Marc Somssich Profile picture
Jan 5, 2022 13 tweets 6 min read Read on X
#PlantScienceClassics #12: The stem cell-maintaining CLAVATA(CLV)-WUSCHEL(WUS) feedback loop. In 1999/2000 the labs of Elliot Meyerowitz & @simonrdg published 2 joint @ScienceMagazine papers describing a self-regulating signaling loop that maintains the stem cells of plants. Title pages of the two Scie...
Plants continue to grow for their entire life due to the activity of stable pools of stem cells. The shoot apical #meristem (SAM) is the stem cell niche responsible for producing all above ground cells and is located at the tip of the plant’s stem. Three sketches. First, an o...
The number of stem cells in the SAM is maintained constant, despite new formed cells continuously leaving the stem cell pool to build new tissues & organs. And like the stem cell number is stable, so are the number of organs formed, e.g. flower organs (see classic #1, ABC model). A screenshot of the first #...
One early Arabidopsis mutant with a visible alteration in stem cell number was clv1, which @OttolineLeyser & Ian Furner showed had a bigger SAM producing more organs, most notably 4 carpels in its flowers instead of 2. A bar graph from the 1992 L...
CLV1 turned out to be a receptor-kinase, & mutations in 2 more clv genes share a similar ‘bigger SAM - more organs’ phenotype. These are the receptor-protein CLV2 & the small protein CLV3. Conversely, mutants of the wus gene seemed to result in stem cell depletion.
With their 1999/2000 papers, the Meyerowitz & Simon labs integrated these proteins into an elegant model that described how plants are able to maintain a stable number of stem cells while continuously producing new cells. First, they showed that CLV3 could be a signal peptide.
CLV3 is expressed in the top cell layers of the center of the SAM, while CLV1 is expressed in a group of cells below, from layer 3 onwards. In clv1 & clv2 mutants, CLV3 expression broadens widely, suggesting loss of a signal that limits both, stem cell number & CLV3 expression. RNA in situ hybridization i...
They conclude that CLV3 is a mobile signal that is secreted from the top layer stem cells, & relays a stem cell limiting signal via CLV1/2 to the cells in the 3rd layer & below. They also suggest that there must be a counteracting stem cell promoting signal, likely from below. One hypothesis that fits th...
They then show that overexpression of CLV3 leads to total loss of stem cells & a wus-like phenotype, indicating that WUS could be the positive regulator counteracting CLV3. Fittingly, CLV3 overexpression completely repressed WUS, while WUS-expression was expanded in clv3 mutants. Figure 4 of the 2000 Scienc...
Based on the results of these 2 papers the authors conclude that the stem cells produce the CLV3 signal which limits WUS expression via CLV1/2. WUS in turn promotes stem cell production, & hence CLV3 production.Strong CLV3 reduces WUS, leading to reduced CLV3 & a recovery of WUS. The 2000 model of the CLV-W...
Curiously though, clv3 mutants have a stronger phenotype than clv1 & clv2 mutants, unexpected if CLV3 signals strictly via the CLV1/2 receptor-complex. To resolve this discrepancy, the @simonrdg lab added another publication in @ThePlantCell in 2008, describing CORYNE (CRN). Title page of the 2008 The ...
CRN is a kinase-protein w/o receptor domain, & hence nicely complements CLV2, a receptor-protein w/o kinase. Accordingly, the authors split up the CLV3 pathway into two parallel pathways, one signalling via the CLV1 receptor-kinase, & one via a CLV2/CRN receptor/kinase complex. The updated model from the ...
Since these publications, the pathways regulating meristem maintenance in plants have been described in ever-increasing detail, with several more receptors, peptides and target genes involved. But at the center remains this elegant feedback loop, holding it all together.

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More from @somssichm

Mar 26, 2023
#PlantScienceClassics #18: Ethylene triple response mutants. 35 years ago Anthony Bleecker et al. exploited the triple response phenotype to identify the #ethylene receptor ETR1. The ethylene story is another example for #PlantBlindness @NobelPrize. doi.org/10.1126/scienc… The Science cover picture of the 1988 issue shows the triple
Ethylene is a gaseous #phytohormone with a wide range of roles from plant development to immunity. Ernest Starling in 1905 defined a hormone as mobile chemical messenger synthesized by a multicellular organism, that has physiological activity distant from the site of synthesis. Excerpt from Ernest Henry Starling’s 1905 paper “The Cro
The effect of ethylene on plants was first noted in the 1900s, when it leaked from illumination gas used in lamps and affected plants nearby. But it was Dimitry Neljubow, who in a series of experiments identified ethlyene as the active substance in the illumination gas in 1905.
Read 17 tweets
Oct 12, 2022
#PlantScienceClassics #17: The Mildew Resistance Locus O (MLO). 80 years ago Rudolf Freisleben & Alfred Lein created the first powdery mildew resistant barley plant. 30yrs ago the gene was mapped, 25yrs ago cloned-yet it's mode of action remains a mystery. doi.org/10.1007/BF0148… A portrait of Rudolf Freisl...
Powdery mildew is a fungal disease of many crop plants, most prominently maybe barley and wheat, where outbreaks can reduce grain quality & yield, and ruin complete harvests. Visible are the fluffy patches formed by the fungus (Blumeria graminis f. sp. hordei). A leaf and awn from a barle...
Freisleben used radiation-induced mutagenesis to create the barley 𝘮𝘭𝘰 mutant, which showed full resistant to this pathogen. A massive agricultural breakthrough!
See also Classic #2, to read about how Emmy Stein has developed this technique in 1921:
Read 12 tweets
Apr 8, 2022
#PlantScienceClassics #16: A linkage map of Arabidopsis thaliana. In 1983 the legendary Maarten Koornneef published a genetic map of A. thaliana, the basis for genetic work & an important contribution towards the acceptance of Arabidopsis as plant model. doi.org/10.1093/oxford… Title page of the 1983 pape...
In the early 1980s scientists finally adopted A. thaliana as model plant. At this point, several mutants were available, but their positions in the genome were mostly unknown. This was years before genome sequences became available,&genetic maps were still based on recombination.
Arabidopsis pioneer György Rédei did linkage analyses with 14 loci in the 1960s, but his genetic map from 1965 suggested 6 linkage groups – 1 more than chromosomes. Curiously, A. D. McKelvie created another map in parallel - & found 4 groups, 1 less than chromosomes. György Rédei’s linkage map,...A. D. McKelvie’s linkage ma...
Read 14 tweets
Mar 31, 2022
#PlantScienceClassics #15 #PlantScienceFails #1: The auxin-independent (axi) Nicotiana tabacum lines. In 1992 Richard Walden et al. (specifically co-worker Inge Czaja) published activation-tagged axi protoplasts @ScienceMagazine that could divide&grow in the absence of any auxin! Title page of the retracted...
The development of plant transformation in the early 1980s (classics #6&13) was inspirational for many scientists. Among them was Richard Walden, who teamed up with plant transformation pioneers Barbara & Thomas Hohn to leverage this advance to develop the “Agroinfection" method. Title page of the 1986 PNAS...
He then joined the next transformation pioneer, Jeff Schell, to develop more such tools. Their first was Activation-Tagging: 4 CaMV 35S enhancers (classic#9) were placed at the RB of the T-DNA. That way, they would overexpress the plant gene next to which the T-DNA was inserted. Figure 1 of the paper showi...
Read 12 tweets
Jan 25, 2022
#PlantScienceClassics #14: Mendelian inheritance. In 1866 Gregor Mendel published his work on dominant/recessive trait inheritance in peas, establishing the hereditary rules on which modern genetics is based. But nobody cared,& his scientific career ended. biodiversitylibrary.org/page/48299076 A portrait of Gregor Mendel...
Mendel had always been interested in nature, and grew/kept and observed plants and bees in his parent’s garden. He later decided to become a monk and teacher. However, he failed teacher’s exam in 1850 & 1856, & eventually settled on being a monk and substitute teacher.
He satisfied his curiosity as a naturalist by keeping and observing plants and bees in the monastery garden, and eventually became interested in how traits are determined through generations. So he started to conduct crossing experiments with mice with grey or white fur. An image of Mendel’s garden...
Read 19 tweets
Jan 20, 2022
Do you know Daisy Roulland-Dussoix? She is one of the discoverers of restriction enzymes, who’s findings paved the way for the development of recombinant DNA and cloning technologies. Accordingly, the finding was rewarded with a #NobelPrize. But the prize didn’t go to her... 🧵👇 A portrait of Daisy Roulland-Dussoix from Wikipedia.
Daisy Roulland-Dussoix worked with Werner Arber to study the mechanism for the observed host-specificity of λ Phages. It was known from an important 1953 paper (Bertani & Weigle) that phages, that had replicated in a certain E. coli strain, could only re-infect the same strain. Title page of the 1953 paper from G. Bertani and J. J. Weigl
Roulland-Dussoix & Arber showed that host-specificity is linked with the phage’s DNA. Using phages carrying radiolabeled DNA, they showed that progeny with 2 parental DNA strands retained specificity, while progeny with newly synthesized daughter strands could adapt to new hosts. Title page of the first paper by Werner Arber and Daisy DussExcerpt from the paper stating ‘(2) All progeny λ K(Pl) p
Read 11 tweets

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