Control of Arabidopsis shoot stem cell homeostasis by two antagonistic CLE peptide signalling pathways.

Stem cell homeostasis in plant shoot meristems requires tight coordination between stem cell proliferation and cell differentiation. In Arabidopsis, stem cells express the secreted dodecapeptide CLAVATA3 (CLV3), which signals through the leucine-rich repeat (LRR)-receptor kinase CLAVATA1 (CLV1) and related CLV1-family members to downregulate expression of the homeodomain transcription factor WUSCHEL (WUS). WUS protein moves from cells below the stem cell domain to the meristem tip and promotes stem cell identity, together with CLV3 expression, generating a negative feedback loop. How stem cell activity in the meristem centre is coordinated with organ initiation and cell differentiation at the periphery is unknown. We show here that the CLE40 gene, encoding a secreted peptide closely related to CLV3, is expressed in the SAM in differentiating cells in a pattern complementary to that of CLV3. CLE40 promotes WUS expression via BAM1, a CLV1-family receptor, and CLE40 expression is in turn repressed in a WUS-dependent manner. Together, CLE40-BAM1-WUS establish a second negative feedback loop. We propose that stem cell homeostasis is achieved through two intertwined pathways that adjust WUS activity and incorporate information on the size of the stem cell domain, via CLV3-CLV1, and on cell differentiation via CLE40-BAM1.

Plants are sessile lifeforms that have evolved many ways to overcome this challenge. For example, they can quickly adapt to their environment, and they can grow new organs, such as leaves and flowers, throughout their lifetime. Stem cells are important precursor cells in plants (and animals) that can divide and specialize into other types of cells to help regrow leaves and flowers. A region in the plant called meristem, which can be found in the roots and shoots, continuously produces new organs in the peripheral zone of the meristem by maintaining a small group of stem cells in the central zone of the meristem. This is regulated by a signalling pathway called CLV and a molecule produced by the stem cells in the central zone, called CLV3. Together, they keep a protein called WUS (found in the deeper meristem known as the organizing zone) at low levels. WUS, in turn, increases the production of stem cells that generate CLV3. However, so far it was unclear how the number of stem cells is coordinated with the rate of organ production in the peripheral zone. To find out more, Schlegel et al. studied cells in the shoot meristems from the thale cress Arabidopsis thaliana. The researchers found that cells in the peripheral zone produce a molecule called CLE40, which is similar to CLV3. Unlike CLV3, however, CLE40 boosts the levels of WUS, thereby increasing the number of stem cells. In return, WUS reduces the production of CLE40 in the central zone and the organizing centre. This system allows meristems to adapt to growing at different speeds. These results help reveal how the activity of plant meristems is regulated to enable plants to grow new structures throughout their life. Together, CLV3 and CLE40 signalling in meristems regulate stem cells to maintain a small population that is able to respond to changing growth rates. This understanding of stem cell control could be further developed to improve the productivity of crops.

The Cell Fate Controlling CLE40 Peptide Requires CNGCs to Trigger Highly Localized Ca2+ Transients in Arabidopsis thaliana Root Meristems.

Communication between plant cells and their biotic environment largely depends on the function of plasma membrane localized receptor-like kinases (RLKs). Major players in this communication within root meristems are secreted peptides, including CLAVATA3/EMBRYO SURROUNDING REGION40 (CLE40). In the distal root meristem, CLE40 acts through the RLK ARABIDOPSIS CRINKLY4 (ACR4) and the leucine-rich repeat (LRR) RLK CLAVATA1 (CLV1) to promote cell differentiation. In the proximal meristem, CLE40 signaling requires the LRR receptor-like protein CLAVATA2 (CLV2) and the membrane localized pseudokinase CORYNE (CRN) and serves to inhibit cell differentiation. The molecular components that act immediately downstream of the CLE40-activated receptors are not yet known. Here, we show that active CLE40 signaling triggers the release of intracellular Ca2+ leading to increased cytosolic Ca2+ concentration ([Ca2+]cyt) in a small subset of proximal root meristem cells. This rise in [Ca2+]cyt depends on the CYCLIC NUCLEOTIDE GATED CHANNELS (CNGCs) 6 and 9 and on CLV1. The precise function of changes in [Ca2+]cyt is not yet known but might form a central part of a fine-tuned response to CLE40 peptide that serves to integrate root meristem growth with stem cell fate decisions and initiation of lateral root primordia.

Receptor-like cytoplasmic kinase MAZZA mediates developmental processes with CLAVATA1 family receptors in Arabidopsis.

The receptor-like kinases (RLKs) CLAVATA1 (CLV1) and BARELY ANY MERISTEMs (BAM1-BAM3) form the CLV1 family (CLV1f), which perceives peptides of the CLV3/EMBRYO SURROUNDING REGION (ESR)-related (CLE) family within various signaling pathways of Arabidopsis thaliana. CLE peptide signaling, which is required for meristem size control, vascular development, and pathogen responses, involves the formation of receptor complexes at the plasma membrane. These complexes comprise RLKs and co-receptors in varying compositions depending on the signaling context, and regulate expression of target genes, such as WUSCHEL (WUS). How the CLE signal is transmitted intracellularly after perception at the plasma membrane is not known in detail. Here, we found that the membrane-associated receptor-like cytoplasmic kinase (RLCK) MAZZA (MAZ) and additional members of the Pti1-like protein family interact in vivo with CLV1f receptors. MAZ, which is widely expressed throughout the plant, localizes to the plasma membrane via post-translational palmitoylation, potentially enabling stimulus-triggered protein re-localization. We identified a role for a CLV1-MAZ signaling module during stomatal and root development, and redundancy could potentially mask other phenotypes of maz mutants. We propose that MAZ, and related RLCKs, mediate CLV1f signaling in a variety of developmental contexts, paving the way towards understanding the intracellular processes after CLE peptide perception.

Optogenetic control of gene expression in plants in the presence of ambient white light.

Optogenetics is the genetic approach for controlling cellular processes with light. It provides spatiotemporal, quantitative and reversible control over biological signaling and metabolic processes, overcoming limitations of chemically inducible systems. However, optogenetics lags in plant research because ambient light required for growth leads to undesired system activation. We solved this issue by developing plant usable light-switch elements (PULSE), an optogenetic tool for reversibly controlling gene expression in plants under ambient light. PULSE combines a blue-light-regulated repressor with a red-light-inducible switch. Gene expression is only activated under red light and remains inactive under white light or in darkness. Supported by a quantitative mathematical model, we characterized PULSE in protoplasts and achieved high induction rates, and we combined it with CRISPR-Cas9-based technologies to target synthetic signaling and developmental pathways. We applied PULSE to control immune responses in plant leaves and generated Arabidopsis transgenic plants. PULSE opens broad experimental avenues in plant research and biotechnology.