The Brassicaceae-specific BSK family member SHORT SUSPENSOR (SSP), however, acts as a patterning cue in the zygote, initiating the apical-basal patterning process in a signal-like manner. The SSP protein has lost a regulatory, intramolecular interaction and activates the MAPKKK YODA signaling pathway constitutively, in principle, enabling the protein to initiate embryonic patterning without receptor activation. We further show that the BSK family members BSK1 and BSK2, both conserved in flowering plants, activate the same signaling pathway in parallel to SSP and might constitute remnants of an older, canonical signaling pathway still active in Arabidopsis.
In flowering plants, the asymmetrical division of the zygote is the first hallmark of apical-basal polarity of the embryo and is controlled by a MAP kinase pathway that includes the MAPKKK YODA (YDA). In Arabidopsis, YDA is activated by the membrane-associated pseudokinase SHORT SUSPENSOR (SSP) through an unusual parent-of-origin effect: SSP transcripts accumulate specifically in sperm cells but are translationally silent. Only after fertilization is SSP protein transiently produced in the zygote, presumably from paternally inherited transcripts. SSP is a recently diverged, Brassicaceae-specific member of the BRASSINOSTEROID SIGNALING KINASE (BSK) family. BSK proteins typically play broadly overlapping roles as receptor-associated signaling partners in various receptor kinase pathways involved in growth and innate immunity. This raises two questions: How did a protein with generic function involved in signal relay acquire the property of a signal-like patterning cue, and how is the early patterning process activated in plants outside the Brassicaceae family, where SSP orthologs are absent? Here, we show that Arabidopsis BSK1 and BSK2, two close paralogs of SSP that are conserved in flowering plants, are involved in several YDA-dependent signaling events, including embryogenesis. However, the contribution of SSP to YDA activation in the early embryo does not overlap with the contributions of BSK1 and BSK2. The loss of an intramolecular regulatory interaction enables SSP to constitutively activate the YDA signaling pathway, and thus initiates apical-basal patterning as soon as SSP protein is translated after fertilization and without the necessity of invoking canonical receptor activation.
Figure 1: Functional importance of BSK family kinases in early embryogenesis. (A) Nomarsky images of transition-stage embryos in whole-mount cleared seeds of wild-type Col-0, yda, bsk1-2, bsk2-2, and ssp-2 single mutants, as well as bsk1 bsk2 double mutants (b1 b2) and bsk1 bsk2 ssp triple mutants (b1 b2 ssp). (Scale bars: 10 μm.) (B) Box plot diagram of suspensor length measurements for >100 embryos. The total number of analyzed embryos is depicted above the x axis. The box plot diagram shows the median as center lines, and the 25th and 75th percentiles are indicated by box limits. Whiskers show 1.5× interquartile distance. Outliers are represented by dots. Statistically significant differences from wild type were determined by the Mann–Whitney U test (**P < 0.001; *P < 0.01). Statistically significant differences in other pairwise comparisons are indicated by brackets. (C) Nomarsky images of embryos at the one-cell stage in whole-mount cleared seeds. The average sizes of the apical (yellow) and basal (green) daughter cells with the SD and number of analyzed embryos are given below the image. Furthermore, significant differences in pairwise comparisons to the indicated reference (Ref) determined by the Mann–Whitney U test (*P < 0.001) are indicated below the image (gray). Apical cells are false-colored in yellow, and basal cells are shown in green. (Scale bars: 10 μm.)