Sang Eun Jun

Arabidopsis thaliana homeobox 12 (ATHB12), a homeodomain‐leucine zipper protein, regulates leaf growth by promoting cell expansion and endoreduplication

Arabidopsis thaliana homeobox 12 (ATHB12), a homeodomain-leucine zipper class I (HD-Zip I) gene, is highly expressed in leaves and stems, and induced by abiotic stresses, but its role in development remains obscure. To understand its function during plant development, we studied the effects of loss and gain of function. Expression of ATHB12 fused to the EAR-motif repression domain (SRDX) - P35 S ::ATHB12SRDX (A12SRDX) and PATHB 12 ::ATHB12SRDX - slowed both leaf and root growth, while the growth of ATHB12-overexpressing seedlings (A12OX) was accelerated. Microscopic examination revealed changes in the size and number of leaf cells. Ploidy was reduced in A12SRDX plants, accompanied by decreased cell expansion and increased cell numbers. By contrast, cell size was increased in A12OX plants, along with increased ploidy and elevated expression of cell cycle switch 52s (CCS52s), which are positive regulators of endoreduplication, indicating that ATHB12 promotes leaf cell expansion and endoreduplication. Overexpression of ATHB12 led to decreased phosphorylation of Arabidopsis thaliana ribosomal protein S6 (AtRPS6), a regulator of cell growth. In addition, induction of ATHB12 in the presence of cycloheximide increased the expression of several genes related to cell expansion, such as EXPANSIN A10 (EXPA10) and DWARF4 (DWF4). Our findings strongly suggest that ATHB12 acts as a positive regulator of endoreduplication and cell growth during leaf development.

Developmental processes of leaf morphogenesis inarabidopsis

The leaf is a suitable subject with which to study plant morphogenesis because of its diversity of shape. Although mechanisms for leaf initiation and lateral morphogenesis have been suggested, the exact means for determining shape remain unclear. Many genes involved in those developmental processes have now been identified. Here, we summarize the early events in the genetic regulation ofArabidopsis leaf formation, including initiation, dorsoventrality, and the spatial and temporal control of cell proliferation and enlargement. We focus on recent progress within the model plantArabidopsis, placing special emphasis on our own findings.

Kip-Related Protein 3 Is Required for Control of Endoreduplication in the Shoot Apical Meristem and Leaves of Arabidopsis

The cell cycle plays an important role in the development and adaptation of multicellular organisms; specifically, it allows them to optimally adjust their architecture in response to environmental changes. Kip-related proteins (KRPs) are important negative regulators of cyclin-dependent kinases (CDKs), which positively control the cell cycle during plant development. The Arabidopsis genome possesses seven KRP genes with low sequence similarity and distinct expression patterns; however, why Arabidopsis needs seven KRP genes and how these genes function in cell cycle regulation are unknown. Here, we focused on the characterization of KRP3, which was found to have unique functions in the shoot apical meristem (SAM) and leaves. KRP3 protein was localized to the SAM, including the ground meristem and vascular tissues in the ground part of the SAM and cotyledons. In addition, KRP3 protein was stabilized when treated with MG132, an inhibitor of the 26S proteasome, indicating that the protein may be regulated by 26S proteasome-mediated protein degradation. KRP3-overexpressing (KRP3 OE) transgenic plants showed reduced organ size, serrated leaves, and reduced fertility. Interestingly, the KRP3 OE transgenic plants showed a significant reduction in the size of the SAM with alterations in cell arrangement. In addition, compared to the wild type, the KRP3 OE transgenic plants had a higher DNA ploidy level in the SAM and leaves. Taken together, our data suggest that KRP3 plays important regulatory roles in the cell cycle and endoreduplication in the SAM and leaves.

Comparative Analysis of the Conserved Functions of Arabidopsis DRL1 and Yeast KTI12

Patterning of the polar axis during the early leaf developmental stage is established by cell-to-cell communication between the shoot apical meristem (SAM) and the leaf primordia. In a previous study, we showed that the DRL1 gene, which encodes a homolog of the Elongator-associated protein KTI12 of yeast, acts as a positive regulator of adaxial leaf patterning and shoot meristem activity. To determine the evolutionally conserved functions of DRL1, we performed a comparison of the deduced amino acid sequence of DRL1 and its yeast homolog, KTI12, and found that while overall homology was low, well-conserved domains were presented. DRL1 contained two conserved plant-specific domains. Expression of the DRL1 gene in a yeast KTI12-deficient yeast mutant suppressed the growth retardation phenotype, but did not rescue the caffeine sensitivity, indicating that the role of Arabidopsis Elongator-associated protein is partially conserved with yeast KTI12, but may have changed between yeast and plants in response to caffeine during the course of evolution. In addition, elevated expression of DRL1 gene triggered zymocin sensitivity, while overexpression of KTI12 maintained zymocin resistance, indicating that the function of Arabidopsis DRL1 may not overlap with yeast KTI12 with regards to toxin sensitivity. In this study, expression analysis showed that class-I KNOX genes were downregulated in the shoot apex, and that YAB and KAN were upregulated in leaves of the Arabidopsis drl1-101 mutant. Our results provide insight into the communication network between the SAM and leaf primordia required for the establishment of leaf polarity by mediating histone acetylation or through other mechanisms.

Overexpression of the downward leaf curling (DLC) gene from melon changes leaf morphology by controlling cell size and shape in Arabidopsis leaves

A plant-specific gene was cloned from melon fruit. This gene was named downward leaf curling (CmDLC) based on the phenotype of transgenic Arabidopsis plants overexpressing the gene. This expression level of this gene was especially upregulated during melon fruit enlargement. Overexpression of CmDLC in Arabidopsis resulted in dwarfism and narrow, epinastically curled leaves. These phenotypes were found to be caused by a reduction in cell number and cell size on the adaxial and abaxial sides of the epidermis, with a greater reduction on the abaxial side of the leaves. These phenotypic characteristics, combined with the more wavy morphology of epidermal cells in overexpression lines, indicate that CmDLC overexpression affects cell elongation and cell morphology. To investigate intracellular protein localization, a CmDLC-GFP fusion protein was made and expressed in onion epidermal cells. This protein was observed to be preferentially localized close to the cell membrane. Thus, we report here a new plant-specific gene that is localized to the cell membrane and that controls leaf cell number, size and morphology.

DRL1 regulates adaxial leaf patterning and shoot apical meristem activity inArabidopsis

Leaf shape is controlled early on by initiation at the shoot apical meristem (SAM), as well as by changes in the rates and planes of cell division and the polarity-dependent differentiation of leaf cells. To elucidate the regulation of this differentiation by signal(s) from the SAM, we screened for mutations in genes that might be involved in these early processes. A novel recessive mutant, 356-2 [identified as a new allele of thedeformed root and leaf1 (drl1) mutant], was isolated from a collection ofDs transposon insertion lines. The356- 2/drl1- 101 mutant produces narrow, filamentous leaves and defective mer-istems. Its palisade cells have a spongy cell-like structure and are fewer in number, indicating that the leaves are abaxialized. Interestingly, some of those filament-like leaves have no vascular tissues inside their blades.DRL1 encodes a protein similar to the yeast elongator-associated protein (EAP) KTI12. The amino acid sequence of DRL1 is universally conserved in prokaryotes and eukaryotes. These facts suggest that DRL1 might positively regulate leaf polarity and SAM activity by controlling cell proliferation and differentiation.

Regulation of cell size and cell number by LANCEOLATA1 gene in Arabidopsis

The processes for leaf development in dicotyledonous plants are surprisingly complex, while the mechanism of controlling and coordinating them is poorly understood. To characterize the fundamental features of the leaf development of Arabidopsis, we first attempted to isolate mutants that alter leaf morphology. Here, leaf morphological mutant of Arabidopsis, lanceolata1 (lan1) which has small and narrow leaves have isolated and characterized. To clarify the function of LAN1 in organ development, we characterized lan1-7 mutant using an anatomical and genetic approach. The lan1-7 mutant had reduced size of foliage leaves and reduced dimensions of stems. A reduction both in cell size and in cell number was evident at the cellular level in the lan1 mutant, revealing that LAN1 gene appears to affect cell division at an earlier stage and cell elongation throughout the development of leaf primordia. From the analysis of heterogeneous plant with lan1 mutation and 35S-AG transgenic plant, AG gene is revealed to regulate leaf morphology under the control of 35S promoter. Thus, MADS-box gene was revealed to have some relationship to that of LAN1 gene at certain stage in leaf development processes.

ORESARA15, a PLATZ transcription factor, mediates leaf growth and senescence in Arabidopsis

Plant leaves undergo a series of developmental changes from leaf primordium initiation through growth and maturation to senescence throughout their life span. Although the mechanisms underlying leaf senescence have been intensively elucidated, our knowledge of the interrelationship between early leaf development and senescence is still fragmentary. We isolated the oresara15-1Dominant (ore15-1D) mutant, which had an extended leaf longevity and an enlarged leaf size, from activation-tagged lines of Arabidopsis. Plasmid rescue identified that ORE15 encodes a PLANT A/T-RICH SEQUENCE- AND ZINC-BINDING PROTEIN family transcription factor. Phenotypes of ore15-1D and ore15-2, a loss-of-function mutant, were evaluated through physiological and anatomical analyses. Microarray, quantitative reverse transcription polymerase chain reaction, and chromatin immunoprecipitation as well as genetic analysis were employed to reveal the molecular mechanism of ORE15 in the regulation of leaf growth and senescence. ORE15 enhanced leaf growth by promoting the rate and duration of cell proliferation in the earlier stage and suppressed leaf senescence in the later stage by modulating the GROWTH-REGULATING FACTOR (GRF)/GRF-INTERACTING FACTOR regulatory pathway. Our study highlighted a molecular conjunction through ORE15 between growth and senescence, which are two temporally separate developmental processes during leaf life span.

MACROPHYLLA/ROTUNDIFOLIA3 gene of Arabidopsis controls leaf index during leaf development

In plants, heteroblasty reflects the morphological adaptation during leaf development according to the external environmental condition and affects the final shape and size of organ. Among parameters displaying heteroblasty, leaf index is an important and typical one to represent the shape and size of simple leaves. Leaf index factor is eventually determined by cell proliferation and cell expansion in leaf blades. Although several regulators and their mechanisms controlling the cell division and cell expansion in leaf development have been studied, it does not fully provide a blueprint of organ formation and morphogenesis during environmental changes. To investigate genes and their mechanisms controlling leaf index during leaf development, we carried out molecular-genetic and physiological experiments using an Arabidopsis mutant. In this study, we identified macrophylla (mac) which had enlarged leaves. In detail, the mac mutant showed alteration in leaf index and cell expansion in direction of width and length, resulting in not only modification of leaf shape but also disruption of heteroblasty. Molecular-genetic studies indicated that mac mutant had point mutation in ROTUDJFOLIA3 (ROT3) gene involved in brassinosteroid biosynthesis and was an allele of rod-I mutant. We named it rnac/rot3-5 mutant. The expression of ROT3 gene was controlled by negative feedback inhibition by the treatment of brassinosteroid hormone, suggesting that ROT3 gene was involved in brassinosteroid biosynthesis. in dark condition, in addition, the expression of ROT3 gene was up-regulated and mac/rot3-5 mutant showed lower response, compare to wild type in petiole elongation. This study suggests that ROT3 gene has an important role in control of leaf index during leaf expansion process for proper environmental adaptation, such as shade avoidance syndrome, via the control of brassinosteroid biosynthesis.