Thomas Greb

A PHD-Polycomb Repressive Complex 2 triggers the epigenetic silencing of FLC during vernalization

Vernalization, the acceleration of flowering by winter, involves cold-induced epigenetic silencing of Arabidopsis FLC. This process has been shown to require conserved Polycomb Repressive Complex 2 (PRC2) components including the Su(z)12 homologue, VRN2, and two plant homeodomain (PHD) finger proteins, VRN5 and VIN3. However, the sequence of events leading to FLC repression was unclear. Here we show that, contrary to expectations, VRN2 associates throughout the FLC locus independently of cold. The vernalization-induced silencing is triggered by the cold-dependent association of the PHD finger protein VRN5 to a specific domain in FLC intron 1, and this association is dependent on the cold-induced PHD protein VIN3. In plants returned to warm conditions, VRN5 distribution changes, and it associates more broadly over FLC, coincident with significant increases in H3K27me3. Biochemical purification of a VRN5 complex showed that during prolonged cold a PHD-PRC2 complex forms composed of core PRC2 components (VRN2, SWINGER [an E(Z) HMTase homologue], FIE [an ESC homologue], MSI1 [p55 homologue]), and three related PHD finger proteins, VRN5, VIN3, and VEL1. The PHD-PRC2 activity increases H3K27me3 throughout the locus to levels sufficient for stable silencing. Arabidopsis PHD-PRC2 thus seems to act similarly to Pcl-PRC2 of Drosophila and PHF1-PRC2 of mammals. These data show FLC silencing involves changed composition and dynamic redistribution of Polycomb complexes at different stages of the vernalization process, a mechanism with greater parallels to Polycomb silencing of certain mammalian loci than the classic Drosophila Polycomb targets.

Strigolactone signaling is required for auxin-dependent stimulation of secondary growth in plants

Long distance cell-to-cell communication is critical for the development of multicellular organisms. In this respect, plants are especially demanding as they constantly integrate environmental inputs to adjust growth processes to different conditions. One example is thickening of shoots and roots, also designated as secondary growth. Secondary growth is mediated by the vascular cambium, a stem cell-like tissue whose cell-proliferating activity is regulated over a long distance by the plant hormone auxin. How auxin signaling is integrated at the level of cambium cells and how cambium activity is coordinated with other growth processes are largely unknown. Here, we provide physiological, genetic, and pharmacological evidence that strigolactones (SLs), a group of plant hormones recently described to be involved in the repression of shoot branching, positively regulate cambial activity and that this function is conserved among species. We show that SL signaling in the vascular cambium itself is sufficient for cambium stimulation and that it interacts strongly with the auxin signaling pathway. Our results provide a model of how auxin-based long-distance signaling is translated into cambium activity and suggest that SLs act as general modulators of plant growth forms linking the control of shoot branching with the thickening of stems and roots.

Strigolactones Suppress Adventitious Rooting in Arabidopsis and Pea

Adventitious root formation is essential for the propagation of many commercially important plant species and involves the formation of roots from nonroot tissues such as stems or leaves. Here, we demonstrate that the plant hormone strigolactone suppresses adventitious root formation in Arabidopsis (Arabidopsis thaliana) and pea (Pisum sativum). Strigolactone-deficient and response mutants of both species have enhanced adventitious rooting. CYCLIN B1 expression, an early marker for the initiation of adventitious root primordia in Arabidopsis, is enhanced in more axillary growth2 (max2), a strigolactone response mutant, suggesting that strigolactones restrain the number of adventitious roots by inhibiting the very first formative divisions of the founder cells. Strigolactones and cytokinins appear to act independently to suppress adventitious rooting, as cytokinin mutants are strigolactone responsive and strigolactone mutants are cytokinin responsive. In contrast, the interaction between the strigolactone and auxin signaling pathways in regulating adventitious rooting appears to be more complex. Strigolactone can at least partially revert the stimulatory effect of auxin on adventitious rooting, and auxin can further increase the number of adventitious roots in max mutants. We present a model depicting the interaction of strigolactones, cytokinins, and auxin in regulating adventitious root formation.

The PHD Finger Protein VRN5 Functions in the Epigenetic Silencing of Arabidopsis FLC

Vernalization, the acceleration of flowering by the prolonged cold of winter, ensures that plants flower in favorable spring conditions. During vernalization in Arabidopsis, cold temperatures repress FLOWERING LOCUS C (FLC) expression in a mechanism involving VERNALIZATION INSENSITIVE 3 (VIN3), and this repression is epigenetically maintained by a Polycomb-like chromatin regulation involving VERNALIZATION 2 (VRN2), a Su(z)12 homolog, VERNALIZATION 1 (VRN1), and LIKE-HETEROCHROMATIN PROTEIN 1. In order to further elaborate how cold repression triggers epigenetic silencing, we have targeted mutations that result in FLC misexpression both at the end of the prolonged cold and after subsequent development. This identified VERNALIZATION 5 (VRN5), a PHD finger protein and homolog of VIN3. Our results suggest that during the prolonged cold, VRN5 and VIN3 form a heterodimer necessary for establishing the vernalization-induced chromatin modifications, histone deacetylation, and H3 lysine 27 trimethylation required for the epigenetic silencing of FLC. Double mutant and FLC misexpression analyses reveal additional VRN5 functions, both FLC-dependent and -independent, and indicate a spatial complexity to FLC epigenetic silencing with VRN5 acting as a common component in multiple pathways.

Interplay of miR164, CUP‐SHAPED COTYLEDON genes and LATERAL SUPPRESSOR controls axillary meristem formation in Arabidopsis thaliana

Aerial architecture in higher plants is established post-embryonically by the inception of new meristems in the axils of leaves. These axillary meristems develop into side shoots or flowers. In Arabidopsis, the NAC domain transcription factors CUP SHAPED COTYLEDON1 (CUC1), CUC2 and CUC3 function redundantly in initiating the shoot apical meristem and establishing organ boundaries. Transcripts of CUC1 and CUC2 are targeted for degradation by miR164. In this study, we show that cuc3-2 mutants are impaired in axillary meristem initiation. Overexpression of miR164 in the cuc3-2 mutant caused an almost complete block of axillary meristem formation. Conversely, mir164 mutants and plants harbouring miR164-resistant alleles of CUC1 or CUC2 developed accessory buds in leaf axils. Collectively, these experiments reveal that, in addition to CUC3, redundant functions of CUC1 and CUC2 as well as miR164 regulation are required for the establishment of axillary meristems. Studies on LAS transcript accumulation in mir164 triple mutants and cuc3-2 plants overexpressing miR164 suggest that regulation of axillary meristem formation by miR164 is mediated through CUC1 and CUC2, which in turn regulate LAS.

WOX4 Imparts Auxin Responsiveness to Cambium Cells in Arabidopsis

Extensive lateral growth of shoot and root axes is mediated mostly by the cambium, a meristematic tissue whose activity depends on the plant hormone auxin. This study reveals that the presence of the transcription factor WOX4 in cambium cells is essential for translating the accumulation of long distance–derived auxin into cell proliferation. Multipotent stem cell populations, the meristems, are fundamental for the indeterminate growth of plant bodies. One of these meristems, the cambium, is responsible for extended root and stem thickening. Strikingly, although the pivotal role of the plant hormone auxin in promoting cambium activity has been known for decades, the molecular basis of auxin responsiveness on the level of cambium cells has so far been elusive. Here, we reveal that auxin-dependent cambium stimulation requires the homeobox transcription factor WOX4. In Arabidopsis thaliana inflorescence stems, 1-N-naphthylphthalamic acid–induced auxin accumulation stimulates cambium activity in the wild type but not in wox4 mutants, although basal cambium activity is not abolished. This conclusion is confirmed by the analysis of cellular markers and genome-wide transcriptional profiling, which revealed only a small overlap between WOX4-dependent and cambium-specific genes. Furthermore, the receptor-like kinase PXY is required for a stable auxin-dependent increase in WOX4 mRNA abundance and the stimulation of cambium activity, suggesting a concerted role of PXY and WOX4 in auxin-dependent cambium stimulation. Thus, in spite of large anatomical differences, our findings uncover parallels between the regulation of lateral and apical plant meristems by demonstrating the requirement for a WOX family member for auxin-dependent regulation of lateral plant growth.

Analysis of secondary growth in the Arabidopsis shoot reveals a positive role of jasmonate signalling in cambium formation

After primary growth, most dicotyledonous plants undergo secondary growth. Secondary growth involves an increase in the diameter of shoots and roots through formation of secondary vascular tissue. A hallmark of secondary growth initiation in shoots of dicotyledonous plants is the initiation of meristematic activity between primary vascular bundles, i.e. in the interfascicular regions. This results in establishment of a cylindrical meristem, namely the vascular cambium. Surprisingly, despite its major implications for plant growth and the accumulation of biomass, the molecular regulation of secondary growth is only poorly understood. Here, we combine histological, molecular and genetic approaches to characterize interfascicular cambium initiation in the Arabidopsis thaliana inflorescence shoot. Using genome-wide transcriptional profiling, we show that stress-related and touch-inducible genes are up-regulated in stem regions where secondary growth takes place. Furthermore, we show that the products of COI1, MYC2, JAZ7 and the touch-inducible gene JAZ10, which are components of the JA signalling pathway, are cambium regulators. The positive effect of JA application on cambium activity confirmed a stimulatory role of JA in secondary growth, and suggests that JA signalling triggers cell divisions in this particular context.

Tackling Drought Stress: RECEPTOR-LIKE KINASES Present New Approaches

Global climate change and a growing population require tackling the reduction in arable land and improving biomass production and seed yield per area under varying conditions. One of these conditions is suboptimal water availability. Here, we review some of the classical approaches to dealing with plant response to drought stress and we evaluate how research on RECEPTOR-LIKE KINASES (RLKs) can contribute to improving plant performance under drought stress. RLKs are considered as key regulators of plant architecture and growth behavior, but they also function in defense and stress responses. The available literature and analyses of available transcript profiling data indeed suggest that RLKs can play an important role in optimizing plant responses to drought stress. In addition, RLK pathways are ideal targets for nontransgenic approaches, such as synthetic molecules, providing a novel strategy to manipulate their activity and supporting translational studies from model species, such as Arabidopsis thaliana, to economically useful crops.

Characterization of Transcriptome Remodeling during Cambium Formation Identifies MOL1 and RUL1 As Opposing Regulators of Secondary Growth

Cell-to-cell communication is crucial for the development of multicellular organisms, especially during the generation of new tissues and organs. Secondary growth—the lateral expansion of plant growth axes—is a highly dynamic process that depends on the activity of the cambium. The cambium is a stem cell–like tissue whose activity is responsible for wood production and, thus, for the establishment of extended shoot and root systems. Attempts to study cambium regulation at the molecular level have been hampered by the limitations of performing genetic analyses in trees and by the difficulty of accessing this tissue in model systems such as Arabidopsis thaliana. Here, we describe the roles of two receptor-like kinases, REDUCED IN LATERAL GROWTH1 (RUL1) and MORE LATERAL GROWTH1 (MOL1), as opposing regulators of cambium activity. Their identification was facilitated by a novel in vitro system in which cambium formation is induced in isolated Arabidopsis stem fragments. By combining this system with laser capture microdissection, we characterized transcriptome remodeling in a tissue- and stage-specific manner and identified series of genes induced during different phases of cambium formation. In summary, we provide a means for investigating cambium regulation in unprecedented depth and present two signaling components that control a process responsible for the accumulation of a large proportion of terrestrial biomass.

Genome‐wide binding‐site analysis of REVOLUTA reveals a link between leaf patterning and light‐mediated growth responses

Unlike the situation in animals, the final morphology of the plant body is highly modulated by the environment. During Arabidopsis development, intrinsic factors provide the framework for basic patterning processes. CLASS III HOMEODOMAIN LEUCINE ZIPPER (HD-ZIPIII) transcription factors are involved in embryo, shoot and root patterning. During vegetative growth HD-ZIPIII proteins control several polarity set-up processes such as in leaves and the vascular system. We have identified several direct target genes of the HD-ZIPIII transcription factor REVOLUTA (REV) using a chromatin immunoprecipitation/DNA sequencing (ChIP-Seq) approach. This analysis revealed that REV acts upstream of auxin biosynthesis and affects directly the expression of several class II HD-ZIP transcription factors that have been shown to act in the shade-avoidance response pathway. We show that, as well as involvement in basic patterning, HD-ZIPIII transcription factors have a critical role in the control of the elongation growth that is induced when plants experience shade. Leaf polarity is established by the opposed actions of HD-ZIPIII and KANADI transcription factors. Finally, our study reveals that the module that consists of HD-ZIPIII/KANADI transcription factors controls shade growth antagonistically and that this antagonism is manifested in the opposed regulation of shared target genes.