Growth of plant zygotes is typically polar and their division asymmetric. In Arabidopsis, the zygote elongates about three-fold before dividing into a small apical and a large basal cell. These two cells then follow fundamentally different developmental programs: while the apical cell and its daughters produce the spherical proembryo, the basal cell and its daughters continue to elongate and to divide transversely, forming the filamentous suspensor.
A molecular cell fate switch
This cell fate decision is regulated by a MAP kinase signaling pathway. Loss of the receptor-like kinase gene SHORT SUSPENSOR (SSP), the MAPKK kinase gene YODA (YDA), the MAP kinase genes MPK3/6 or the predicted transcription factor GROUNDED (GRD) all suppress elongation of the zygote as well as formation of the suspensor. Hyperactive variants of the YDA MAPKK kinase or some of the ten Arabidopsis MAPK kinases have the opposite effect, promoting extensive elongation of the zygote and exaggerated suspensor growth. In the most severely affected cases, the zygote develops into a filament of cells, as if no apical cell is specified or its development arrested. These findings suggest that the YDA MAP kinase cascade acts as part of a cell fate switch.
Embryo phenotypes associated with abnormal MAP kinase signaling. Wild type zygotes elongate three-fold before dividing asymmetrically. Mutations in the YDA MAPKK kinase suppress elongation elongation of the zygote (left two panels). Subsequently, the basal daughter cell of yda mutants fails to produce a suspensor (compare to wt embryo shown to the right). Hyperactive variants of YDA have the opposite effect, often causing the formation of stalk-like structures (rightmost panel).
Activation of the YDA MAP kinase cascade by a unique parent-of-origin effect
Which are the signals that mediate activation of the YDA MAP kinase cascade in the zygote? Recent work on the SHORT SUSPENSOR (SSP) has provided some surprising clues. Genetic analysis places SSP upstream of the YDA MAP kinase cascade, and forced expression of SSP protein in leaves is sufficient to trigger YDA-dependent signaling. SSP messenger RNA is only found in the sperm cells of pollen, but the sperm cells do not contain detectable amounts of SSP protein. Instead, SSP protein seems to accumulate transiently after fertilization in the zygote. Furthermore, ssp mutations show a unique parent-of-origin effect. Reciprocal crosses reveal that the phenotype of the developing embryo is strictly determined by the genotype of the pollen: when wild type pollen is crossed to homozygous mutant plants, all the progeny develops normally; conversely, crosses of mutant pollen to wild type plants produce all abnormal, mutant-looking progeny.
Expression of the SSP gene. mRNA accumulates in the two sperm cells of pollen (left panel), but protein is only detected in the zygote. Fluorescently tagged SSP protein weakly outlines in the zygote (center panel), and a fluorescentr reporter of SSP transcription/translation crossed via the pollen accumulates in the nuclei of the zygote and the endosperm (right panel).
On the basis of these findings we propose that SSP links activation of the YDA MAP kinase cascade to fertilization. According to this working model, SSP mRNA is produced in sperm cells but remains untranslated. Instead, the mRNA is delivered to the zygote, where it becomes translated. Transient accumulation of SSP protein then triggers activation of YDA signaling.
Where do we go from here?
A number of key questions surrounding this signaling event remain open:
For how long after fertilization and in which cells is the YDA MAP kinase active? Can we visualize YDA signaling in live embryos, or can we find other means to map signaling with a cellular resolution?
How, mechanistically, does SSP protein trigger activation of the MAP kinase cascade?
What are the direct targets of the MAP kinase cascade? Is the predicted transcription factor GRD one of them?
How does the action of these targets impact elongation and division of the zygote?