Across all kingdoms of life, cells secrete an extracellular polymer mesh that in turn feeds back onto them. This entails physical connections between the plasma membrane and the polymer mesh. In plant cells, one connection stands out: the Hechtian strand which, during plasmolysis, reflects the existence of a physical link between the plasma membrane of the retracting protoplast and the cell wall. The Hechtian strand is part of a larger structure, which we call the Hechtian structure, that comprises the Hechtian strand, the Hechtian reticulum and the Hechtian attachment sites. Although it has been observed for more than 100 years, its molecular composition and biological functions remain ill-described. A comprehensive characterization of the Hechtian structure is a critical step towards understanding this plasma membrane-cell wall connection and its relevance in cell signaling. This short review intends to highlight the main features of the Hechtian structure, in order to provide a clear framework for future research in this under-explored and promising field.

Water stress challenges plasma membrane – wall attachment. When plants are exposed to strong hyperosmotic stress, water exits the cell leading to plasmolysis, where the plasma membrane detaches partially from the cell wall and reveals discrete and persistent attachment sites in the form of Hechtian strands. Although these structures have been observed in several species for more than a century, the molecular bases behind plasma membrane – wall attachment remains elusive. Through a screen of candidate proteins, we reveal that the overexpression of two lectin receptor-like proteins (LecRK-I.9 and LecTM) increases the density of plasma membrane – wall connections in N. Benthamiana. The extracellular lectin domain of LecRK-I.9 was able to remain in the wall during plasmolysis. Conversely, deletion of the lectin domain in the LecRK-I.9 overexpressor restored the density of Hechtian strands to WT levels. In Arabidopsis, upon hyperosmotic stress, LecRK-I.9 formed largely immobile clusters, whereas lectin-deleted versions of LecRK-I.9 clusters were mobile. Cluster density correlated with predicted tensile stress levels and mechanical reinforcement in the wall before plasmolysis, consistent with a scenario in which attachment of the lectin domain also reflects the wall properties. Last, while overexpressing LecRK-I.9 conferred resistance to low water potential conditions, deletion of the lectin domain in the LecRK-I.9 overexpressor restored a WT response to water stress. Altogether, this demonstrates that the lectin domain of LecRK-I.9 creates persistent plasma membrane – wall attachment sites, with physiological relevance for plant resistance to water stress.

Summary: Rapid hypocotyl elongation allows buried seedlings to reach the surface, where light triggers de-etiolation and inhibits hypocotyl growth mainly by phytochromes A, B and cryptochromes 1, 2. Dynamic phosphorylation/dephosphorylation events provide a mechanism to rapidly transduce light signals. Only recently we have begun to uncover the earliest phospho-signaling responders to light. Here, we report a large-scale phosphoproteomic analysis and identify 20 proteins that change their phosphorylation pattern after 20 min of white light pulse compared to darkness. Microtubule-associated proteins (MAPs) were highly overrepresented in this group. Among them, we studied CIP7 (COP1-INTERACTING-PROTEIN-7), which presented microtubule (MT) localization, in contrast to what was previously described. Phosphorylated isoform in Serine 915 (Sp ⁹¹⁵ ) of CIP7 was detected in etiolated seedlings but undetectable after a light pulse in the presence of photoreceptors, while its expression decays with long light exposure. The short hypocotyl phenotype and rearrangement of MTs in etiolated cip7 mutants are complemented by CIP7-YFP and the phospho-mimetic CIP7 ^S915D -YFP, but not the phospho-null CIP7 ^S915A -YFP suggesting that Sp ⁹¹⁵ CIP7 is the active isoform that promotes hypocotyl elongation thorough MT reorganisation in darkness. Our results reveal that the small repertory of proteins that changes the phosphorylation status after a rapid light signal is tightly focused on MAPs; suggesting that phospho-regulation of microtubule-base processes are early targets during de-etiolation. The evidence on Sp ⁹¹⁵ CIP7 supports this idea.

We developed a pipeline to standardize and integrate publicly available transcriptomic, proteomic, and phosphoproteomic datasets from Arabidopsis thaliana etiolated seedlings. The pipeline is broadly adaptable, as its database schema and processing scripts can be applied to datasets from any organism or experimental condition. We applied it to etiolated seedlings to uncover protein kinases and protein phosphatases relevant to early skotomorphogenic development, a stage in which seedlings grow underground and rely on precisely regulated signaling networks to ensure survival and successful emergence. This multi-omics approach revealed a comprehensive landscape of protein expression at this developmental stage; however, we observed widespread discrepancies between transcript and protein abundance, pointing to post-transcriptional and/or post-translational regulation. This cross-study minimizes experimental bias and allows the detection of phosphorylation events that may have been overlooked in single-condition studies. A search of kinase specific motifs revealed that RxxS motifs were significantly enriched among phosphorylated peptides in protein phosphatases and microtubule-associated proteins, suggesting regulation by calcium dependent protein kinases (CPKs). CPK3 and CPK9 were identified as the most relevant isoforms in the proteome and phosphokinome, supporting their central role in skotomorphogenesis. Our results underscore the importance of phosphorylation-mediated signaling in coordinating early seedling development and provide a resource for dissecting regulatory networks and candidate genes involved in skotomorphogenesis.

Because of its role as an interface, the plasma membrane cell wall nexus is a signaling hub in all walled organisms. It is also a key structural component, being essential to maintain the integrity of the cell. In plants, many plasma membrane proteins have been shown to bind cell wall components, but with no obvious structural role. Conversely, the formation of Hechtian strands has revealed the existence of stiff structural hotspots, where plasma membrane is glued to the cell wall, even in the absence of plasmodesmata. However, the molecular factors behind this behavior remain elusive. Here, we bridge these two angles. Through a screen of candidate proteins, we reveal that the receptor-like kinase LecRKI.9 plays the role of a strong molecular glue at the plant cell cortex. Through deletion experiments, we find that the adhesive function of LecRKI.9 requires the presence of a lectin domain. Upon hyperosmotic stress, LecRKI.9 formed immobile clusters, whereas lectin-deleted versions of LecRKI.9 clusters were mobile. Cluster size and density correlated with the expected magnitude of mechanical stress in the wall before plasmolysis, further suggesting that the development of stiff adhesive hotspots through LecRKI.9 is regulated by stress in the wall. Altogether, this provides a scenario in which a subclass of lectin receptor-like kinases patterns the plasma membrane through stiff attachment sites, in response to stress.