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During the terminal stage of stomatal development, the R2R3‐MYB transcription factors FOUR LIPS (FLP/MYB124) and MYB88 limit guard mother cell division by repressing the transcript levels of multiple cell‐cycle genes. In Arabidopsis thaliana possessing the weak allele flp‐1, an extra guard mother cell division results in two stomata having direct contact. Here, we identified an ethylmethane sulfonate‐mutagenized mutant, flp‐1 xs01c, which exhibited more severe defects than flp‐1 alone, producing giant tumor‐like cell clusters. XS01C, encoding F‐BOX STRESS‐INDUCED 4 (FBS4), is preferentially expressed in epidermal stomatal precursor cells. Overexpressing FBS4 rescued the defective stomatal phenotypes of flp‐1 xs01c and flp‐1 mutants. The deletion or substitution of a conserved residue (Proline166) within the F‐box domain of FBS4 abolished or reduced, respectively, its interaction with Arabidopsis Skp1‐Like1 (ASK1), the core subunit of the Skp1/Cullin/F‐box E3 ubiquitin ligase complex. Furthermore, the FBS4 protein physically interacted with CYCA2;3 and induced its degradation through the ubiquitin‐26S proteasome pathway. Thus, in addition to the known transcriptional pathway, the terminal symmetric division in stomatal development is ensured at the post‐translational level, such as through the ubiquitination of target proteins recognized by the stomatal lineage F‐box protein FBS4. [ABSTRACT FROM AUTHOR]

Proteomic analysis was applied to generating the map of Arabidopsis mature pollen proteins and analyzing the differentially expressed proteins that are potentially involved in the regulation of Arabidopsis pollen germination. By applying 2-D electrophoresis and silver staining, we resolved 499 and 494 protein spots from protein samples extracted from pollen grains and pollen tubes, respectively. Using the matrix-assisted laser desorption ionization time-of-flight mass spectrometry method, we identified 189 distinct proteins from 213 protein spots expressed in mature pollen or pollen tubes, and 75 new identified proteins that had not been reported before in research into the Arabidopsis pollen proteome. Comparative analysis revealed that 40 protein spots exhibit reproducible significant changes between mature pollen and pollen tubes. And 21 proteins from 17 downregulated and six upregulated protein spots were identified. Functional category analysis indicated that these differentially expressed proteins mainly involved in signaling, cellular structure, transport, defense/stress responses, transcription, metabolism, and energy production. The patterns of changes at protein level suggested the important roles for energy metabolism-related proteins in pollen tube growth, accompanied by the activation of the stress response pathway and modifications to the cell wall. [ABSTRACT FROM AUTHOR]

Phosphorous is one of the essential mineral elements for plant growth and development. Typically, the shoots of plant seedlings usually turn a dark-brown or purple colour under low-Pi stress. Using protein 2-D gel and peptide mass fingerprinting mapping (PMF) methods, a cytoplasmic glyceralde- hyde-3-phosphate dehydrogenase GapC-2 was identified as a low-Pi responsive protein in Arabidopsis plants. Expression of AtGapC-2 protein was significantly decreased after 4 d of low-Pi stress. Two in- dependent T-DNA insertion lines of GapC-2 gene (At1g13440) showed a hypersensitive phenotype in response to low-Pi stress compared with wild type plants, while the transgenic complementation lines of the mutants showed a similar phenotype to the wild type. These results indicate that AtGapC-2 may play an important role in Arabidopsis responses to low-Pi stress, possibly by regulation of glycoly- sis-associated "Pi-pool" and accumulation of anthocyanin pigments in plants. [ABSTRACT FROM AUTHOR]