Previously, the GRF-INTERACTING FACTOR1 (GIF1)/ANGUSTIFOLIA3 (AN3) transcription coactivator gene, a member of a small gene family comprising three genes, was characterized as a positive regulator of cell proliferation in lateral organs, such as leaves and flowers, of Arabidopsis (Arabidopsis thaliana). As yet, it remains unclear how GIF1/AN3 affects the cell proliferation process. In this study, we demonstrate that the other members of the GIF gene family, GIF2 and GIF3, are also required for cell proliferation and lateral organ growth, as gif1, gif2, and gif3 mutations cause a synergistic reduction in cell numbers, leading to small lateral organs. Furthermore, GIF1, GIF2, and GIF3 overexpression complemented a cell proliferation defect of the gif1 mutant and significantly increased lateral organ growth of wild-type plants as well, indicating that members of the GIF gene family are functionally redundant. Kinematic analysis on leaf growth revealed that the gif triple mutant as well as other strong gif mutants developed leaf primordia with fewer cells, which was due to the low rate of cell proliferation, eventually resulting in earlier exit from the proliferative phase of organ growth. The low proliferative activity of primordial leaves was accompanied by decreased expression of cell cycle-regulating genes, indicating that GIF genes may act upstream of cell cycle regulators. Analysis of gif double and triple mutants clarified a previously undescribed role of the GIF gene family: gif mutants had small vegetative shoot apical meristems, which was correlated with the development of small leaf primordia. gif triple mutants also displayed defective structures of floral organs. Taken together, our results suggest that the GIF gene family plays important roles in the control of cell proliferation via cell cycle regulation and in other developmental properties that are associated with shoot apical meristem function.

Previously, we identified a GROWTH-REGULATING FACTOR gone family, comprising nine members, which encodes putative transcription factors in Arabidopsis thaliana. The grfl grf2 grf3 triple mutants produced partially fused cotyaedons and developed small leaves due to a reduction in cell numbers. To understand the functional role of another member of this gone family, GRF4, we have new identified a grf4 null mutant and constructed a quadruple mutant by crossing it to the grf triple mutant. The quadruple mutant has much smaller leaves than its parental ~utants, with this reduced size again due to fewer cells. Interestinglyp the quadruple mutant displays not enny a much stronger fusion of cotyledons but also the phenotype of the shoot meristemless mutant. The aberrant cotyledons of the quadruple mutants result from a fusion of cotyledon primordia during embryogenesis. These results suggest that GRF4 is required for both Meal ce|[ proliferation and the embryonic development of cotyledons and the shoot apical meristem.

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Transcription factors are key regulators of gene expression and play pivotal roles in all aspects of living organisms. Therefore, identification and functional characterization of transcription factors is a prerequisite step toward understanding life. This article reviews molecular and biological functions of the two transcription regulator families, GROWTH-REGULATING FACTOR (GRF) and GRF-INTERACTING FACTOR (GIF), which have only recently been recognized. A myriad of experimental evidence clearly illustrates that GRF and GIF are bona fide partner proteins and form a plant-specific transcriptional complex. One of the most conspicuous outcomes from this research field is that the GRF-GIF duo endows the primordial cells of vegetative and reproductive organs with a meristematic specification state, guaranteeing the supply of cells for organogenesis and successful reproduction. It has recently been shown that GIF1 proteins, also known as ANGUSTIFOLIA3, recruit chromatin remodelling complexes to target genes, and that AtGRF expression is directly activated by the floral identity factors, APETALA1 and SEPALLATA3, providing an important insight into understanding of the action of GRF-GIF. Moreover, GRF genes are extensively subjected to post-transcriptional control by microRNA396, revealing the presence of a complex regulatory circuit in regulation of plant growth and development by the GRF-GIF duo.

Development of the functional endodermis of Arabidopsis thaliana roots is controlled, in part, by GRAS transcription factors, namely SHORT-ROOT (SHR), SCARECROW (SCR), and SCARECROW-LIKE 23 (SCL23). Recently, it has been shown that the SHR-SCR-SCL23 regulatory module is also essential for specification of the endodermis (known as the bundle sheath) in leaves. Nevertheless, compared with what is known about the role of the SHR-SCR-SCL23 regulatory network in roots, the molecular interactions of SHR, SCR, and SCL23 are much less understood in shoots. Here, we show that SHR forms protein complexes with SCL23 to regulate transcription of SCL23 in shoots, similar to the regulation mode of SCR expression. Our results indicate that SHR acts as master regulator to directly activate the expression of SCR and SCL23. In the SHR-SCR-SCL23 network, we found a previously uncharacterized negative feedback loop whereby SCL23 modulates SHR levels. Through molecular, genetic, physiological, and morphological analyses, we also reveal that the SHR-SCR-SCL23 module plays a key role in the formation of the endodermis (known as the starch sheath) in hypocotyls. Taken together, our results provide new insights into the regulatory role of the SHR-SCR-SCL23 network in the endodermis development in both roots and shoots.

Edited by Ulf-Ingo FlüggeKeywords: DEFECTIVE IN ANTHER DEHISCENCE1-like acylhydrolase Phospholipase Galactolipase Chloroplast targeting Alternative RNA splicing a b s t r a c tIn Arabidopsis, there are at least seven class I acylhydrolase members, which have a putative N-terminal chloroplast-targeting signal. Here, we show that all seven class I proteins are localized to the chloroplasts and hydrolyze phosphatidylcholine at the sn-1 position. However, based on their activities toward various lipids, Arabidopsis class I enzymes could be further divided into three sub-groups by substrate specificity, one with phospholipase-specific activity, another with phospholipase and galactolipase activities, and the other with broad lipolytic activity toward phosphatidylcholine, galactolipids, and triacylglycerol. These results suggest that the three sub-groups of class I acylhydrolases have specific roles in chloroplasts.

GROWTH-REGULATING FACTORs (GRFs) are sequence-specific DNA-binding transcription factors that regulate various aspects of plant growth and development. GRF proteins interact with a transcription cofactor, GRF-INTERACTING FACTOR (GIF), to form a functional transcriptional complex. For its activities, the GRF-GIF duo requires the SWITCH2/SUCROSE NONFERMENTING2 chromatin remodeling complex. One of the most conspicuous roles of the duo is conferring the meristematic potential on the proliferative and formative cells during organogenesis. GRF expression is post-transcriptionally down-regulated by microRNA396 (miR396), thus constructing the GRF-GIF-miR396 module and fine-tuning the duo’s action. Since the last comprehensive review articles were published over three years ago, many studies have added further insight into its action and elucidated new biological roles. The current review highlights recent advances in our understanding of how the GRF-GIF-miR396 module regulates plant growth and development. In addition, I revise the previous view on the evolutionary origin of the GRF gene family.

Reproductive success of angiosperms relies on the precise development of the gynoecium and the anther, because their primary function is to bear and to nurture the embryo sac/female gametophyte and pollen, in which the egg and sperm cells, respectively, are generated. It has been known that the GRF-INTERACTING FACTOR (GIF) transcription co-activator family of Arabidopsis thaliana (Arabidopsis) consists of three members and acts as a positive regulator of cell proliferation. Here, we demonstrate that GIF proteins also play an essential role in development of reproductive organs and generation of the gamete cells. The gif1 gif2 gif3 triple mutant, but not the single or double mutants, failed to establish normal carpel margin meristem (CMM) and its derivative tissues, such as the ovule and the septum, resulting in a split gynoecium and no observable embryo sac. The gif triple mutant also displayed severe structural and functional defects in the anther, producing neither microsporangium nor pollen grains. Therefore, we propose that the GIF family of Arabidopsis is a novel and essential component required for the cell specification maintenance during reproductive organ development and, ultimately, for the reproductive competence.