Gyung-Tae Kim

The ANGUSTIFOLIA gene of Arabidopsis, a plant CtBP gene, regulates leaf-cell expansion, the arrangement of cortical microtubules in leaf cells and expression of a gene involved in cell-wall formation

We previously showed that the ANGUSTIFOLIA (AN) gene regulates the width of leaves of Arabidopsis thaliana, by controlling the polar elongation of leaf cells. In the present study, we found that the abnormal arrangement of cortical microtubules (MTs) in an leaf cells appeared to account entirely for the abnormal shape of the cells. It suggested that the AN gene might regulate the polarity of cell growth by controlling the arrangement of cortical MTs. We cloned the AN gene using a map‐based strategy and identified it as the first member of the CtBP family to be found in plants. Wild‐type AN cDNA reversed the narrow‐leaved phenotype and the abnormal arrangement of cortical MTs of the an‐1 mutation. In the animal kingdom, CtBPs self‐associate and act as co‐repressors of transcription. The AN protein can also self‐associate in the yeast two‐hybrid system. Furthermore, microarray analysis suggested that the AN gene might regulate the expression of certain genes, e.g. the gene involved in formation of cell walls, MERI5. A discussion of the molecular mechanisms involved in the leaf shape regulation is presented based on our observations.

The BLADE-ON-PETIOLE 1 gene controls leaf pattern formation through the modulation of meristematic activity in Arabidopsis.

The plant leaf provides an ideal system to study the mechanisms of organ formation and morphogenesis. The key factors that control leaf morphogenesis include the timing, location and extent of meristematic activity during cell division and differentiation. We identified an Arabidopsis mutant in which the regulation of meristematic activities in leaves was aberrant. The recessive mutant allele blade-on-petiole1-1 (bop1-1) produced ectopic, lobed blades along the adaxial side of petioles of the cotyledon and rosette leaves. The ectopic organ, which has some of the characteristics of rosette leaf blades with formation of trichomes in a dorsoventrally dependent manner, was generated by prolonged and clustered cell division in the mutant petioles. Ectopic, lobed blades were also formed on the proximal part of cauline leaves that lacked a petiole. Thus, BOP1 regulates the meristematic activity of leaf cells in a proximodistally dependent manner. Manifestation of the phenotypes in the mutant leaves was dependent on the leaf position. Thus, BOP1 controls leaf morphogenesis through control of the ectopic meristematic activity but within the context of the leaf proximodistality, dorsoventrality and heteroblasty. BOP1 appears to regulate meristematic activity in organs other than leaves, since the mutation also causes some ectopic outgrowths on stem surfaces and at the base of floral organs. Three class I knox genes, i.e., KNAT1, KNAT2 and KNAT6, were expressed aberrantly in the leaves of the bop1-1 mutant. Furthermore, the bop1-1 mutation showed some synergistic effect in double mutants with as1-1 or as2-2 mutation that is known to be defective in the regulation of meristematic activity and class I knox gene expression in leaves. The bop1-1 mutation also showed a synergistic effect with the stm-1 mutation, a strong mutant allele of a class I knox gene, STM. We, thus, suggest that BOP1 promotes or maintains a developmentally determinate state in leaf cells through the regulation of class I knox genes.

The Histidine Kinases CYTOKININ-INDEPENDENT1 and ARABIDOPSIS HISTIDINE KINASE2 and 3 Regulate Vascular Tissue Development in Arabidopsis Shoots

The development and activity of the procambium and cambium, which ensure vascular tissue formation, is critical for overall plant architecture and growth. However, little is known about the molecular factors affecting the activity of vascular meristems and vascular tissue formation. Here, we show that the His kinase CYTOKININ-INDEPENDENT1 (CKI1) and the cytokinin receptors ARABIOPSIS HISTIDINE KINASE2 (AHK2) and AHK3 are important regulators of vascular tissue development in Arabidopsis thaliana shoots. Genetic modifications of CKI1 activity in Arabidopsis cause dysfunction of the two-component signaling pathway and defects in procambial cell maintenance. CKI1 overexpression in protoplasts leads to cytokinin-independent activation of the two-component phosphorelay, and intracellular domains are responsible for the cytokinin-independent activity of CKI1. CKI1 expression is observed in vascular tissues of inflorescence stems, and CKI1 forms homodimers both in vitro and in planta. Loss-of-function ahk2 and ahk3 mutants and plants with reduced levels of endogenous cytokinins show defects in procambium proliferation and an absence of secondary growth. CKI1 overexpression partially rescues ahk2 ahk3 phenotypes in vascular tissue, while the negative mutation CKI1H405Q further accentuates mutant phenotypes. These results indicate that the cytokinin-independent activity of CKI1 and cytokinin-induced AHK2 and AHK3 are important for vascular bundle formation in Arabidopsis.

The CURLY LEAF gene controls both division and elongation of cells during the expansion of the leaf blade in Arabidopsis thaliana

Abstract. The CURLY LEAF (CLF ) gene in Arabidopsis thaliana (L.) Heynh. is required for stable repression of a floral homeotic gene, AGAMOUS in leaves and stems To clarify the function of CLF in organ development, we characterized clf mutants using an anatomical and genetic approach. The clf mutants had normal roots, hypocotyls, and cotyledons, but the foliage leaves and the stems had reduced dimensions. A decrease both in the extent of cell elongation and in the number of cells was evident in the clf mutant leaves, suggesting that the CLF gene might be involved in the division and elongation of cells during leaf morphogenesis. An analysis of the development of clf mutant leaves revealed that the period during which cell division or cell elongation occurred was of normal duration, while the rates of both cell production and cell elongation were lower than in the wild type. Two phases in the elongation of cells were also recognized from this analysis. From analysis of an angustifolia clf double mutant, we found that the two phases of elongation of leaf cells were regulated independently by each gene. Thus, the CLF gene appears to affect cell division at an earlier stage and cell elongation throughout the development of leaf primordia.

The rice narrow leaf2 and narrow leaf3 loci encode WUSCHEL‐related homeobox 3A (OsWOX3A) and function in leaf, spikelet, tiller and lateral root development

· In order to understand the molecular genetic mechanisms of rice (Oryza sativa) organ development, we studied the narrow leaf2 narrow leaf3 (nal2 nal3; hereafter nal2/3) double mutant, which produces narrow-curly leaves, more tillers, fewer lateral roots, opened spikelets and narrow-thin grains. · We found that narrow-curly leaves resulted mainly from reduced lateral-axis outgrowth with fewer longitudinal veins and more, larger bulliform cells. Opened spikelets, possibly caused by marginal deformity in the lemma, gave rise to narrow-thin grains. · Map-based cloning revealed that NAL2 and NAL3 are paralogs that encode an identical OsWOX3A (OsNS) transcriptional activator, homologous to NARROW SHEATH1 (NS1) and NS2 in maize and PRESSED FLOWER in Arabidopsis. · OsWOX3A is expressed in the vascular tissues of various organs, where nal2/3 mutant phenotypes were displayed. Expression levels of several leaf development-associated genes were altered in nal2/3, and auxin transport-related genes were significantly changed, leading to pin mutant-like phenotypes such as more tillers and fewer lateral roots. OsWOX3A is involved in organ development in rice, lateral-axis outgrowth and vascular patterning in leaves, lemma and palea morphogenesis in spikelets, and development of tillers and lateral roots.

LONGIFOLIA1 and LONGIFOLIA2, two homologous genes, regulate longitudinal cell elongation in Arabidopsis.

Plants have diversified their leaf morphologies to adapt to diverse ecological niches. The molecular components responsible for regulating leaf morphology, however, have not been fully elucidated. By screening Arabidopsis activation-tagging lines, we identified a dominant mutant, which we designated longifolia1-1D (lng1-1D). lng1-1D plants were characterized by long petioles, narrow but extremely long leaf blades with serrated margins, elongated floral organs, and elongated siliques. The elongated leaves of the mutant were due to increased polar cell elongation rather than increased cell proliferation. Molecular characterization revealed that this phenotype was caused by overexpression of the novel gene LNG1, which was found to have a homolog, LNG2,in Arabidopsis. To further examine the role of the LNG genes, we characterized lng1 and lng2 loss-of-function mutant lines. In contrast to the elongated leaves of lng1-1D plants, the lng1 and lng2 mutants showed slightly decreased leaf length. Furthermore, the lng1-3 lng2-1 double mutant showed further decreased leaf length associated with less longitudinal polar cell elongation. The leaf widths in lng1-3 lng2-1 mutant plants were similar to those in wild type, implying that the role of LNG1 and LNG2 on polar cell elongation is similar to that of ROTUNDIFOLIA3 (ROT3). However, analysis of a lng1-3 lng2-1 rot3-1 triple mutant and of a lng1-1D rot3-1 double mutant indicated that LNG1 and LNG2 promote longitudinal cell elongation independently of ROT3. Taken together, these findings indicate that LNG1 and LNG2 are new components that regulate leaf morphology by positively promoting longitudinal polar cell elongation independently of ROT3 in Arabidopsis.

Photomorphogenesis of leaves: shade-avoidance and differentiation of sun and shade leaves

Leaf shape is an important factor in optimal plant growth, because leaves are the main photosynthetic organs. Plants exhibit plasticity in leaf shape and structure, allowing them to optimize photosynthetic efficiency. In Arabidopsis thaliana(L.) Heynh., several types of leaves develop differentially, according to light intensity and quality. When shaded, the expansion of leaf lamina is inhibited, while the petiole elongation is enhanced. This phenomenon is part of the so-called shade-avoidance syndrome. Under low light, A. thaliana develops shade leaves with only one layer of palisade tissue, whereas under high light, it develops sun leaves that have nearly two complete layers of palisade tissue. Although the molecular mechanisms of these photomorphogenic phenomena in leaves are not well understood, recent studies of A. thaliana have provided some insight. For example, some cytochrome P450s may be involved in the specific control of the petiole length during photomorphogenesis. On the other hand, switching between sun and shade leaves is regulated by long-distance signaling from mature leaves in Chenopodium album. Here we provide an overview of the mechanisms of photomorphogenesis in leaves based on recent findings.

Heteroblasty in Arabidopsis thaliana (L.) Heynh

Abstract. Heteroblasty in Arabidopsis thaliana was analyzed in a variety of plants with mutations in leaf morphology using a tissue-specific β-glucuronidase gene marker. Some mutants exhibited their mutant phenotypes specifically in foliage leaves. The phenotypes associated with the foliage-leaf-specific mutations were also found to be induced ectopically in cotyledons in the presence of the lec1 mutation. Moreover, the features of an emf1lec1 double mutant showed that cotyledons can be partially converted into carpelloids. When heteroblastic traits were examined in foliage leaves in the presence of certain mutations or natural deviations by histochemical analysis of the expression of the tissue-specific marker gene, it was found that ectopic expression of the developmental program for the first foliage leaves in lec1 cotyledons seemed to affect the heteroblastic features of the first set of foliage leaves, while foliage leaves beyond the third position appeared normal. Similarly, in wild-type plants, discrepancies in heteroblastic features, relative to standard features, of foliage leaves at early positions seemed to be eliminated in foliage leaves at later positions. These results suggest that heteroblasty in foliage leaves might be affected in part by the heteroblastic stage of the preceding foliage leaves but is finally controlled autonomously at each leaf position.

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.

Structurally related Arabidopsis ANGUSTIFOLIA is functionally distinct from the transcriptional corepressor CtBP

ANGUSTIFOLIA (AN) controls leaf morphology in the plant Arabidopsis thaliana. Previous studies on sequence similarity demonstrated that the closest proteins to AN are members of animal C-terminal-binding proteins (CtBPs) found in nematodes, arthropods, and vertebrates. Drosophila CtBP (dCtBP) functions as a transcriptional corepressor for deoxyribonucleic acid (DNA)-binding repressors containing the short amino acid motif, PXDLS, to regulate tissue specification and segmentation during early embryogenesis. It has previously been shown that AN was thought to repress transcription similar to the function of CtBPs; however, AN lacks some of the structural features that are conserved in animal CtBPs. In this paper, we examined whether AN is functionally related to dCtBP. Firstly, we re-examined sequence similarity among AN and various CtBPs from several representative species in the plant and animal kingdoms. Secondly, yeast two-hybrid assays demonstrated that AN failed to interact with an authentic CtBP-interacting factor, adenovirus E1A oncoprotein bearing the PXDLS motif. Thirdly, AN tethered to DNA was unable to repress the expression of reporter genes in transgenic Drosophila embryos. Fourthly, overexpression assays suggested that dCtBP and AN function differently in Drosophila tissues. Finally, AN failed to rescue the zygotic lethality caused by dCtBP loss-of-function. These data, taken together, suggest that AN is functionally distinct from dCtBP. Likely, ancestral CtBPs acquired corepressor function (capability of both repression and binding to repressors containing the PXDLS motif) after the animal–plant divergence but before the protostome–deuterostome split. We therefore propose to categorize AN as a subfamily member within the CtBP/BARS/RIBEYE/AN superfamily.