During plant reproduction, the central cell of the female gametophyte becomes fertilized to produce the endosperm, a storage tissue that nourishes the developing embryo within the seed. The molecular mechanisms controlling the specification and differentiation of the central cell are poorly understood. We identified a female gametophyte mutant in Arabidopsis thaliana, fem111, that is affected in central cell development. In fem111 female gametophytes, the central cell's nucleolus and vacuole fail to mature properly. In addition, endosperm development is not initiated after fertilization of fem111 female gametophytes. fem111 contains a T-DNA insertion in AGAMOUS-LIKE80 (AGL80). FEM111/AGL80 is a member of the MADS box family of genes that likely encode transcription factors. An AGL80–green fluorescent protein fusion protein is localized to the nucleus. Within the ovule and seed, FEM111/AGL80 is expressed exclusively in the central cell and uncellularized endosperm. FEM111/AGL80 expression is also detected in roots, leaves, floral stems, anthers, and young flowers by real-time RT-PCR. FEM111/AGL80 is required for the expression of two central cell–expressed genes, DEMETER and DD46, but not for a third central cell–expressed gene, FERTILIZATION-INDEPENDENT SEED2. Together, these data suggest that FEM111/AGL80 functions as a transcription factor within the central cell gene regulatory network and controls the expression of downstream genes required for central cell development and function.
The synergid cells within the female gametophyte are essential for reproduction in angiosperms. MYB98 encodes an R2R3-MYB protein required for pollen tube guidance and filiform apparatus formation by the synergid cells. To test the predicted function of MYB98 as a transcriptional regulator, we determined its subcellular localization and examined its DNA binding properties. We show that MYB98 binds to a specific DNA sequence (TAAC) and that a MYB98-green fluorescent protein fusion protein localizes to the nucleus, consistent with a role in transcriptional regulation. To identify genes regulated by MYB98, we tested previously identified synergid-expressed genes for reduced expression in myb98 female gametophytes and identified 16 such genes. We dissected the promoter of one of the downstream genes, DD11, and show that it contains a MYB98 binding site required for synergid expression, suggesting that DD11 is regulated directly by MYB98. To gain insight into the functions of the downstream genes, we chose five genes and determined the subcellular localization of the encoded proteins. We show that these five proteins are secreted into the filiform apparatus, suggesting that they play a role in either the formation or the function of this unique structure. Together, these data suggest that MYB98 functions as a transcriptional regulator in the synergid cells and activates the expression of genes required for pollen tube guidance and filiform apparatus formation.
SUMMARYCytokinins are a class of mitogenic plant hormones that play an important role in most aspects of plant development, including shoot and root growth, vascular and photomorphogenic development and leaf senescence. A model for cytokinin perception and signaling has emerged that is similar to bacterial twocomponent phosphorelays. In this model, binding of cytokinin to the extracellular domain of the Arabidopsis histidine kinase (AHKs) receptors induces autophosphorylation within the intracellular histidine-kinase domain. The phosphoryl group is subsequently transferred to cytosolic Arabidopsis histidine phosphotransfer proteins (AHPs), which have been suggested to translocate to the nucleus in response to cytokinin treatment, where they then transfer the phosphoryl group to nuclear-localized response regulators (Type-A and Type-B ARRs). We examined the effects of cytokinin on AHP subcellular localization in Arabidopsis and, contrary to expectations, the AHPs maintained a constant nuclear/cytosolic distribution following cytokinin treatment. Furthermore, mutation of the conserved phosphoacceptor histidine residue of the AHP, as well as disruption of multiple cytokinin signaling elements, did not affect the subcellular localization of the AHP proteins. Finally, we present data indicating that AHPs maintain a nuclear/cytosolic distribution by balancing active transport into and out of the nucleus. Our findings suggest that the current models indicating relocalization of AHP protein into the nucleus in response to cytokinin are incorrect. Rather, AHPs actively maintain a consistent nuclear/cytosolic distribution regardless of the status of the cytokinin response pathway.
To gain insight into the processes controlling leaf development, we characterized an Arabidopsis mutant, varicose(vcs), with leaf and shoot apical meristem defects. The vcsphenotype is temperature dependent; low temperature growth largely suppressed defects, whereas high growth temperatures resulted in severe leaf and meristem defects. VCS encodes a putative WD-domain containing protein,suggesting a function involving protein-protein interactions. Temperature shift experiments indicated that VCS is required throughout leaf development,but normal secondary vein patterning required low temperature early in leaf development. The low-temperature vcs phenotype is enhanced in axr1-3 vcs double mutants and in vcs mutants grown in the presence of polar auxin transport inhibitors, however, vcs has apparently normal auxin responses. Taken together, these observations suggest a role for VCS in leaf blade formation.
SummaryThe female gametophyte contains two synergid cells that play a role in many steps of the angiosperm reproductive process, including pollen tube guidance. At their micropylar poles, the synergid cells have a thickened and elaborated cell wall: the filiform apparatus that is thought to play a role in the secretion of the pollen tube attractant(s). MYB98 regulates an important subcircuit of the synergid gene regulatory network (GRN) that functions to activate the expression of genes required for pollen tube guidance and filiform apparatus formation. The MYB98 subcircuit comprises at least 83 downstream genes, including 48 genes within four gene families (CRP810, CRP3700, CRP3730 and CRP3740) that encode Cys-rich proteins. We show that the 11 CRP3700 genes, which include DD11 and DD18, are regulated by a common cis-element, GTAACNT, and that a multimer of this sequence confers MYB98-dependent synergid expression. The GTAACNT element contains the MYB98-binding site identified in vitro, suggesting that the 11 CRP3700 genes are direct targets of MYB98. We also show that five of the CRP810 genes, which include DD2, lack a functional GTAACNT element, suggesting that they are not directly regulated by MYB98. In addition, we show that the five CRP810 genes are regulated by the cis-element AACGT, and that a multimer of this sequence confers synergid expression. Together, these results suggest that the MYB98 branch of the synergid GRN is multi-tiered and, therefore, contains at least one additional downstream transcription factor.
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