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Our anatomical analysis revealed that a dry maize seed contains four to five embryonic leaves at different developmental stages. Rudimentary kranz structure (KS) is apparent in the first leaf with a substantial density, but its density decreases toward younger leaves. Upon imbibition, leaf expansion occurs rapidly with new KSs initiated from the palisade-like ground meristem cells in the middle of the leaf. In parallel to the anatomical analysis, we obtained the time course transcriptomes for the embryonic leaves in dry and imbibed seeds every 6 h up to hour 72. Over this time course, the embryonic leaves exhibit transcripts of 30,255 genes at a level that can be regarded as “expressed.” In dry seeds, ∼25,500 genes are expressed, showing functional enrichment in transcription, RNA processing, protein synthesis, primary metabolic pathways, and calcium transport. During the 72-h time course, ∼13,900 genes, including 590 transcription factor genes, are differentially expressed. Indeed, by 30 h postimbibition, ∼2,200 genes expressed in dry seeds are already down-regulated, and ∼2,000 are up-regulated. Moreover, the top 1% expressed genes at 54 h or later are very different from those before 30 h, reflecting important developmental and physiological transitions. Interestingly, clusters of genes involved in hormone metabolism, signaling, and responses are differentially expressed at various time points and TF gene expression is also modular and stage specific. Our dataset provides an opportunity for hypothesizing the timing of regulatory actions, particularly in the context of KS development.

WUSCHEL-related homeobox (WOX) transcription factors play a crucial role in lateral organ development in several plant species; however, their precise functions in soybean (Glycine max [L.] Merr.) were unclear. Here, we identified two independent multi-leaflet soybean mutants, mlw48-8 and mlw48-161, from a CRISPR/Cas9-engineered mutant library in the Williams 82 background. Both mutants exhibited irregular leaf margins, and the upper leaves were narrow and almost lanceolate at maturity. Molecular analysis revealed that these are allelic mutants with independent mutations in the WUSCHEL-related homeobox1 (GmWOX1A) gene. A transcriptome analysis demonstrated that GmWOX1A modulates the expression of auxin- and leaf development–related genes. Yeast two-hybrid and split-luciferase complementation imaging assays revealed that GmWOX1A interacts with the YABBY family protein GmYAB5, providing further evidence of its potential involvement in leaf development. Notably, the mlw48-161 mutant showed an increased seed number per plant. Consequently, our study not only provides valuable insights into the role of GmWOX1A in soybean leaf development but also offers a potential strategy for high-yield breeding.

In plants, asymmetric cell divisions result in distinct cell fates forming large and small daughter cells, adding to the cellular diversity in an organ. SCARECROW (SCR), a GRAS domain-containing transcription factor controls asymmetric periclinal cell divisions in flowering plants by governing radial patterning of ground tissue in roots and cell proliferation in leaves. Though SCR homologs are present across land plant lineages, the current understanding of their role in cellular patterning and leaf development is mostly limited to flowering plants. Our phylogenetic analysis identified three SCR homologs in moss Physcomitrium patens, amongst which PpSCR1 showed highest expression in gametophores and its promoter activity was prominent at the mid-vein and the flanking leaf blade cells pointing towards its role in leaf development. Notably, out of the three SCR homologs, only the ppscr1 knock-out lines developed slender leaves with four times narrower leaf blade and three times thicker mid-vein. Detailed histology studies revealed that slender leaf phenotype is either due to the loss of anticlinal cell divisions or failure of periclinal division suppression in the leaf blade. RNA-Seq analyses revealed that genes responsible for cell division and differentiation are expressed differentially in the mutant. PpSCR1 overexpression lines exhibited significantly wider leaf lamina, further reconfirming the role in leaf development. Together, our data suggests that PpSCR1 is involved in the leaf blade and mid-vein development of moss and that its role in the regulation of cell division and proliferation is ancient and conserved among flowering plants and mosses.Key messageThe GRAS domain containing protein PpSCR1 regulates asymmetric cell divisions and governs leaf blade and mid-vein development in moss Physcomitrium patens.

TEOSINTE-BRANCHED/CYCLOIDEA/PCF (TCP) proteins are plant-specific transcription factors (TFs) that play a pivotal role in leaf development by controlling cell proliferation and differentiation. In this study, the authors systematically analyzed the phylogeny, sequence structure, domain feature and expression profiles of TCP genes in Liriodendron chinense, an ornamental tree species with peculiar leaf shape. A total of 17 LcTCP genes were identified in L. chinense genome, which could be grouped into two classes according to their features in the TCP domain. RT-qPCR analysis showed that the expression levels of four TCP genes in Class I (LcTCP21, LcTCP9, LcTCP19a, and LcTCP19b) and three genes in Class II (LcTCP4a, LcTCP4b, and LcTCP24) were consistently higher than those of the other LcTCP genes during leaf development. Degradome data analysis revealed that three LcTCP genes, LcTCP4a, LcTCP4b, and LcTCP24, are targeted by lch-miR319c. Further, LcTCP4a/b and LcTCP24 differed significantly in their expression levels between leaf buds and lobed leaves. However, the expression patterns of LcTCP21 and LcTCP9 contrasted with those of LcTCP19a and LcTCP19b, implying that leaf development in L. chinense may be regulated by a balance between the antagonistic roles of Class I and Class II LcTCP genes. Furthermore, overexpression of LcTCP4 in Arabidopsis thaliana caused a tendency of leaf margin smoothness, and down-regulated the expression levels of genes involved in cell division, AtCYCD3,1 and AtKNOLLE, indicating that LcTCP4 may influence leaf margin shape by inhibiting cell proliferation. Overall, this study provided a comprehensive assessment of the LcTCP gene family and serves as a cornerstone for subsequent functional verification of the LcTCP genes in regulating the leaf development of L. chinense.

Premise of the study: How leaf shape is regulated is a long-standing question in botany. For diverse groups of dicotyledon species, lamina folding along the veins and geometry of the space available for the primordia can explain the palmate leaf morphology. Dubbed the kirigami theory, this hypothesis of fold-dependent leaf shape regulation has remained largely theoretical. Using Acer pseudoplatanus, we investigated the mechanisms behind the two key processes of kirigami leaf development. Methods: Cytological examination and quantitative analyses were used to examine the course of the vein-dependent lamina folding. Surgical ablation and tissue culturing were employed to test the effects of physical constraints on primordia growth. The final morphology of leaves growing without steric constraints were predicted mathematically. Key results: The cytological examination showed that the lamina's abaxial side along the veins grows substantially more than the adaxial side. The abaxial hypergrowth along the veins and the lamina extension correlated with the lamina folding. When a primordium was released from the physical constraints imposed by the other primordia, it rapidly grew into the newly available space, while maintaining the curvature inward. The morphology of such a leaf was predicted to lack symmetry in the lobe shapes. Conclusions: The enhanced growth on the abaxial side of the lamina along the veins is likely to drive lamina folding. The surgical ablation provided clear support for the space-filling nature of leaf growth; thus, steric constraints play a role in determination of the shapes of folded leaves and probably also of the final leaf morphology.

In evergreen tropical forests, the extent, magnitude, and controls on photosynthetic seasonality are poorly resolved and inadequately represented in Earth system models. Combining camera observations with ecosystem carbon dioxide fluxes at forests across rainfall gradients in Amazônia, we show that aggregate canopy phenology, not seasonality of climate drivers, is the primary cause of photosynthetic seasonality in these forests. Specifically, synchronization of new leaf growth with dry season litterfall shifts canopy composition toward younger, more light-use efficient leaves, explaining large seasonal increases (~27%) in ecosystem photosynthesis. Coordinated leaf development and demography thus reconcile seemingly disparate observations at different scales and indicate that accounting for leaf-level phenology is critical for accurately simulating ecosystem-scale responses to climate change.

• Synergistic improvement in leaf photosynthetic area and rate is essential for enhancing crop yield. However, reduction in leaf area occurs earlier than that in the photosynthetic rate under potassium (K) deficiency stress. • The photosynthetic capacity and anatomical characteristics of oilseed rape (Brassica napus) leaves in different growth stages under different K levels were observed to clarify the mechanism regulating this process. • Increased mesophyll cell size and palisade tissue thickness, in K-deficient leaves triggered significant enlargement of mesophyll cell area per transverse section width (S/W), in turn inhibiting leaf expansion. However, there was only a minor difference in chloroplast morphology, likely because of K redistribution from vacuole to chloroplast. As K stress increased, decreased mesophyll surface exposed to intercellular space and chloroplast density induced longer distances between neighbouring chloroplasts (Dchl-chl ) and decreased the chloroplast surface area exposed to intercellular space (Sc/S); conversely this induced a greater limitation imposed by the cytosol on CO₂ transport, further reducing the photosynthetic rate. • Changes in S/W associated with mesophyll cell morphology occurred earlier than changes in Sc/S and Dchl-chl , inducing a decrease in leaf area before photosynthetic rate reduction. Adequate K nutrition simultaneously increases photosynthetic area and rate, thus enhancing crop yield.

Comparisons of concepts in monocot and eudicot leaf development are presented, with attention to the morphologies and mechanisms separating these angiosperm lineages. Monocot and eudicot leaves are distinguished by the differential elaborations of upper and lower leaf zones, the formation of sheathing/nonsheathing leaf bases and vasculature patterning. We propose that monocot and eudicot leaves undergo expansion of mediolateral domains at different times in ontogeny, directly impacting features such as venation and leaf bases. Furthermore, lineage-specific mechanisms in compound leaf development are discussed. Although models for the homologies of enigmatic tissues, such as ligules and stipules, are proposed, tests of these hypotheses are rare. Likewise, comparisons of stomatal development are limited to Arabidopsis and a few grasses. Future studies may investigate correlations in the ontogenies of parallel venation and linear stomatal files in monocots, and the reticulate patterning of veins and dispersed stoma in eudicots. Although many fundamental mechanisms of leaf development are shared in eudicots and monocots, variations in the timing, degree and duration of these ontogenetic events may contribute to key differences in morphology. We anticipate that the incorporation of an ever-expanding number of sequenced genomes will enrich our understanding of the developmental mechanisms generating eudicot and monocot leaves.

CONTENTS: Summary 926 I. Introduction 927 II. What triggers a plant to leaf-out? 928 III. Variation in leaf-out among species 929 IV. Leaf-out and climate change 932 V. Conclusions 937 Acknowledgements 937 References 937 SUMMARY: Leafing-out of woody plants begins the growing season in temperate forests and is one of the most important drivers of ecosystem processes. There is substantial variation in the timing of leaf-out, both within and among species, but the leaf development of almost all temperate tree and shrub species is highly sensitive to temperature. As a result, leaf-out times of temperate forests are valuable for observing the effects of climate change. Analysis of phenology data from around the world indicates that leaf-out is generally earlier in warmer years than in cooler years and that the onset of leaf-out has advanced in many locations. Changes in the timing of leaf-out will affect carbon sequestration, plant-animal interactions, and other essential ecosystem processes. The development of remote sensing methods has expanded the scope of leaf-out monitoring from the level of an individual plant or forest to an entire region. Meanwhile, historical data have informed modeling and experimental studies addressing questions about leaf-out timing. For most species, onset of leaf-out will continue to advance, although advancement may be slowed for some species because of unmet chilling requirements. More information is needed to reduce the uncertainty in predicting the timing of future spring onset.

Q-type C₂H₂ zinc finger proteins (ZFPs) form a subfamily of transcription factors that contain a plant-specific QALGGH amino acid motif. A total of 47 expressed Q-type C₂H₂ zinc finger genes in bread wheat (Triticum aestivum) (designated TaZFP) were identified from the current databases. Protein sequence analysis for the presence of ERF-associated amphiphilic repressor (EAR) motif sequences from known transcriptional repressors revealed that 26% of the TaZFP subfamily members contain an EAR motif. Quantitative RT-PCR analysis of the mRNA distribution of 44 TaZFP genes in various organs revealed that 30 genes were predominantly expressed in the roots. The majority of the TaZFP genes showed significant changes in their mRNA levels during leaf development and aging. Expression of 37 TaZFP genes in the leaves and roots responded to drought stress at least in one organ with 74% of the drought-responsive TaZFP genes being down-regulated in the drought-stressed roots. In contrast, only 6 out of the 44 TaZFP genes showed expression changes in the leaves with sucrose treatment. Expression of 50% of the drought-responsive TaZFP genes in the leaves (16 genes analysed) did not respond to ABA treatment, indicating that some TaZFP genes are involved in ABA-independent signalling pathways. These results indicate that the Q-type TaZFP subfamily is likely to have an important role in wheat roots and is enriched with members that are potentially involved in regulating cellular activities during changes of the physiological status of plant cells, as it occurs during drought stress or leaf development/aging.