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Key message This review summarizes recent knowledge on functions of WUS and WUS-related homeobox (WOX) transcription factors in diverse signaling pathways governing shoot meristem biology and several other aspects of plant dynamics. Transcription factors (TFs) are master regulators involved in controlling different cellular and biological functions as well as diverse signaling pathways in plant growth and development. WUSCHEL (WUS) is a homeodomain transcription factor necessary for the maintenance of the stem cell niche in the shoot apical meristem, the differentiation of lateral primordia, plant cell totipotency and other diverse cellular processes. Recent research about WUS has uncovered several unique features including the complex signaling pathways that further improve the understanding of vital network for meristem biology and crop productivity. In addition, several reports bridge the gap between WUS expression and plant signaling pathway by identifying different WUS and WUS-related homeobox (WOX) genes during the formation of shoot (apical and axillary) meristems, vegetative-to-embryo transition, genetic transformation, and other aspects of plant growth and development. In this respect, the WOX family of TFs comprises multiple members involved in diverse signaling pathways, but how these pathways are regulated remains to be elucidated. Here, we review the current status and recent discoveries on the functions of WUS and newly identified WOX family members in the regulatory network of various aspects of plant dynamics.

485 I. 485 II. 486 III. 491 IV. 491 V. 495 495 References 495 SUMMARY: Boundaries, established and maintained in different regions of the plant body, have diverse functions in development. One role is to separate different cell groups, for example the differentiating cells of a leaf primordium from the pluripotent cells of the apical meristem. Boundary zones are also established during compound leaf development, to separate young leaflets from each other, and in many other positions of the plant body. Recent studies have demonstrated that different boundary zones share similar properties. They are characterized by a low rate of cell divisions and specific patterns of gene expression. In addition, the levels of the plant hormones auxin and brassinosteroids are down‐regulated in boundary zones, resulting in a low differentiation level of boundary cells. This feature seems to be crucial for a second important role of boundary zones, the formation of new meristems. The primary shoot meristem, as well as secondary and ectopic shoot meristems, initiate from boundary cells that exhibit competence for meristem formation.

Grasses have evolved complex inflorescences, where the primary unit is the specialized short branch called a spike-let. Detailed studies of the cumulative action of the genetic regulators that direct the progressive change in axillary meristem identity and their terminal differentiation are crucial to understanding the complexities of the inflorescence and the development of a determinate floret. Grass florets also pose interesting questions concerning the morphologies and functions of organs as compared to other monocots and eudicots. In this review, we summarize our current knowledge of the regulation of the transitions that occur in grass inflorescence meristems, and of the specification of floret meristems and their determinate development. We primarily use rice as a model, with appropriate comparisons to other crop models and to the extensively studied eudicot Arabidopsis. The role of MADS-domain transcription factors in floral organ patterning is well documented in many eudicots and in grasses. However, there is evidence to suggest that some of these rice floral regulators have evolved distinctive functions and that other grass species-specific factors and regulatory pathways occur – for example the LOFSEP ‘E’ class genes OsMADS1 and OsMAD34, and ramosa genes. A better understanding of these systems and the epigenetic regulators and hormone signaling pathways that interact with them will provide new insights into the rice inflorescence meristem and the differentiation of its floret organs, and should indicate genetic tools that can be used to control yield-related traits in both rice and other cereal crops.

Flower morphology results from the interaction of an established genetic program, the influence of external forces induced by pollination systems, and physical forces acting before, during and after initiation. Floral ontogeny, as the process of development from a meristem to a fully developed flower, can be approached either from a historical perspective, as a “recapitulation of the phylogeny” mainly explained as a process of genetic mutations through time, or from a physico-dynamic perspective, where time, spatial pressures, and growth processes are determining factors in creating the floral morphospace. The first (historical) perspective clarifies how flower morphology is the result of development over time, where evolutionary changes are only possible using building blocks that are available at a certain stage in the developmental history. Flowers are regulated by genetically determined constraints and development clarifies specific transitions between different floral morphs. These constraints are the result of inherent mutations or are induced by the interaction of flowers with pollinators. The second (physico-dynamic) perspective explains how changes in the physical environment of apical meristems create shifts in ontogeny and this is reflected in the morphospace of flowers. Changes in morphology are mainly induced by shifts in space, caused by the time of initiation (heterochrony), pressure of organs, and alterations of the size of the floral meristem, and these operate independently or in parallel with genetic factors. A number of examples demonstrate this interaction and its importance in the establishment of different floral forms. Both perspectives are complementary and should be considered in the understanding of factors regulating floral development. It is suggested that floral evolution is the result of alternating bursts of physical constraints and genetic stabilization processes following each other in succession. Future research needs to combine these different perspectives in understanding the evolution of floral systems and their diversification.

Key messageThis review compares the molecular mechanisms of stem cell control in the shoot apical meristems of mosses and angiosperms and reveals the conserved features and evolution of plant stem cells.The establishment and maintenance of pluripotent stem cells in the shoot apical meristem (SAM) are key developmental processes in land plants including the most basal, bryophytes. Bryophytes, such as Physcomitrium (Physcomitrella) patens and Marchantia polymorpha, are emerging as attractive model species to study the conserved features and evolutionary processes in the mechanisms controlling stem cells. Recent studies using these model bryophyte species have started to uncover the similarities and differences in stem cell regulation between bryophytes and angiosperms. In this review, we summarize findings on stem cell function and its regulation focusing on different aspects including hormonal, genetic, and epigenetic control. Stem cell regulation through auxin, cytokinin, CLAVATA3/EMBRYO SURROUNDING REGION-RELATED (CLE) signaling and chromatin modification by Polycomb Repressive Complex 2 (PRC2) and PRC1 is well conserved. Several transcription factors crucial for SAM regulation in angiosperms are not involved in the regulation of the SAM in mosses, but similarities also exist. These findings provide insights into the evolutionary trajectory of the SAM and the fundamental mechanisms involved in stem cell regulation that are conserved across land plants.

The development of the plant root system is highly plastic, which allows the plant to adapt to various environmental stresses. Salt stress inhibits root elongation by reducing the size of the root meristem. However, the mechanism underlying this process remains unclear. In this study, we explored whether and how auxin and nitric oxide (NO) are involved in salt-mediated inhibition of root meristem growth in Arabidopsis (Arabidopsis thaliana) using physiological, pharmacological, and genetic approaches. We found that salt stress significantly reduced root meristem size by down-regulating the expression ofPINFORMED (PIN)genes, thereby reducing auxin levels. In addition, salt stress promoted AUXIN RESISTANT3 (AXR3)/INDOLE-3-ACETIC ACID17 (IAA17) stabilization, which repressed auxin signaling during this process. Furthermore, salt stress stimulated NO accumulation, whereas blocking NO production with the inhibitorNω -nitro-L-arginine-methylester compromised the salt-mediated reduction of root meristem size,PINdown-regulation, and stabilization of AXR3/IAA17, indicating that NO is involved in salt-mediated inhibition of root meristem growth. Taken together, these findings suggest that salt stress inhibits root meristem growth by repressingPINexpression (thereby reducing auxin levels) and stabilizing IAA17 (thereby repressing auxin signaling) via increasing NO levels.

The forms resulting from growth processes are highly sensitive to the nature of the driving impetus, and to the local properties of the medium, in particular, its isotropy or anisotropy. In turn, these local properties can be organized by growth. Here, we consider a growing plant tissue, the shoot apical meristem of Arabidopsis thaliana. In plants, the resistance of the cell wall to the growing internal turgor pressure is the main factor shaping the cells and the tissues. It is well established that the physical properties of the walls depend on the oriented deposition of the cellulose microfibrils in the extracellular matrix or cell wall; this order is correlated to the highly oriented cortical array of microtubules attached to the inner side of the plasma membrane. We used oryzalin to depolymerize microtubules and analyzed its influence on the growing meristem. This had no short-term effect, but it had a profound impact on the cell anisotropy and the resulting tissue growth. The geometry of the cells became similar to that of bubbles in a soap froth. At a multicellular scale, this switch to a local isotropy induced growth into spherical structures. A theoretical model is presented in which a cellular structure grows through the plastic yielding of its walls under turgor pressure. The simulations reproduce the geometrical properties of a normal tissue if cell division is included. If not, a \"cell froth\" very similar to that observed experimentally is obtained. Our results suggest strong physical constraints on the mechanisms of growth regulation.

Complex multicellular organisms evolved on Earth in an oxygen-rich atmosphere 1 ; their tissues, including stem-cell niches, require continuous oxygen provision for efficient energy metabolism 2 . Notably, the maintenance of the pluripotent state of animal stem cells requires hypoxic conditions, whereas higher oxygen tension promotes cell differentiation 3 . Here we demonstrate, using a combination of genetic reporters and in vivo oxygen measurements, that plant shoot meristems develop embedded in a low-oxygen niche, and that hypoxic conditions are required to regulate the production of new leaves. We show that hypoxia localized to the shoot meristem inhibits the proteolysis of an N-degron-pathway 4 , 5 substrate known as LITTLE ZIPPER 2 (ZPR2)—which evolved to control the activity of the class-III homeodomain-leucine zipper transcription factors 6 – 8 —and thereby regulates the activity of shoot meristems. Our results reveal oxygen as a diffusible signal that is involved in the control of stem-cell activity in plants grown under aerobic conditions, which suggests that the spatially distinct distribution of oxygen affects plant development. In molecular terms, this signal is translated into transcriptional regulation by the N-degron pathway, thereby linking the control of metabolic activity to the regulation of development in plants. Hypoxia in the shoot meristem of Arabidopsis links the regulation of metabolic activity to development by inhibiting proteolysis of a substrate of the N-degron pathway, which controls class-III homeodomain-leucine zipper transcription factors.

In plants, roots are essential for water and nutrient acquisition. MicroRNAs (miRNAs) regulate their target mRNAs by transcript cleavage and/or inhibition of protein translation and are known as major post‐transcriptional regulators of various developmental pathways and stress responses. In Arabidopsis thaliana, four isoforms of miR169 are encoded by 14 different genes and target diverse mRNAs, encoding subunits A of the NF‐Y transcription factor complex. These miRNA isoforms and their targets have previously been linked to nutrient signalling in plants. By using mimicry constructs against different isoforms of miR169 and miR‐resistant versions of NF‐YA genes we analysed the role of specific miR169 isoforms in root growth and branching. We identified a regulatory node involving the particular miR169defg isoform and NF‐YA2 and NF‐YA10 genes that acts in the control of primary root growth. The specific expression of MIM169defg constructs altered specific cell type numbers and dimensions in the root meristem. Preventing miR169defg‐regulation of NF‐YA2 indirectly affected laterial root initiation. We also showed that the miR169defg isoform affects NF‐YA2 transcripts both at mRNA stability and translation levels. We propose that a specific miR169 isoform and the NF‐YA2 target control root architecture in Arabidopsis.

Inflorescence structures result from the activities of meristems, which coordinate both the renewal of stem cells in the center and organ formation at the periphery. The fate of a meristem is specified at its initiation and changes as the plant develops. During rice inflorescence development, newly formed meristems acquire a branch meristem (BM) identity, and can generate further meristems or terminate as spikelets. Thus, the form of rice inflorescence is determined by a reiterative pattern of decisions made at the meristems. In the dominant gain-of-function mutant tawawa1-D, the activity of the inflorescence meristem (IM) is extended and spikelet specification is delayed, resulting in prolonged branch formation and increased numbers of spikelets. In contrast, reductions in TAWAWA1 (TAW1) activity cause precocious IM abortion and spikelet formation, resulting in the generation of small inflorescences. TAW1 encodes a nuclear protein of unknown function and shows high levels of expression in the shoot apical meristem, the IM, and the BMs. TAW1 expression disappears from incipient spikelet meristems (SMs). We also demonstrate that members of the SHORT VEGETATIVE PHASE subfamily of MADS-box genes function downstream of TAW1. We thus propose that TAW1 is a unique regulator of meristem activity in rice and regulates inflorescence development through the promotion of IM activity and suppression of the phase change to SM identity.