Trichomes are differentiated epidermal cells on above ground organs of nearly all land plants. They play important protective roles as structural defenses upon biotic attacks such as herbivory, ...oviposition and fungal infections, and against abiotic stressors such as drought, heat, freezing, excess of light, and UV radiation. The pattern and density of trichomes is highly variable within natural population suggesting tradeoffs between traits positively affecting fitness such as resistance and the costs of trichome production. The spatial distribution of trichomes is regulated through a combination of endogenous developmental programs and external signals. This review summarizes the current understanding on the molecular basis of the natural variation and the role of phytohormones and environmental stimuli on trichome patterning.
Kinesins are versatile nano‐machines that utilize variable non‐motor domains to tune specific motor microtubule encounters. During plant cytokinesis, the kinesin‐12 orthologs, PHRAGMOPLAST ORIENTING ...KINESIN (POK)1 and POK2, are essential for rapid centrifugal expansion of the cytokinetic apparatus, the phragmoplast, toward a pre‐selected cell plate fusion site at the cell cortex. Here, we report on the spatio‐temporal localization pattern of POK2, mediated by distinct protein domains. Functional dissection of POK2 domains revealed the association of POK2 with the site of the future cell division plane and with the phragmoplast during cytokinesis. Accumulation of POK2 at the phragmoplast midzone depends on its functional POK2 motor domain and is fine‐tuned by its carboxy‐terminal region that also directs POK2 to the division site. Furthermore, POK2 likely stabilizes the phragmoplast midzone via interaction with the conserved microtubule‐associated protein MAP65‐3/PLEIADE, a well‐established microtubule cross‐linker. Collectively, our results suggest that dual localized POK2 plays multiple roles during plant cell division.
Synopsis
Kinesin‐12 POK2 demarcates the plant cell division site from prophase onward and accumulates at the phragmoplast midzone, where it interacts with the microtubule cross‐linker MAP65‐3. This dual localization pattern confers guidance of the phragmoplast and accurate cell plate positioning during plant cell division.
Distinct protein domains mediate the dual localization of POK2.
Interaction with MAP65‐3 via two distinct binding sites retains POK2 at the phragmoplast midzone to ensure proper phragmoplast expansion.
POK2 and POK1 act redundantly to maintain the division site.
The Arabidopsis kinesin‐12 motor protein POK2 demarcates the cell division site and accumulates at the phragmoplast midzone, where it interacts with the microtubule cross‐linker MAP65‐3. This dual localization pattern confers guidance of the phragmoplast and accurate cell plate positioning during plant cell division.
Interest in transgenerational epigenetic inheritance has intensified with the boosting of knowledge on epigenetic mechanisms regulating gene expression during development and in response to internal ...and external signals such as biotic and abiotic stresses. Starting with an historical background of scantily documented anecdotes and their consequences, we recapitulate the information gathered during the last 60
years on naturally occurring and induced epialleles and paramutations in plants. We present the major players of epigenetic regulation and their importance in controlling stress responses. The effect of diverse stressors on the epigenetic status and its transgenerational inheritance is summarized from a mechanistic viewpoint. The consequences of transgenerational epigenetic inheritance are presented, focusing on the knowledge about its stability, and in relation to genetically fixed mutations, recombination, and genomic rearrangement. We conclude with an outlook on the importance of transgenerational inheritance for adaptation to changing environments and for practical applications. This article is part of a Special Issue entitled “Epigenetic control of cellular and developmental processes in plants”.
► The relevance of plant adaptation for coping with adverse environments ► Insights into mechanisms by which biotic and abiotic stress might impinge on the epigenome of plants ► The potential role of inherited epigenetic adjustments for plant adaptation
In plants, regulation of cellulose synthesis is fundamental for morphogenesis and plant growth. Cellulose is synthesized at the plasma membrane, and the orientation of synthesis is guided by cortical ...microtubules; however, the guiding mechanism is currently unknown. We show that the conditional root elongation pom2 mutants are impaired in cell elongation, fertility, and microtubule-related functions. Map-based cloning of the POM-POM2 locus revealed that it is allelic to CELLULOSE SYNTHASE INTERACTING1 CSI1). Fluorescently tagged POM2/CSI1s associated with both plasma membrane-located cellulose synthases (CESAs) and post-Golgi CESA-containing compartments. Interestingly, while CESA insertions coincided with cortical microtubules in the pom2/csi1 mutants, the microtubule-defined movement of the CESAs was significantly reduced in the mutant. We propose that POM2/CSI1 provides a scaffold between the CESAs and cortical microtubules that guide cellulose synthesis.
Plants, as sessile organisms, need to sense and adapt to heterogeneous environments and have developed sophisticated responses by changing their cellular physiology, gene regulation, and genome ...stability. Recent work dem- onstrated heritable stress effects on the control of genome stability in plants--a phenomenon that was suggested to be of epigenetic nature. Here, we show that temperature and UV-B stress cause immediate and heritable changes in the epi- genetic control of a silent reporter gene in Arabidopsis. This stress-mediated release of gene silencing correlated with pronounced alterations in histone occupancy and in histone H3 acetylation but did not involve adjustments in DNA meth- ylation. We observed transmission of stress effects on reporter gene silencing to non-stressed progeny, but this effect was restricted to areas consisting of a small number of cells and limited to a few non-stressed progeny generations. Further- more, stress-induced release of gene silencing was antagonized and reset during seed aging. The transient nature of this phenomenon highlights the ability of plants to restrict stress-induced relaxation of epigenetic control mechanisms, which likely contributes to safeguarding genome integrity.
Cell walls are not only a protective barrier surrounding protoplasts but serve as signaling platform between the extracellular environment and the intracellular physiology. Ions of heavy metals and ...trace elements, summarized to metal ions, bind to cell wall components, trigger their modification and provoke growth responses. To examine if metal ions trigger cell wall sensing receptor like kinases (RLKs) of the
Catharanthus roseus
RLK1-like (
Cr
RLK1L) family we employed a molecular genetic approach. Quantitative transcription analyses show that
HERCULES1 (HERK1)
,
THESEUS1 (THE1)
, and
FERONIA (FER)
were differently regulated by cadmium (Cd), nickel (Ni), and lead (Pb). Growth responses were quantified for roots and etiolated hypocotyls of related mutants and overexpressors on Cd, copper (Cu), Ni, Pb, and zinc (Zn) and revealed a complex pattern of gene specific, overlapping and antagonistic responses. Root growth was often inversely affected to hypocotyl elongation. For example, both
HERK
genes seem to negatively regulate hypocotyl elongation upon Cd, Ni, Zn, and Pb while they support root growth on Cd, Cu, and Ni. The different
THE1
alleles exhibited a similar effect between roots and hypocotyls on Ni, where the loss-of-function mutant was more tolerant while the gain of function mutants were hypersensitive indicating that
THE1
is mediating Ni specific inhibition of hypocotyl elongation in the dark. In contrast hypocotyl elongation of the knock-out mutant,
fer-4
, was hypersensitive to Ni but exhibited a higher tolerance to Cd, Cu, Pb, and Zn. These data indicate an antagonistic action between THE1 and FER in relation to hypocotyl elongation upon excess of Ni. FERs function as receptor for rapid alkalinization factors (RALFs) was tested with the indicator bromocresol purple. While
fer-4
roots strongly acidified control and metal ion containing media, the etiolated hypocotyls alkalized the media which is consistent with the already shorter hypocotyl of
fer-4
. No other
CrRLK1L
mutant exhibited this phenotype except of the THE1:GFP overexpressor on Ni suggesting that THE1 might be involved in Ni induced and hypocotyl specific RALF signaling and growth regulating pathway. Overall, our findings establish a molecular link between metal ion stress, growth and the cell wall integrity sensors of the
Cr
RLK1L family.
Cell death is intrinsically linked with immunity. Disruption of an immune-activated MAPK cascade, consisting of MEKK1, MKK1/2, and MPK4, triggers cell death and autoimmunity through the ...nucleotide-binding leucine-rich repeat (NLR) protein SUMM2 and the MAPK kinase kinase MEKK2. In this study, we identify a Catharanthus roseus receptor-like kinase 1-like (CrRLK1L), named LETUM2/MEDOS1 (LET2/MDS1), and the glycosylphosphatidylinositol (GPI)-anchored protein LLG1 as regulators of mekk1-mkk1/2-mpk4 cell death. LET2/MDS1 functions additively with LET1, another CrRLK1L, and acts genetically downstream of MEKK2 in regulating SUMM2 activation. LET2/MDS1 complexes with LET1 and promotes LET1 phosphorylation, revealing an intertwined regulation between different CrRLK1Ls. LLG1 interacts with the ectodomain of LET1/2 and mediates LET1/2 transport to the plasma membrane, corroborating its function as a co-receptor of LET1/2 in the mekk1-mkk1/2-mpk4 cell death pathway. Thus, our data suggest that a trimeric complex consisting of two CrRLK1Ls LET1, LET2/MDS1, and a GPI-anchored protein LLG1 that regulates the activation of NLR SUMM2 for initiating cell death and autoimmunity.
Directed plant cell growth is governed by deposition and alterations of cell wall components under turgor pressure. A key regulatory element of anisotropic growth, and hence cell shape, is the ...directional deposition of cellulose microfibrils. The microfibrils are synthesized by plasma membrane-located cellulose synthase complexes that co-align with and move along cortical microtubules. That the parallel relation between cortical microtubules and extracellular microfibrils is causal has been named the alignment hypothesis. Three recent studies revealed that the previously identified pom2 mutant codes for a large cellulose synthases interacting (CSI1) protein which also binds cortical microtubules. This review summarizes these findings, provides structure–function models and discusses the inferred mechanisms in the context of plant growth.
In multicellular organisms, temporal and spatial regulation of cell proliferation is central for generating organs with defined sizes and morphologies. For establishing and maintaining the ...post‐mitotic quiescent state during cell differentiation, it is important to repress genes with mitotic functions. We found that three of the Arabidopsis MYB3R transcription factors synergistically maintain G2/M‐specific genes repressed in post‐mitotic cells and restrict the time window of mitotic gene expression in proliferating cells. The combined mutants of the three repressor‐type MYB3R genes displayed long roots, enlarged leaves, embryos, and seeds. Genome‐wide chromatin immunoprecipitation revealed that MYB3R3 binds to the promoters of G2/M‐specific genes and to E2F target genes. MYB3R3 associates with the repressor‐type E2F, E2FC, and the RETINOBLASTOMA RELATED proteins. In contrast, the activator MYB3R4 was in complex with E2FB in proliferating cells. With mass spectrometry and pairwise interaction assays, we identified some of the other conserved components of the multiprotein complexes, known as DREAM/dREAM in human and flies. In plants, these repressor complexes are important for periodic expression during cell cycle and to establish a post‐mitotic quiescent state determining organ size.
Synopsis
Activator and DREAM‐like repressor R1R2R3‐Myb transcription factors act coordinately to regulate mitotic target genes and organ growth in Arabidopsis thaliana.
R1R2R3‐Mybs with repressor functions maintain G2/M‐specific genes repressed in post‐mitotic cells and restrict mitotic gene expression in proliferating cells.
Transcriptional cell cycle regulation at G2/M impacts on plant organ growth.
R1R2R3‐Myb activators and repressors form distinct multi‐protein complexes.
Repressive R1R2R3‐Mybs are similar to the DREAM/dREAM complexes in human and fly.
Activator and DREAM‐like repressor R1R2R3‐Myb transcription factors act coordinately to regulate mitotic target genes and organ growth in Arabidopsis thaliana.
The profile of protein sorting into multivesicular bodies (MVBs) has risen recently with the identification of three heteromeric complexes known as ESCRT-I,-II,–III (Endosomal Sorting Complex ...Required for Transport). Genetic analyses in yeast have identified up to 15 soluble class E VPS (vacuolar protein sorting) proteins that have been assigned to the ESCRT machinery and function in cargo recognition and sorting, complex assembly, vesicle formation and dissociation. Despite their functional importance in yeast and mammalian cells, little is known about their presence and function in other organisms including plants. We have made use of the fully sequenced genomes of
Arabidopsis thaliana and
Oryza sativa,
Drosophila melanogaster and
Caenorhabditis elegans to explore the identity, structural characteristics and phylogenetic relationships of proteins assigned to the ESCRT machinery.
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