At 18 hours after infection by the fungal pathogen, powdery mildew (Erysiphe cichoracearum), a leaf sample was stained with propidium iodide to highlight fungal structures. The plant plasma membrane is visualized with a GFP-tagged marker (Q8, arrows). The image shown is a z-series projection.

The haustorium (H) is a fungal feeding structure formed in the plant epidermal cell (Ep) about 20 hours after penetration by an appressorium (A). The mature haustorium has expanded lobe structures (arrowheads) and a nucleus (nc). A host-derived extrahaustorial membrane (EHM) encases the haustorium separating it from plant cytoplasm (dotted line). The characteristics of the EHM are largely unknown. Biochemical evidence suggests that the EHM is a unique membrane, different from the plant plasma membrane. The discontinuity of the plasma membrane protein::GFP signal at the haustorial neck (yellow arrow), where the EHM starts, provides the clear optical evidence suporting the distinct nature of the EHM. A fungal conidium is also visible in this image.

Advances in synthetic chemistry have made it possible to create numerous small molecules with potential biological activity. A library of 10,000 unique compounds was screened for effects on root growth. This compound induced the formation of lateral roots at the root/hypocotyl junction of a 7-day old Arabidopsis seedling. Two well-formed lateral roots are visible at the lower left immediately below a fused set of lateral roots. The curled tube-like structures on the right are root hairs and root epidermal cells that have peeled away as the lateral roots emerged from the internal tissues.

Anthocyanin pigments provide most of the red, purple, and blue colors of flowers, and this pigmentation provides a natural "reporter gene" assay for the activities of transposable elements in many plants. Here a transposon disrupts the an9 gene of Petunia; an9 encodes a glutathione S-transferase that acts as a carrier protein for anthocyanins in the cytoplasm. The plant is an an9-mutable homozygote. The allele with the transposon lacks the AN9 protein, hence anthocyanin accumulates in the cytoplasm where it confers only a pale pink color. In some cells, the transposon excises from the mutable an9 allele in a manner that restores gene function. In cells with functional AN9 the pigment is transferred to the vacuole; in this acidic cellular compartment the pigmentation is much more intense and red. The timing of transposon excision can be assessed by the size of the sectors: most excisions occur late in development and result in small red sectors because there are only a few cell divisions of the revertant cell. In this particular flower, however, an excision event occurred early in plant life such that half of the floral meristem that produced this flower was revertant. As a consequence, some progeny seed collected from this flower will be true-breeding red. (Walbot Lab)

The BRONZE2 protein of maize encodes a glutathione S-transferase that performs the same carrier protein function as AN9 in Petunia. BZ2 and AN9 are an interesting case of convergent evolution of function: the two proteins are only 11% similar yet they can reciprocally complement. More closely related GST proteins of either Petunia or maize fail to complement in the original host plant or in transient assays in the alternate host.

An interesting aspect of host control of Mu transposons is illustrated in this figure. Using a mutable bz2::Mu reporter allele only small revertant sectors are observed. This is in contrast with the Petunia transposon behavior that can excise at any time during plant development. Although the transcripts encoding transposase and the transposase protein are constitutively expressed, excision activity is restricted to the final few cell divisions throughout the plant. Does the host confer developmental timing by altering the transposase protein, by expressing gene products required for transposition only late in development, by changing chromatin configuration during cellular differentiation? These questions can be generalized to all aspects of cellular differentiation -- in a field of cells progressing through a developmental program, how do the cells know that only a few more cell divisions will occur? How do plant organs and tissues achieve a final cell number, despite the variation in the actual number of cells within successive organs such as leaves? Another important aspect of somatic excision activity is that in most cases Mu elements excise without reinserting. If this behavior occurred in reproductive cells all the Mu elements would be lost. Yet, Mu elements are very effective mutagens and can increase their copy number in progeny of a Mutator plant. (Walbot Lab)

The BRONZE2 protein of maize encodes a glutathione S-transferase that performs the same carrier protein function as AN9 in Petunia. BZ2 and AN9 are an interesting case of convergent evolution of function: the two proteins are only 11% similar yet they can reciprocally complement. More closely related GST proteins of either Petunia or maize fail to complement in the original host plant or in transient assays in the alternate host. Observing maize anthers with the bz2::MuDR allele indicates that transposon excision in the outer layers of the anther occurs late in development. This deduction is possible because the resulting purple sectors on a bronze background are uniformly small. Mu elements exhibit high frequency, late excision throughout the plant. In the cell divisions before cells are specified to initiate meiosis, Mutator activity is also occuring, but the biochemical outcome is different. Instead of excision, an alternative transposase protein catalyzes a net replicative Mu element movement. This means that the original site (such as bz2::MuDR) retains it's transposon while new copies of the transposon are inserted elsewhere; the genetic consequences are a high forward mutation frequency without germinal (heritable) revertant alleles. The MuDR element actually encodes three distinct transposase proteins as a result of alternative splicing events. The transposase responsble for insertion events utilizes a novel start site of transcription, a different first intron, and frameshift translation for synthesis. (Walbot Lab)