a four-and two-fold increase in rubber and triterpene content, respectively. Synthesis of PHB and over-expression of ACL also changed the amount of the cutin monomer octadecadien-1,18-dioic acid, revealing an unsuspected link between cytosolic acetyl-CoA and cutin biosynthesis. Together, these results reveal the complexity of ACL regulation and its central role in influencing the carbon flux to metabolic pathways using cytosolic acetyl-CoA, including wax and polyisoprenoids.

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Salicylic acid methyltransferase (SAMT), benzoic acid methyltransferase (BAMT) and theobromine methyltransferase (TH) (henceforth, SABATH) family proteins belong to a unique class of methyltransferase that can methylate small molecular compounds including indole-3-acidic acid (IAA), salicylic acid (SA) and jasmonic acid (JA), in plants. Here we report that the GAMT2 protein, which has 34.2% similarity with IAMT1 in the amino acid sequence, can methylate gibberellic acid (GA). Bioinformatics analysis suggests that GAMT2 may be able to methylate one molecule larger than SA. GAMT2 is predominantly expressed in the developing seed embryo and endosperm in Arabidopsis. During seed germination, the expression of GAMT2 decreases until the cotyledons expand out of the seed coat. Overexpression of GAMT2 in Arabidopsis resulted in multiple phenotypes, including dwarfism, retarded growth, late flowering, and reduced fertility, which are similar to the phenotypes of GA-deficient mutants. Seed germination assay showed that GAMT2 overexpression in plants was hypersensitive to GA biosynthesis inhibitor (ancymidol) and abscisic acid (ABA) treatments, whereas the GAMT2 null mutant (SALK_075450) was slightly insensitive to such treatments, suggesting that GAMT2 may methylate GA or ABA. Enzyme activity analysis indicated that GAMT2 was able to methylate GA 3 into Methyl-GA 3 in vitro, but could not methylate ABA. Microarray analysis on GAMT2 overexpression plants suggested that Methyl-GA may be an inactive form of GA in Arabidopsis. These data suggest that GAMT2 is involved in seed maturation and germination by modulating GA activity.

In plants, one of the most common modifications of secondary metabolites is methylation catalyzed by various methyltransferases. Recently, a new class of methyltransferases, the SABATH family of methyltransferases, was found to modify phytohormones and other small molecules. The SABATH methyltransferases share little sequence similarity with other well characterized methyltransferases. Arabidopsis has 24 members of the SABATH methyltransferase genes, and a subset of them has been shown to catalyze the formation of methyl esters with phytohormones and other small molecules. Physiological and genetic analyses show that methylation of phytohormones plays important roles in regulating various biological processes in plants, including stress responses, leaf development, and seed maturation/germination. In this review, we focus on phytohormone methylation by the SABATH family methyltransferases and the implication of these modifications in plant development.phytohormone, SABATH family methyltransferase, methylation, AtJMT, AtBSMT, AtIAMT1, AtGAMT1, AtGAMT2, enzyme activity Citation:Qu L J, Li S, Xing S F. Methylation of phytohormones by the SABATH methyltransferases.Methylation is a key modification that plays important roles in many biological processes including gene regulation and metabolic control. In plants, methylation is far more widespread. Methylation of small organic compounds is regarded as one of the most common endogenous modifications in plants. Most specialized plant metabolites are converted to their ester forms by methylation. Methylated compounds make up the assistant components of the cell wall, and are also found in the scent of different species of flowers and fragrant herbs [1−4]. Methylated chemicals are involved in different plant developmental or defense processes, including cell expansion, seed maturation, and interand intracellular plant signal transduction. The methylation reaction is known to be catalyzed by methyltransferases (MTs), which belong to the MT superfamily [5]. MTs are grouped into 4 types, i.e. C-, N-, S-and O-methyltransferases, based on the atom to which the methyl group is attached. MTs are also classified into other groups, e.g., SMOMT, HCCoA OMT, and NMT [6,7], on the basis of sequence similarity and the substrates catalyzed by the enzymes. A new family of MTs has recently been identified and designated as the SABATH family. The SABATH family is named after the first identified enzymes (SAMT, BAMT and Theobromine synthase) in this family [8−10]. All SABATH proteins characterized thus far have a molecular mass ranging from 40 to 49 kD. Chromatographic evidence indicates that the native enzyme functions as a dimer [5]. All the proteins have been found to act in the cytoplasm, but no detailed sub-cellular localization has been studied for any of the SABATH methyltransferases [11]. Like most of the other methyltransferases, the SABATH methyltransferases' activity is also dependent on S-adenosyl-Lmethionine (SAM) to provide the methyl group (Figure 1).The SABATH family is uniqu...