Behavior of Leaf Meristems and Their Modification

Functional characterisation of BnaA02.TOP1α and BnaC02.TOP1α involved in true leaf biomass accumulation in Brassica napus L

Leaves, as primary photosynthetic organs essential for high crop yield and quality, have attracted significant attention. The functions of DNA topoisomerase 1α (TOP1α) in various biological processes, including leaf development, in Brassica napus remain unknown. Here, four paralogs of BnaTOP1α, namely BnaA01.TOP1α, BnaA02.TOP1α, BnaC01.TOP1α and BnaC02.TOP1α, were identified and cloned in the B. napus inbred line 'K407'. Expression pattern analysis revealed that BnaA02.TOP1α and BnaC02.TOP1α, but not BnaA01.TOP1α and BnaC01.TOP1α, were persistently and highly expressed in B. napus true leaves. Preliminary analysis in Arabidopsis thaliana revealed that BnaA02.TOP1α and BnaC02.TOP1α paralogs, but not BnaA01.TOP1α and BnaC01.TOP1α, performed biological functions. Targeted mutations of four BnaTOP1α paralogs in B. napus using the CRISPR-Cas9 system revealed that BnaA02.TOP1α and BnaC02.TOP1α served as functional paralogs and redundantly promoted true leaf number and size, thereby promoting true leaf biomass accumulation. Moreover, BnaA02.TOP1α modulated the levels of endogenous gibberellins, cytokinins and auxins by indirectly regulating several genes related to their metabolism processes. BnaA02.TOP1α directly activated BnaA03.CCS52A2 and BnaC09.AN3 by facilitating the recruitment of RNA polymerase II and modulating H3K27me3, H3K36me2 and H3K36me3 levels at these loci and indirectly activated the BnaA08.PARL1 expression, thereby positively controlling the true leaf size in B. napus. Additionally, BnaA02.TOP1α indirectly activated the BnaA07.PIN1 expression to positively regulate the true leaf number. These results reveal the important functions of BnaTOP1α and provide insights into the regulatory network controlling true leaf biomass accumulation in B. napus.

Cytogenetic and Molecular Effects of Kaolin's Foliar Application in Grapevine (Vitis vinifera L.) under Summer's Stressful Growing Conditions

Grapevine varieties from "Douro Superior" (NE Portugal) experience high temperatures, solar radiation, and water deficit during the summer. This summer's stressful growing conditions induce nucleic acids, lipids, and protein oxidation, which cause cellular, physiological, molecular, and biochemical changes. Cell cycle anomalies, mitosis delay, or cell death may occur at the cellular level, leading to reduced plant productivity. However, the foliar application of kaolin (KL) can mitigate the impact of abiotic stress by decreasing leaf temperature and enhancing antioxidant defence. Hence, this study hypothesised that KL-treated grapevine plants growing in NE Portugal would reveal, under summer stressful growing conditions, higher progression and stability of the leaf mitotic cell cycle than the untreated (control) plants. KL was applied after veraison for two years. Leaves, sampled 3 and 5 weeks later, were cytogenetically, molecularly, and biochemically analysed. Globally, integrating these multidisciplinary data confirmed the decreased leaf temperature and enhanced antioxidant defence of the KL-treated plants, accompanied by an improved regularity and completion of the leaf cell cycle relative to the control plants. Nevertheless, the KL efficacy was significantly influenced by the sampling date and/or variety. In sum, the achieved results confirmed the hypothesis initially proposed.

Suspension culture of stem cells established of Calendula officinalis L

Plant stem cell cultures have so far been established in only a few plant species using cambial meristematic cells. The presence of stem cells or stem cell-like cells in other organs and tissues of the plant body, as well as the possibility of de novo generation of meristematic cells from differentiated cells, allow to consider the establishment of stem cell cultures in a broader range of species. This study aimed to establish a stem cell culture of the medicinal plant Calendula officinalis L. Callus tissues were induced from leaf and root explants, and already at this stage, stem and dedifferentiated cells could be identified. The cell suspension cultures established both from the root- and leaf-derived calli contained a high proportion of stem cells (92-93% and 72-73%, respectively). The most effective combination of growth regulators for the development of stem cells in calli as well as cell cultures was 1.0 mg/L 2,4-D and 0.5 mg/L BAP. The highest amount of stem cells (5.60-5.72 × 105) was in cell suspension derived from the roots. An effective protocol for the establishment of marigold stem cell suspension culture was developed. The ratio of root-derived stem cells against dedifferentiated cells exceeded 90%.

The role of Class Ⅱ KNOX family in controlling compound leaf patterning in Medicago truncatula

Compound leaf development requires the coordination of genetic factors, hormones, and other signals. In this study, we explored the functions of Class Ⅱ KNOTTED-like homeobox (KNOXII) genes in the model leguminous plant Medicago truncatula. Phenotypic and genetic analyses suggest that MtKNOX4, 5 are able to repress leaflet formation, while MtKNOX3, 9, 10 are not involved in this developmental process. Further investigations have shown that MtKNOX4 represses the CK signal transduction, which is downstream of MtKNOXⅠ-mediated CK biosynthesis. Additionally, two boundary genes, FUSED COMPOUND LEAF1 (orthologue of Arabidopsis Class M KNOX) and NO APICAL MERISTEM (orthologue of Arabidopsis CUP-SHAPED COTYLEDON), are necessary for MtKNOX4-mediated compound leaf formation. These findings suggest, that among the members of MtKNOXⅡ, MtKNOX4 plays a crucial role in integrating the CK pathway and boundary regulators, providing new insights into the roles of MtKNOXⅡ in regulating the elaboration of compound leaves in M. truncatula.

Applying quantitative spatial phenotypes analysis to the investigation of peltate glandular trichomes development pattern in Perilla frutescens

BACKGROUND

Glandular trichomes, often referred to as "phytochemical factories", plays a crucial role in plant growth and metabolism. As the site for secretion and storage, the development of glandular trichomes is related to the dynamic biosynthesis of specialised metabolites. The study aims to explore the relationship between spatial phenotype and dynamic metabolism of glandular trichomes, and establish a novel approach for the exploration and study of the regulatory mechanism governing the development of glandular trichomes.

RESULTS

In this study, we proposed a technical route based on the relative deviation value to distinguish the peltate glandular trichomes (PGTs) from the background tissues and extract their spatial phenotype. By defining glandular trichome developmental stages based on the leaf vein growth axis, we found that young PGTs were densely distributed near the proximal end of growth axis of the leaf veins, where perillaketone, a primary metabolite of PGTs, is predominantly accumulated. Conversely, mature PGTs are typically found near the distal end of the mid-vein growth axis and the lateral end of the secondary vein growth axis, where the accumulation rate of isoegomaketone and egomaketone exceeds that of perillaketone in PGTs. We further identified spatial phenotypic parameters, Lsum and d, as independent variables to construct a linear regression model that illustrates the relationship between the spatial phenotypes and metabolite content of PGTs, including perillaketone (R2 = 0.698), egomaketone (R2 = 0.593), isoegomaketone (R2 = 0.662) and the sum of the amount (R2 = 0.773).

CONCLUSIONS

This model proved that the development of PGTs was correlated with the growth of the entire leaf, and the development stage of PGTs can be identifined by spatial phenotypes based on the leaf veins. In conclusion, the findings of this study enhance our understanding of correlation between spatial phenotype and development of glandular trichomes and offer a new approach to explore and study the regulatory mechanism of glandular trichome development.

A Common Molecular Signature Indicates the Pre-Meristematic State of Plant Calli

In response to different degrees of mechanical injury, certain plant cells re-enter the division cycle to provide cells for tissue replenishment, tissue rejoining, de novo organ formation, and/or wound healing. The intermediate tissue formed by the dividing cells is called a callus. Callus formation can also be induced artificially in vitro by wounding and/or hormone (auxin and cytokinin) treatments. The callus tissue can be maintained in culture, providing starting material for de novo organ or embryo regeneration and thus serving as the basis for many plant biotechnology applications. Due to the biotechnological importance of callus cultures and the scientific interest in the developmental flexibility of somatic plant cells, the initial molecular steps of callus formation have been studied in detail. It was revealed that callus initiation can follow various ways, depending on the organ from which it develops and the inducer, but they converge on a seemingly identical tissue. It is not known, however, if callus is indeed a special tissue with a defined gene expression signature, whether it is a malformed meristem, or a mass of so-called "undifferentiated" cells, as is mostly believed. In this paper, I review the various mechanisms of plant regeneration that may converge on callus initiation. I discuss the role of plant hormones in the detour of callus formation from normal development. Finally, I compare various Arabidopsis gene expression datasets obtained a few days, two weeks, or several years after callus induction and identify 21 genes, including genes of key transcription factors controlling cell division and differentiation in meristematic regions, which were upregulated in all investigated callus samples. I summarize the information available on all 21 genes that point to the pre-meristematic nature of callus tissues underlying their wide regeneration potential.

Diversity of tomato leaf form provides novel insights into breeding

Tomato (Solanum lycopersicum L.) is cultivated widely globally. The crop exhibits tremendous morphological variations because of its long breeding history. Apart from the commercial tomato varieties, wild species and heirlooms are grown in certain regions of the world. Since the fruit constitutes the edible part, much of the agronomical research is focused on it. However, recent studies have indicated that leaf morphology influences fruit quality. As leaves are specialized photosynthetic organs and the vascular systems transport the photosynthetic products to sink organs, the architectural characteristics of the leaves have a strong influence on the final fruit quality. Therefore, comprehensive research focusing on both the fruit and leaf morphology is required for further tomato breeding. This review summarizes an overview of knowledge of the basic tomato leaf development, morphological diversification, and molecular mechanisms behind them and emphasizes its importance in breeding. Finally, we discuss how these findings and knowledge can be applied to future tomato breeding.

Overexpression of the Liriodendron tulipifera BOP2 Gene (LtuBOP2) Affects Leaf Margin Development in Transgenic Arabidopsis thaliana

BLADE-ON-PETIOLE 2 (BOP2) plays a pivotal role in leaf morphogenesis. Liriodendron tulipifera is a suitable model for exploring the molecular mechanisms underlying leaf serration formation, which are largely unknown. Here, we isolated the full-length LtuBOP2 gene and its promoter from L. tulipifera and characterized its function in leaf morphogenesis through multidimensional approaches. The spatiotemporal expression pattern of LtuBOP2 indicated the high expression of LtuBOP2 in stems and leaf buds. We constructed LtuBOP2 promoter, fused the promoter sequences to the β-glucuronidase (GUS) gene, and then transformed them into Arabidopsis thaliana. Histochemical GUS staining results indicated that GUS activity was higher in petioles and the main vein. LtuBOP2 overexpression in A. thaliana caused moderate serration in the leaf tip, owing to the increased number of abnormal lamina epidermal cells and defective vascular tissue, thus indicating a novel role of BOP2. The ectopic expression of LtuBOP2 in A. thaliana promoted the expression of the lateral organ boundary gene ASYMMETRIC LEAVES2 (AS2) and inhibited JAGGED (JAG) and CUP-SHAPED COTYLEDON2 (CUC2) expression to establish leaf proximal-distal polarity. Moreover, LtuBOP2 participated in leaf serration formation by promoting the antagonistic relationship between KNOX I and hormones during leaf margin development. Our findings revealed the role of LtuBOP2 in the proximal-distal polarity formation and development of leaf margin morphology, providing new insights into the regulatory mechanisms of the leaf formation development of L. tulipifera.

The function of BoTCP25 in the regulation of leaf development of Chinese kale

XG Chinese kale (Brassica oleracea cv. 'XiangGu') is a variety of Chinese kale and has metamorphic leaves attached to the true leaves. Metamorphic leaves are secondary leaves emerging from the veins of true leaves. However, it remains unknown how the formation of metamorphic leaves is regulated and whether it differs from normal leaves. BoTCP25 is differentially expressed in different parts of XG leaves and respond to auxin signals. To clarify the function of BoTCP25 in XG Chinese kale leaves, we overexpressed BoTCP25 in XG and Arabidopsis, and interestingly, its overexpression caused Chinese kale leaves to curl and changed the location of metamorphic leaves, whereas heterologous expression of BoTCP25 in Arabidopsis did not show metamorphic leaves, but only an increase in leaf number and leaf area. Further analysis of the expression of related genes in Chinese kale and Arabidopsis overexpressing BoTCP25 revealed that BoTCP25 could directly bind the promoter of BoNGA3, a transcription factor related to leaf development, and induce a significant expression of BoNGA3 in transgenic Chinese kale plants, whereas this induction of NGA3 did not occur in transgenic Arabidopsis. This suggests that the regulation of Chinese kale metamorphic leaves by BoTCP25 is dependent on a regulatory pathway or elements specific to XG and that this regulatory element may be repressed or absent from Arabidopsis. In addition, the expression of miR319's precursor, a negative regulator of BoTCP25, also differed in transgenic Chinese kale and Arabidopsis. miR319's transcrips were significantly up-regulated in transgenic Chinese kale mature leaves, while in transgenic Arabidopsis, the expression of miR319 in mature leaves was kept low. In conclusion, the differential expression of BoNGA3 and miR319 in the two species may be related to the exertion of BoTCP25 function, thus partially contributing to the differences in leaf phenotypes between overexpressed BoTCP25 in Arabidopsis and Chinese kale.

Is auxin enough? Cytokinins and margin patterning in simple leaves

The interplay between auxin and cytokinins affects facets of plant development as different as ovule formation and lateral root initiation. Moreover, cytokinins favor complexity in the development of Solanum lycopersicum and Cardamine hirsuta compound leaves. Nevertheless, no role has been proposed for cytokinins in patterning the margins of the simple leaves of Arabidopsis thaliana, a process that is assumed to be sufficiently explained by auxin localization. Here, we discuss evidence supporting the hypothesis that cytokinins play a role in simple leaf margin morphogenesis via crosstalk with auxin, as occurs in other plant developmental events. Indeed, mutant or transgenic arabidopsis plants defective in cytokinin biosynthesis or signaling, or with increased cytokinin degradation have leaf margins less serrated than the wild type.

Molecular mechanism of leaf adaxial upward curling caused by BpPIN3 suppression in Betula pendula

Leaves are one of the vegetative organs of plants that are essential for plant growth and development. PIN-FORMED (PINs) gene is an indoleacetic acid (IAA) transporter that plays a critical role in leaf development. To determine the function of BpPIN3 in leaf polarity formation in Betula pendula, the transgenic lines with BpPIN3 overexpression (OE) and BpPIN3-reduced expression (RE) were analyzed using the Agrobacterium-mediated method. The RE lines displayed the characteristics of leaf margin adaxial upward curling, with lower expression of BpPIN3 resulting in greater rolling. Tissue localization of IAA in the auxin GUS reporter system proved that auxin in the RE was mainly distributed in the secondary veins, palisade tissues, and epidermal cells in the leaf margin area. The auxin content in the leaf margin area was significantly greater than that in the main vein tissue. The cell density of the palisade tissue and the ratio of palisade tissue to spongy tissue in the curled leaf margin of the RE lines were found to be significantly decreased. RNA-seq analysis revealed that the RE hormone-signaling pathway genes were significantly enriched compared with those of the OE and WT lines; in particular, the auxin response-related genes SAURs (i.e., SAUR23, SAUR24, SAUR28, and SAUR50) and GH3.10 were found to be significantly upregulated. qRT-PCR analysis indicated that BpPIN3 expression at the leaf margin was significantly lower than that near the main vein in the RE lines. In contrast, the expression levels of SAURs and GH3.10 were significantly higher than those near the midrib. In conclusion, BpPIN3 regulates the expression of auxin response-related genes and the polar transport of auxin to change the polar form of the proximal and distal axes of birch leaves.

Position of meristems and the angles of the cell division plane regulate the uniqueness of lateral organ shape

Leaf meristem is a cell proliferative zone present in the lateral organ primordia. In this study, we examined how cell proliferative zones in primordia of planar floral organs and polar auxin transport inhibitor (PATI)-treated leaf organs differ from those of non-treated foliage leaves of Arabidopsis thaliana, with a focus on the accumulation pattern of ANGUSTIFOLIA3 (AN3) protein, a key element for leaf meristem positioning. We found that PATI-induced leaf shape changes were correlated with cell division angle but not with meristem positioning/size or AN3 localisation. In contrast, different shapes between sepals and petals compared with foliage leaves were associated with both altered meristem position, due to altered AN3 expression patterns, and different distributions of cell division angles. A numerical simulation showed that meristem position majorly affected the final shape but biased cell division angles had a minor effect. Taken together, these results suggest that the unique shapes of different lateral organs depend on the position of the meristem in the case of floral organs and cell division angles in the case of leaf organs with different auxin flow.

Genetic analysis of a DROOPING LEAF mutant allele dl-6 associated with a twisted and folded leaf base caused by a deficiency in midrib development in rice

The failure of midrib formation in rice leaf blades results in the drooping leaf (dl) phenotype. A normal DROOPING LEAF (DL) gene is necessary for leaf homeotic transformation, which affects midrib and pistil development. Genetic analysis was performed on a new drooping leaf (dl) mutant named dl-6 in rice. The dl-6 allelic mutant exhibited drooping leaves that were severely folded and twisted at the base but had normal flower structure. The dl-6 allele is a nuclear recessive trait that fits a 3:1 Mendelian segregation ratio. The dl-6 mutant leaves displayed an abnormal main vein (midrib-less) with undeveloped aerenchyma and vascular bundles, resulting in severe leaf drooping. The lack of a midrib in dl-6 caused weak mechanical support, which resulted in folding at the collar junction of the leaf base and downward bending. Through genetic mapping, the dl-6 allele was identified at approximately 28.2 cM on rice chromosome 3. The allele was caused by mutations within the DL (LOC_Os03g11600.1) gene, with specific amino acid substitutions and additions in the encoded protein of the YABBY transcription factor. The dl-6 mutant is a recessive allele encoding a dysfunctional YABBY transcription factor that regulates leaf midrib development and aerenchymatous clear cell structures, leading to a drooping leaf phenotype in rice.

Elucidating the pleiotropic effects of sesame KANADI1 locus on leaf and capsule development

Autonomous seed dispersal is a critical trait for wild plants in natural ecosystems; however, for domesticated crop-plants it can lead to significant yield losses. While seed shattering was a major selection target during the initial domestication of many crops, this trait is still targeted in breeding programs, especially in 'orphan crops' such as sesame, whose capsules dehisce upon ripening. Here we used a mapping population derived from a cross between wild-type (dehiscent) × indehiscent lines to test the hypothesis that the selection against indehiscent alleles in sesame is a consequence of complex genetic interactions associated with yield reduction. We identified a major pleiotropic locus, SiKANADI1, associated with abnormal hyponastic leaf and indehiscent capsule, and genetically dissected its underlying mechanism using a set of near-isogenic lines. Transcriptional, anatomical and physiological information shed light, for the first time, on the polar regulatory gene network in sesame. The pleiotropic effect of SiKANADI1 on leaf and capsule structure and its influence on photosynthetic capacity and final yield are thoroughly characterized. Overall, our results provide new insights on the genetic and morphological mechanisms regulating capsule indehiscence in sesame, and discuss their evolutionary consequences and potential for future sesame breeding.

Genomes of leafy and leafless Platanthera orchids illuminate the evolution of mycoheterotrophy

To improve our understanding of the origin and evolution of mycoheterotrophic plants, we here present the chromosome-scale genome assemblies of two sibling orchid species: partially mycoheterotrophic Platanthera zijinensis and holomycoheterotrophic Platanthera guangdongensis. Comparative analysis shows that mycoheterotrophy is associated with increased substitution rates and gene loss, and the deletion of most photoreceptor genes and auxin transporter genes might be linked to the unique phenotypes of fully mycoheterotrophic orchids. Conversely, trehalase genes that catalyse the conversion of trehalose into glucose have expanded in most sequenced orchids, in line with the fact that the germination of orchid non-endosperm seeds needs carbohydrates from fungi during the protocorm stage. We further show that the mature plant of P. guangdongensis, different from photosynthetic orchids, keeps expressing trehalase genes to hijack trehalose from fungi. Therefore, we propose that mycoheterotrophy in mature orchids is a continuation of the protocorm stage by sustaining the expression of trehalase genes. Our results shed light on the molecular mechanism underlying initial, partial and full mycoheterotrophy.

Genome-Wide Identification and Expression Pattern Analysis of KNOX Gene Family in Orchidaceae

The establishment of lateral organs and subsequent plant architecture involves factors intrinsic to the stem apical meristem (SAM) from which they are derived. KNOTTED1-LIKE HOMEOBOX (KNOX) genes are a family of plant-specific homeobox transcription factors that especially act in determining stem cell fate in SAM. Although KNOXs have been studied in many land plants for decades, there is a dearth of knowledge on KNOX's role in Orchidaceae, the largest and most diverse lineage of flowering plants. In this study, a total of 32 putative KNOX genes were identified in the genomes of five orchid species and further designated into two classes (Class I and Class II) based on phylogenetic relationships. Sequence analysis showed that most orchid KNOX proteins retain four conserved domains (KNOX1, KNOX2, ELK, and Homeobox_KN). Comparative analysis of gene structure showed that the exon-intron structure is conserved in the same clade but most orchids exhibited longer intron, which may be a unique feature of Orchidaceae. Cis-elements identified in the promoter region of orchid KNOXs were found mostly enriched in a function of light responsiveness, followed by MeJA and ABA responsiveness, indicative of their roles in modulating light and phytohormones. Collinear analysis unraveled a one-to-one correspondence among KNOXs in orchids, and all KNOX genes experienced strong purifying selection, indicating the conservation of this gene family has been reinforced across the Orchidaceae lineage. Expression profiles based on transcriptomic data and real-time reverse transcription-quantitative PCR (RT-qPCR) revealed a stem-specific expression of KNOX Class I genes and a broader expression pattern of Class II genes. Taken together, our results provided a comprehensive analysis to uncover the underlying function of KNOX genes in Orchidaceae.

Transcriptome and DNA methylome analyses provide insight into the heterosis in flag leaf of inter-subspecific hybrid rice

Flag leaf heterosis of inter-subspecific hybrid rice is suggested to be related to leaf area, gene expression pattern and allele-specific expression, putatively related to DNA methylation differences between the hybrid and its parents. Inter-subspecific hybrid rice combinations of indica × japonica have great potential to broaden genetic diversity and enhance the heterosis. The genetic and epigenetic molecular mechanism of its heterosis is not completely understood. Here, the dissection of gene expression and epigenetic regulation of an elite inter-subspecific hybrid rice were reported. In the hybrid, plant height, flag leaf area and Pn showed significant heterosis at the heading stage. Chloroplast-related differentially expressed genes (DEGs) and 530 allele-specific expression genes in hybrid were identified. Analysis of the genome-wide distribution of DNA methylation (5-methylcytosine, 5mC) and its association with transcription showed that there were variant DNA methylation maps and that the regulation of gene expression levels was negatively regulated by DNA methylation in the inter-subspecific hybrid rice. Differentially methylated DEGs were significantly enriched in photosynthetic functions. Moreover, distinct 5mC sequence contexts and distinct functional elements (promoter/gene body) may have different influences on heterosis related genes. The data identified heterosis related molecular mechanisms in inter-subspecific hybrid rice and suggested that epigenetic changes could extensively influence the flag leaf gene expression and heterosis.

The legume-specific transcription factor E1 controls leaf morphology in soybean

BACKGROUND

The leaf is a determinate organ essential for photosynthesis, whose size and shape determine plant architecture and strongly affect agronomic traits. In soybean, the molecular mechanism of leaf development is not well understood. The flowering repressor gene E1, which encodes a legume-specific B3-like protein, is known to be the gene with the largest influence on soybean flowering and maturity. However, knowledge of its potential other functions remains poor.

RESULTS

Here, we identified a novel function of E1 protein in leaf development. Unifoliolate leaves of E1-overexpression (E1-OE) lines were smaller and curlier than those of wild type DongNong 50 (DN50) and Williams 82 (W82). Transverse histological sections showed disorganized cells and significantly elevated palisade tissue number, spongy tissue number, and bulliform cell number in E1-OE lines. Our results indicate that E1 binds to the promoters of the leaf- development-related CINCINNATA (CIN)-like TEOSINTE BRANCHED1/CYCLOIDEA/PROLIFERATING CELL FACTOR (TCP) transcription factor genes to negatively regulate their expression.

CONCLUSIONS

Our findings identify E1 as an important new factor in soybean leaf development.

The Diverse Manifestations of Regeneration and Why We Need to Study Them

For hundreds of years, the question of why some organisms can regenerate missing body parts while others cannot has remained poorly understood. This has been due in great part to the inability to genetically, molecularly, and cellularly dissect this problem for most of the history of the field. It has only been in the past 20-30 years that important mechanistic advances have been made in methodologies that introduce loss and gain of gene function in animals that can regenerate. However, we still have a very incomplete understanding of how broadly regenerative abilities may be dispersed across species and whether or not such properties share a common evolutionary origin, which may have emerged independently or both. Understanding regeneration, therefore, will require rigorously practiced fundamental, curiosity-driven, discovery research. Expanding the number of research organisms used to study regeneration allows us to uncover aspects of this problem we may not yet know exist and simultaneously increases our chances of solving this long-standing problem of biology.

The leaf meristem enigma: The relationship between the plate meristem and the marginal meristem

Leaf organogenesis is governed by the spatiotemporal activity of the leaf meristem, which has far greater mitotic activity than the shoot apical meristem. The two types of leaf meristems, the plate meristem and the marginal meristem, are distinguished by the location and longevity of their cell proliferative activity. Most leaf lamina outgrowth depends on the plate meristem. The presence of the marginal meristem was a matter of debate in classic anatomy, but recent genetic analyses of leaf growth in Arabidopsis thaliana confirmed its short-lived activity. Several genes key for the regulation of the two meristem types have been identified, and at least superficially, the systems appear to function independently, as they are regulated by different transcription factors and microRNAs. However, many of the details of these regulatory systems, including how the expression of these key factors is spatially regulated, remain unclear. One major unsolved question is the relationship between the plate meristem and the marginal meristem. Here, I present an overview of our current understanding of this topic and discuss questions that remain to be answered.

Leaflet initiation and blade expansion are separable in compound leaf development

Compound leaves are composed of multiple separate blade units termed leaflets. In tomato (Solanum lycopersicum) compound leaves, auxin promotes both leaflet initiation and blade expansion. However, it is unclear how these two developmental processes interact. With highly variable complexity, tomato compound leaves provide an ideal system to address this question. In this study, we obtained and analyzed mutants of the WUSCHEL-RELATED HOMEOBOX (WOX) family gene SlLAM1 from tomato, whose orthologs in tobacco (Nicotiana sylvestris) and other species are indispensable for blade expansion. We show that SlLAM1 is expressed in the middle and marginal domains of leaves, and is required for blade expansion in leaflets. We demonstrate that sllam1 mutants cause a delay of leaflet initiation and slightly alter the arrangement of first-order leaflets, whereas the overall leaflet number is comparable to that of wild-type leaves. Analysis of the genetic interactions between SlLAM1 and key auxin signaling components revealed an epistatic effect of SlLAM1 in determining the final leaf form. Finally, we show that SlLAM1 is also required for floral organ growth and affects the fertility of gametophytes. Our data suggest that SlLAM1 promotes blade expansion in multiple leaf types, and leaflet initiation can be largely uncoupled from blade expansion during compound leaf morphogenesis.

Fine mapping of a leaf flattening gene Bralcm through BSR-Seq in Chinese cabbage (Brassica rapa L. ssp. pekinensis)

Leaf flattening influences plant photosynthesis, thereby affecting product yield and quality. Here, we obtained a stably inherited leaf crinkled mutant (lcm), derived from the Chinese cabbage doubled haploid (DH) ‘FT’ line using EMS mutagenesis combined with isolated microspore culture. The crinkled phenotype was controlled by a single recessive nuclear gene, namely Bralcm, which was preliminarily mapped to chromosome A01 by bulked segregant analysis RNA-seq, and further between markers SSRS-1 and IndelD-20 using 1,575 recessive homozygous individuals in F₂ population by a map-based cloning method. The target region physical distance was 126.69 kb, containing 23 genes; the marker SSRMG-4 co-segregated with the crinkled trait. Further, we found SSRMG-4 to be located on BraA01g007510.3C, a homolog of AHA2, which encodes H⁺-ATPase2, an essential enzyme in plant growth and development. Sequence analysis indicated a C to T transition in exon 7 of BraA01g007510.3C, resulting in a Thr (ACT) to Ile (ATT) amino acid change. Genotyping revealed that the leaf crinkled phenotype fully co-segregated with this SNP within the recombinants. qRT-PCR demonstrated that BraA01g007510.3C expression in lcm mutant leaves was dramatically higher than that in wild-type ‘FT’. Thus, BraA01g007510.3C is a strong candidate gene for Bralcm, and AHA2 is possibly associated with leaf flattening in Chinese cabbage.

Molecular networks regulating cell division during Arabidopsis leaf growth

Leaves are the primary organs for photosynthesis, and as such have a pivotal role for plant growth and development. Leaf development is a multifactorial and dynamic process involving many genes that regulate size, shape, and differentiation. The processes that mainly drive leaf development are cell proliferation and cell expansion, and numerous genes have been identified that, when ectopically expressed or down-regulated, increase cell number and/or cell size during leaf growth. Many of the genes regulating cell proliferation are functionally interconnected and can be grouped into regulatory modules. Here, we review our current understanding of six important gene regulatory modules affecting cell proliferation during Arabidopsis leaf growth: ubiquitin receptor DA1-ENHANCER OF DA1 (EOD1), GROWTH REGULATING FACTOR (GRF)-GRF-INTERACTING FACTOR (GIF), SWITCH/SUCROSE NON-FERMENTING (SWI/SNF), gibberellin (GA)-DELLA, KLU, and PEAPOD (PPD). Furthermore, we discuss how post-mitotic cell expansion and these six modules regulating cell proliferation make up the final leaf size.

Expression Profiles of ANGUSTIFOLIA3 and SHOOT MERISTEMLESS, Key Genes for Meristematic Activity in a One-Leaf Plant Monophyllaea glabra, Revealed by Whole-Mount In Situ Hybridization

Members of the genus Monophyllaea are unique in that they produce no new organ during the vegetative phase in the shoot; instead, one of the cotyledons grows indeterminately. The mechanism of this unique trait is unclear, in part because of the lack of suitable assessment techniques. We therefore established a whole-mount in situ hybridization technique, a powerful means of examining spatial patterns in gene expression, for Monophyllaea glabra. By using this, we examined the expression pattern of a SHOOT MERISTEMLESS (STM) ortholog, which is indispensable for the formation and maintenance of the shoot apical meristem (SAM) in typical angiosperms. Expression was confined to the groove meristem (GM), which corresponds to the SAM. We also assessed the expression pattern of ANGUSTIFOLIA3 (AN3), a key promoter for cell division in the leaf meristem. It was expressed not only in the basal meristem (BM) tissue with active cell division in the basal part of the growing cotyledon but also in the GM. The findings suggest that the unusual gene expression pattern of the GM underpins the fuzzy morphogenesis of Monophyllaea.

Identification of TCP13 as an Upstream Regulator of ATHB12 during Leaf Development

Leaves grow by distinct phases controlled by gene regulatory networks including many transcription factors. Arabidopsis thaliana homeobox 12 (ATHB12) promotes leaf growth especially during the cell expansion phase. In this study, we identify TCP13, a member of the TCP transcription factor family, as an upstream inhibitor of ATHB12. Yeast one-hybrid screening using a 1.2-kb upstream region of ATHB12 resulted in the isolation of TCP13 as well as other transcription factors. Transgenic plants constitutively expressing TCP13 displays a significant reduction in leaf cell size especially during the cell expansion period, while repression of TCP13 and its paralogs (TCP5 and TCP17) result in enlarged leaf cells, indicating that TCP13 and its paralogs inhibit leaf development, mainly at the cell expansion phase. Its expression pattern during leaf expansion phase is opposite to ATHB12 expression. Consistently, the expression of ATHB12 and its downstream genes decreases when TCP13 was overexpressed, and increases when the expression of TCP13 and its paralogs is repressed. In chromatin immunoprecipitation assays using TCP13-GFP plants, a fragment of the ATHB12 upstream region that contains the consensus sequence for TCP binding is strongly enriched. Taken together, these findings indicate that TCP13 and its paralogs inhibit leaf growth by repressing ATHB12 expression.

Heat stress tolerance assayed in four wine-producing grapevine varieties using a cytogenetic approach

The degree of tolerance to heat stress (HS) differs among grapevine varieties. HS affects the duration and the regularity of the cell cycle in plants. The cytogenetic studies in grapevine are scarce, and the consequences of HS in the mitosis are barely known. This work intends to evaluate the consequences of induced HS in the mitotic cell cycle and chromosomes of four wine-producing varieties: Touriga Franca (TF), Touriga Nacional (TN), Rabigato and Viosinho using a cytogenetic approach. HS (1h at 42 ºC) was induced in plants of the four grapevine varieties that grew in vitro for 11 months. Plants of the same varieties and with equal age were used as control (maintained in vitro at 25 ºC). Three plants per variety and treatment (control and HS) were analysed. After HS, root-tips were collected in all plants and immediately fixed to be used for the preparation of mitotic chromosome spreads. In total, 6,116 root-tip cells were scored. Among the 5,973 dividing cells, 24.33% showed different types of irregularities in all mitotic phases (prophase, metaphase, anaphase, and telophase). After HS, the mitotic index (MI) decreased in the varieties TF and Viosinho, and increased in TN and Rabigato, relatively to the control. However, the average values of MI did not show statistically significant differences (p ˃ 0.05) among varieties, treatments and for the variety x treatment interaction. The percentage of dividing cells with anomalies (%DCA) increased after HS in all varieties relatively to the control. The average values of %DCA presented statistically significant differences (p < 0.05) only between treatments. As far as we know, this work constitutes the first cytogenetic evaluation of the HS effects in the mitotic cell cycle and chromosomes of grapevine using meristematic cells of root-tips. TN has been considered tolerant to various abiotic stresses (drought and excessive heat and light) based on other methodologies. TF and Viosinho have been referred as tolerant to abiotic stress without deeper studies available, and till the development of our investigation, the sensibility of Rabigato was unknown. Among the varieties analysed in this work, Rabigato revealed to be the less tolerant to HS. This research can be useful for selection of grapevine clones more tolerant to HS for commercialization and for the improvement of the economic sustainability.

QTL fine-mapping of soybean (Glycine max L.) leaf type associated traits in two RILs populations

BACKGROUND

The different leaf type associated traits of soybean (Glycine max L.) including leaf area, leaf length, leaf width, leaf shape and petiole length are considered to be associated with seed yield. In order to identify quantitative trait loci (QTLs) affecting leaf type traits, two advanced recombinant inbred line (RIL, ZH, Zhonghuang 24 × Huaxia 3; GB, Guizao 1 × Brazil 13) populations were introduced to score phenotypic values in plants across nine different environments (years, seasons, locations and soybean growth stages). Two restriction site-associated DNA sequencing (RAD-seq) based high-density genetic linkage maps with an average distance of 1.00 centimorgan (cM) between adjacent bin markers were utilized for QTL fine mapping.

RESULTS

Correlation analysis showed that most of the traits were correlated with each other and regulated both by hereditary and environmental factors. A total of 190 QTLs were identified for leaf type associated traits in the two populations, of which 14 loci were found to be environmentally stable. Moreover, these detected QTLs were categorized into 34 QTL hotspots, and four important QTL hotspots with phenotypic variance ranging from 3.89-23.13% were highlighted. Furthermore, Glyma04g05840, Glyma19g37820, Glyma14g07140 and Glyma19g39340 were predicted in the intervals of the stable loci and important QTL hotspots for leaf type traits by adopting Gene Ontology (GO) enrichment analysis.

CONCLUSIONS

Our findings of the QTLs and the putative genes will be beneficial to gain new insights into the genetic basis for soybean leaf type traits and may further accelerate the breeding process for reasonable leaf type soybean.

One-leaf plants in the Gesneriaceae: Natural mutants of the typical shoot system

The aerial part of seed plants is called the shoot, which is composed of stems, leaves, and axial buds. These are produced by indeterminate activity in the shoot apical meristem (SAM), whereas the morphogenesis of leaves depends on determinate activity of leaf meristems. However, one-leaf plants in the Gesneriaceae family (eudicots) do not have a typical SAM and do not produce new organs when in the vegetative phase. Instead, they have one cotyledon whose growth is indeterminate. This peculiar development is supported by the groove meristem, which corresponds to the canonical SAM, and the basal meristem, which corresponds to the typical leaf meristem. However, the former does not produce any organ and the latter is active indeterminately. Gene expression and physiological analyses have been conducted in an effort to determine the molecular nature of this peculiar organogenesis. This review summarizes the current understanding of the development of one-leaf plants to provide future perspectives in this field of research.

Effects of heat stress in the leaf mitotic cell cycle and chromosomes of four wine-producing grapevine varieties

Grapevine varieties respond differentially to heat stress (HS). HS ultimately reduces the photosynthesis and respiratory performance. However, the HS effects in the leaf nuclei and mitotic cells of grapevine are barely known. This work intends to evaluate the HS effects in the leaf mitotic cell cycle and chromosomes of four wine-producing varieties: Touriga Franca (TF), Touriga Nacional (TN), Rabigato, and Viosinho. In vitro plants with 11 months were used in a stepwise acclimation and recovery (SAR) experimental setup comprising different phases: heat acclimation period (3 h-32 °C), extreme HS (1 h-42 °C), and two recovery periods (3 h-32 °C and 24 h-25 °C), and compared to control plants (maintained in vitro at 25 °C). At the end of each SAR phase, leaves were collected, fixed, and used for cell suspensions and chromosome preparations. Normal and abnormal interphase and mitotic cells were observed, scored, and statistically analyzed in all varieties and treatments (control and SAR phases). Different types of chromosomal anomalies in all mitotic phases, treatments, and varieties were found. In all varieties, the percentage of dividing cells with anomalies (%DCA) after extreme HS increased relative to control. TF and Viosinho were considered the most tolerant to HS. TF showed a gradual MI reduction from heat acclimation to HS and the lowest %DCA after HS and 24 h of recovery. Only Viosinho reached the control values after the long recovery period. Extrapolating these data to the field, we hypothesize that during consecutive hot summer days, the grapevine plants will not have time or capacity to recover from the mitotic anomalies caused by high temperatures.

A hidden link between leaf development and senescence

Leaf senescence is the final step of leaf development and is usually accompanied by visible color changes from green to yellow or brown. Unlike the senescence of the whole body of animals and unicellular organisms, which is often associated with death, leaf senescence in plants requires highly integrative processes towards cell death with nutrient recycling and storage. Since leaf senescence plays pivotal roles in the production of plant biomass and grain yield, the mechanisms of degradation and relocation of macromolecules as well as the regulation of signaling and biosynthetic pathways have received much attention. The importance of the plant hormone ethylene in the onset of leaf senescence has been clearly documented. However, research has increasingly demonstrated that the function of ethylene in the regulation of leaf senescence is dependent on leaf development. This review raises the issue of how ethylene requires developmental regulators and focuses on the developmental aspect of leaf senescence. It also emphasizes the remarkable impact that developmental regulators have on regulating the onset of leaf senescence.

Mechanisms Underlying the Environmentally Induced Plasticity of Leaf Morphology

The primary function of leaves is to provide an interface between plants and their environment for gas exchange, light exposure and thermoregulation. Leaves have, therefore a central contribution to plant fitness by allowing an efficient absorption of sunlight energy through photosynthesis to ensure an optimal growth. Their final geometry will result from a balance between the need to maximize energy uptake while minimizing the damage caused by environmental stresses. This intimate relationship between leaf and its surroundings has led to an enormous diversification in leaf forms. Leaf shape varies between species, populations, individuals or even within identical genotypes when those are subjected to different environmental conditions. For instance, the extent of leaf margin dissection has, for long, been found to inversely correlate with the mean annual temperature, such that Paleobotanists have used models based on leaf shape to predict the paleoclimate from fossil flora. Leaf growth is not only dependent on temperature but is also regulated by many other environmental factors such as light quality and intensity or ambient humidity. This raises the question of how the different signals can be integrated at the molecular level and converted into clear developmental decisions. Several recent studies have started to shed the light on the molecular mechanisms that connect the environmental sensing with organ-growth and patterning. In this review, we discuss the current knowledge on the influence of different environmental signals on leaf size and shape, their integration as well as their importance for plant adaptation.

Molecular Mechanisms of Leaf Morphogenesis

Plants maintain the ability to form lateral appendages throughout their life cycle and form leaves as the principal lateral appendages of the stem. Leaves initiate at the peripheral zone of the shoot apical meristem and then develop into flattened structures. In most plants, the leaf functions as a solar panel, where photosynthesis converts carbon dioxide and water into carbohydrates and oxygen. To produce structures that can optimally fulfill this function, plants precisely control the initiation, shape, and polarity of leaves. Moreover, leaf development is highly flexible but follows common themes with conserved regulatory mechanisms. Leaves may have evolved from lateral branches that are converted into determinate, flattened structures. Many other plant parts, such as floral organs, are considered specialized leaves, and thus leaf development underlies their morphogenesis. Here, we review recent advances in the understanding of how three-dimensional leaf forms are established. We focus on how genes, phytohormones, and mechanical properties modulate leaf development, and discuss these factors in the context of leaf initiation, polarity establishment and maintenance, leaf flattening, and intercalary growth.

Identification and Characterization of the WOX Family Genes in Five Solanaceae Species Reveal Their Conserved Roles in Peptide Signaling

Members of the plant-specific WOX (WUSCHEL-related homeobox) transcription factor family have been reported to play important roles in peptide signaling that regulates stem cell maintenance and cell fate specification in various developmental processes. Even though remarkable advances have been made in studying WOX genes in Arabidopsis, little is known about this family in Solanaceae species. A total of 45 WOX members from five Solanaceae species were identified, including eight members from Solanum tuberosum, eight from Nicotiana tomentosiformis, 10 from Solanum lycopersicum, 10 from Nicotiana sylvestris and nine from Nicotiana tabacum. The newly identified WOX members were classified into three clades and nine subgroups based on phylogenetic analysis using three different methods. The patterns of exon-intron structure and motif organization of the WOX proteins agreed with the phylogenetic results. Gene duplication events and ongoing evolution were revealed by additional branches on the phylogenetic tree and the presence of a partial WUS-box in some non-WUS clade members. Gene expression with or without CLE (clavata3 (clv3)/embryo surrounding region-related) peptide treatments revealed that tobacco WOX genes showed similar or distinct expression patterns compared with their Arabidopsis homologues, suggesting either functional conservation or divergence. Expression of Nicotiana tabacum WUSCHEL (NtabWUS) in the organizing center could rescue the wus-1 mutant phenotypes in Arabidopsis, implying conserved roles of the Solanaceae WOX proteins in peptide-mediated regulation of plant development.

A novel antisense long noncoding RNA, TWISTED LEAF, maintains leaf blade flattening by regulating its associated sense R2R3-MYB gene in rice

Natural antisense long noncoding RNAs (lncRNAs) are widespread in many organisms. However, their biological functions remain largely unknown, particularly in plants. We report the identification and characterization of an endogenous lncRNA, TWISTED LEAF (TL), which is transcribed from the opposite strand of the R2R3 MYB transcription factor gene locus, OsMYB60, in rice (Oryza sativa). TL and OsMYB60 were found to be coexpressed in many different tissues, and the expression level of TL was higher than that of OsMYB60. Downregulation of TL by RNA interference (RNAi) and overexpression of OsMYB60 resulted in twisted leaf blades in transgenic rice. The expression level of OsMYB60 was significantly increased in TL-RNAi transgenic plants. This suggests that TL may play a cis-regulatory role on OsMYB60 in leaf morphological development. We also determined that the antisense transcription suppressed the sense gene expression by mediating chromatin modifications. We further discovered that a C2H2 transcription factor, OsZFP7, is an OsMYB60 binding partner and involved in leaf development. Taken together, these findings reveal that the cis-natural antisense lncRNA plays a critical role in maintaining leaf blade flattening in rice. Our study uncovers a regulatory mechanism of lncRNA in plant leaf development.

The Arabidopsis arc5 and arc6 mutations differentially affect plastid morphology in pavement and guard cells in the leaf epidermis

Chloroplasts, or photosynthetic plastids, multiply by binary fission, forming a homogeneous population in plant cells. In Arabidopsis thaliana, the division apparatus (or division ring) of mesophyll chloroplasts includes an inner envelope transmembrane protein ARC6, a cytoplasmic dynamin-related protein ARC5 (DRP5B), and members of the FtsZ1 and FtsZ2 families of proteins, which co-assemble in the stromal mid-plastid division ring (FtsZ ring). FtsZ ring placement is controlled by several proteins, including a stromal factor MinE (AtMinE1). During leaf mesophyll development, ARC6 and AtMinE1 are necessary for FtsZ ring formation and thus plastid division initiation, while ARC5 is essential for a later stage of plastid division. Here, we examined plastid morphology in leaf epidermal pavement cells (PCs) and stomatal guard cells (GCs) in the arc5 and arc6 mutants using stroma-targeted fluorescent proteins. The arc5 PC plastids were generally a bit larger than those of the wild type, but most had normal shapes and were division-competent, unlike mutant mesophyll chloroplasts. The arc6 PC plastids were heterogeneous in size and shape, including the formation of giant and mini-plastids, plastids with highly developed stromules, and grape-like plastid clusters, which varied on a cell-by-cell basis. Moreover, unique plastid phenotypes for stomatal GCs were observed in both mutants. The arc5 GCs rarely lacked chlorophyll-bearing plastids (chloroplasts), while they accumulated minute chlorophyll-less plastids, whereas most GCs developed wild type-like chloroplasts. The arc6 GCs produced large chloroplasts and/or chlorophyll-less plastids, as previously observed, but unexpectedly, their chloroplasts/plastids exhibited marked morphological variations. We quantitatively analyzed plastid morphology and partitioning in paired GCs from wild-type, arc5, arc6, and atminE1 plants. Collectively, our results support the notion that ARC5 is dispensable in the process of equal division of epidermal plastids, and indicate that dysfunctions in ARC5 and ARC6 differentially affect plastid replication among mesophyll cells, PCs, and GCs within a single leaf.

OLIGOCELLULA1/HIGH EXPRESSION OF OSMOTICALLY RESPONSIVE GENES15 Promotes Cell Proliferation With HISTONE DEACETYLASE9 and POWERDRESS During Leaf Development in Arabidopsis thaliana

Organ size regulation is dependent on the precise spatial and temporal regulation of cell proliferation and cell expansion. A number of transcription factors have been identified that play a key role in the determination of aerial lateral organ size, but their functional relationship to various chromatin modifiers has not been well understood. To understand how leaf size is regulated, we previously isolated the oligocellula1 (oli1) mutant of Arabidopsis thaliana that develops smaller first leaves than the wild type (WT) mainly due to a reduction in the cell number. In this study, we further characterized oli1 leaf phenotypes and identified the OLI1 gene as well as interaction partners of OLI1. Detailed characterizations of leaf development suggested that the cell proliferation rate in oli1 leaf primordia is lower than that in the WT. In addition, oli1 was associated with a slight delay of the progression from the juvenile to adult phases of leaf traits. A classical map-based approach demonstrated that OLI1 is identical to HIGH EXPRESSION OF OSMOTICALLY RESPONSIVE GENES15 (HOS15). HOS15/OLI1 encodes a homolog of human transducin β-like protein1 (TBL1). TBL1 forms a transcriptional repression complex with the histone deacetylase (HDAC) HDAC3 and either nuclear receptor co-repressor (N-CoR) or silencing mediator for retinoic acid and thyroid receptor (SMRT). We found that mutations in HISTONE DEACETYLASE9 (HDA9) and a switching-defective protein 3, adaptor 2, N-CoR, and transcription factor IIIB-domain protein gene, POWERDRESS (PWR), showed a small-leaf phenotype similar to oli1. In addition, hda9 and pwr did not further enhance the oli1 small-leaf phenotype, suggesting that these three genes act in the same pathway. Yeast two-hybrid assays suggested physical interactions, wherein PWR probably bridges HOS15/OLI1 and HDA9. Earlier studies suggested the roles of HOS15, HDA9, and PWR in transcriptional repression. Consistently, transcriptome analyses showed several genes commonly upregulated in the three mutants. From these findings, we propose a possibility that HOS15/OLI1, PWR, and HDA9 form an evolutionary conserved transcription repression complex that plays a positive role in the regulation of final leaf size.

Roles of miR319 and TCP Transcription Factors in Leaf Development

Sophisticated regulation of gene expression, including microRNAs (miRNAs) and their target genes, is required for leaf differentiation, growth, and senescence. The impact of miR319 and its target TEOSINTE BRANCHED1, CYCLOIDEA, and PROLIFERATING CELL NUCLEAR ANTIGEN BINDING FACTOR (TCP) genes on leaf development has been extensively investigated, but the redundancies of these gene families often interfere with the evaluation of their function and regulation in the developmental context. Here, we present the genetic evidence of the involvement of the MIR319 and TCP gene families in Arabidopsis (Arabidopsis thaliana) leaf development. Single mutations in MIR319A and MIR319B genes moderately inhibited the formation of leaf serrations, whereas double mutations increased the extent of this inhibition and resulted in the formation of smooth leaves. Mutations in MIR319 and gain-of-function mutations in the TCP4 gene conferred resistance against miR319 and impaired the cotyledon boundary and leaf serration formation. These mutations functionally associated with CUP-SHAPED COTYLEDON genes, which regulate the cotyledon boundary and leaf serration formation. In contrast, loss-of-function mutations in miR319-targeted and nontargeted TCP genes cooperatively induced the formation of serrated leaves in addition to changes in the levels of their downstream gene transcript. Taken together, these findings demonstrate that the MIR319 and TCP gene families underlie robust and multilayer control of leaf development. This study also provides a framework toward future researches on redundant miRNAs and transcription factors in Arabidopsis and crop plants.

Spatial Auxin Signaling Controls Leaf Flattening in Arabidopsis

The flattening of leaves to form broad blades is an important adaptation that maximizes photosynthesis. However, the molecular mechanism underlying this process remains unclear. The WUSCHEL-RELATED HOMEOBOX (WOX) genes WOX1 and PRS are expressed in the leaf marginal domain to enable leaf flattening, but the nature of WOX expression establishment remains elusive. Here, we report that adaxial-expressed MONOPTEROS (MP) and abaxial-enriched auxin together act as positional cues for patterning the WOX domain. MP directly binds to the WOX1 and PRS promoters and activates their expression. Furthermore, redundant abaxial-enriched ARF repressors suppress WOX1 and PRS expression, also through direct binding. In particular, we show that ARF2 is redundantly required with ARF3 and ARF4 to maintain the abaxial identity. Taken together, these findings explain how adaxial-abaxial polarity patterns the mediolateral axis and subsequent lateral expansion of leaves.

BIG LEAF is a regulator of organ size and adventitious root formation in poplar

Here we report the discovery through activation tagging and subsequent characterization of the BIG LEAF (BL) gene from poplar. In poplar, BL regulates leaf size via positively affecting cell proliferation. Up and downregulation of the gene led to increased and decreased leaf size, respectively, and these phenotypes corresponded to increased and decreased cell numbers. BL function encompasses the early stages of leaf development as native BL expression was specific to the shoot apical meristem and leaf primordia and was absent from the later stages of leaf development and other organs. Consistently, BL downregulation reduced leaf size at the earliest stages of leaf development. Ectopic expression in mature leaves resulted in continued growth most probably via sustained cell proliferation and thus the increased leaf size. In contrast to the positive effect on leaf growth, ectopic BL expression in stems interfered with and significantly reduced stem thickening, suggesting that BL is a highly specific activator of growth. In addition, stem cuttings from BL overexpressing plants developed roots, whereas the wild type was difficult to root, demonstrating that BL is a positive regulator of adventitious rooting. Large transcriptomic changes in plants that overexpressed BL indicated that BL may have a broad integrative role, encompassing many genes linked to organ growth. We conclude that BL plays a fundamental role in control of leaf size and thus may be a useful tool for modifying plant biomass productivity and adventitious rooting.

Overexpression of the WOX gene STENOFOLIA improves biomass yield and sugar release in transgenic grasses and display altered cytokinin homeostasis

Lignocellulosic biomass can be a significant source of renewable clean energy with continued improvement in biomass yield and bioconversion strategies. In higher plants, the leaf blade is the central energy convertor where solar energy and CO2 are assimilated to make the building blocks for biomass production. Here we report that introducing the leaf blade development regulator STENOFOLIA (STF), a WOX family transcription factor, into the biofuel crop switchgrass, significantly improves both biomass yield and sugar release. We found that STF overexpressing switchgrass plants produced approximately 2-fold more dry biomass and release approximately 1.8-fold more solubilized sugars without pretreatment compared to controls. The biomass increase was attributed mainly to increased leaf width and stem thickness, which was also consistent in STF transgenic rice and Brachypodium, and appeared to be caused by enhanced cell proliferation. STF directly binds to multiple regions in the promoters of some cytokinin oxidase/dehydrogenase (CKX) genes and represses their expression in all three transgenic grasses. This repression was accompanied by a significant increase in active cytokinin content in transgenic rice leaves, suggesting that the increase in biomass productivity and sugar release could at least in part be associated with improved cytokinin levels caused by repression of cytokinin degrading enzymes. Our study provides a new tool for improving biomass feedstock yield in bioenergy crops, and uncovers a novel mechanistic insight in the function of STF, which may also apply to other repressive WOX genes that are master regulators of several key plant developmental programs.

The Arabidopsis RING-Type E3 Ligase TEAR1 Controls Leaf Development by Targeting the TIE1 Transcriptional Repressor for Degradation

The developmental plasticity of leaf size and shape is important for leaf function and plant survival. However, the mechanisms by which plants form diverse leaves in response to environmental conditions are not well understood. Here, we identified TIE1-ASSOCIATED RING-TYPE E3 LIGASE1 (TEAR1) and found that it regulates leaf development by promoting the degradation of TCP INTERACTOR-CONTAINING EAR MOTIF PROTEIN1 (TIE1), an important repressor of CINCINNATA (CIN)-like TEOSINTE BRANCHED1/CYCLOIDEA/PCF (TCP) transcription factors, which are key for leaf development. TEAR1 contains a typical C3H2C3-type RING domain and has E3 ligase activity. We show that TEAR1 interacts with the TCP repressor TIE1, which is ubiquitinated in vivo and degraded by the 26S proteasome system. We demonstrate that TEAR1 is colocalized with TIE1 in nuclei and negatively regulates TIE1 protein levels. Overexpression of TEAR1 rescued leaf defects caused by TIE1 overexpression, whereas disruption of TEAR1 resulted in leaf phenotypes resembling those caused by TIE1 overexpression or TCP dysfunction. Deficiency in TEAR partially rescued the leaf defects of TCP4 overexpression line and enhanced the wavy leaf phenotypes of jaw-5D We propose that TEAR1 positively regulates CIN-like TCP activity to promote leaf development by mediating the degradation of the TCP repressor TIE1.

Breaking the mold: understanding the evolution and development of lateral organs in diverse plant models

The formation of complex three-dimensional shape differs significantly between plants and animals due to the presence of the cell wall in the former, which prevents all cell migration. Instead, in lateral plant organs such as leaves or petals, shape is controlled by a series of developmental phases in which the organ acquires polarity, cells undergo proliferation, and, lastly, cells expand to their final shape and size. Although these processes were first described based on mutagenesis approaches in major model systems like Arabidopsis thaliana, further insight into their complexity is best provided by studies of natural variation in organ shape in alternative model systems that sample a broader range of plant form. Weaving together work from both forward and evolutionary genetics, this review focuses on how modification in polarity establishment, cell proliferation and cell expansion drives modifications in the fundamental lateral organ developmental program to create diversity in shape.