Asseck, L. Y. | bioRxiv Science

bioRxiv2026

Before auxin distribution matters: Prepatterning of the Arabidopsis embryo axis by MPK6-dependent AUX/IAA8 phosphorylation

In growth and development of plants, the signaling molecule auxin plays an instrumental role. Many patterning processes are controlled by auxin signaling in a self-organizing matter. After fertilization, such a patterning process needs to be started for the first time during plant life. Contrary to previous models, we show that the first differential auxin response that separates embryonic from extra-embryonic development in the two zygotic daughter cells happens independently of a detectable auxin gradient. Instead, differential phosphorylation of the transcriptional inhibitor Aux/IAA8 by MAP kinase signaling leads to differences in auxin sensitivity between the two daughter cells, initiating embryonic development in the apical cell. IAA8 phosphorylation in the basal cell prevents proteasomal degradation, suppressing canonical auxin responses, which initiates extra-embryonic development. MAP kinase signaling therefore precedes auxin gradients and prepatterns the apical-basal axis independently of an auxin gradient.

plant biology↗

bioRxiv2019

Distinct ROPGEFs successively drive polarization and outgrowth of root hairs

Root hairs are tubular protrusions of the root epidermis that significantly enlarge the exploitable soil volume in the rhizosphere. Trichoblasts, the cell type responsible for root hair formation, switch from cell elongation to tip growth through polarization of the growth machinery to a pre-defined root hair initiation domain (RHID) at the plasma membrane. The emergence of this polar domain resembles the establishment of cell polarity in other eukaryotic systems [1-3]. Rho-type GTPases of plants (ROPs) are among the first molecular determinants of the RHID [4, 5] and later play a central role in polar growth [6]. Numerous studies have elucidated mechanisms that position the RHID in the cell [7-9] or regulate ROP activity [10-18]. The molecular players that target ROPs to the RHID and initiate outgrowth, however, have not been identified. We dissected the timing of the growth machinery assembly in polarizing hair cells and found that positioning of molecular players and outgrowth are temporally separate processes that are each controlled by specific ROP guanine nucleotide exchange factor (GEFs). A functional analysis of trichoblast-specific GEFs revealed GEF3 to be required for normal ROP polarization and thus efficient root hair emergence, while GEF4 predominantly regulates subsequent tip growth. Ectopic expression of GEF3 induced the formation of spatially confined, ROP-recruiting domains in other cell types, demonstrating the role of GEF3 to serve as a membrane landmark during cell polarization. Our findings suggest that morphogenetic programs in plants employ distinct regulatory modules for the alignment and activation of the cellular growth machinery.

cell biology↗