The gene for mitochondrial citrate synthase (CS) was isolated from Daucus carota (DcCS) and introduced into Arabidopsis thaliana (strain WS) using Agrobacterium tumefaciens-mediated transformation. Characteristics of citrate excretion were compared between T3 transgenic plants, which were derived from the initial transgenic plants by self-fertilization and homozygous for DcCS, and the control plants that had no DcCS. The highest CS activity 0.78 micromol protein min(-1) exhibited by the transgenic plants was about threefold greater than that found in the control plants (0.23-0.28 micromol protein min(-1)). Western analysis of the transgenic plants showed two CS signals corresponding to signals obtained from both D. carota and A. thaliana. Thus, it appears that the CS polypeptides by ectopic expression of DcCS were processed into the mature form and localized in the mitochondria of A. thaliana. The signal corresponding to the mature form of DcCS were greater in the transgenic plants having higher levels of CS activity. When the transgenic plants were grown in Al-phosphate media, a correlation between the levels of CS activity and the amounts of citrate excreted into the medium. The highest value (5.1 nmol per plant) was about 2.5-fold greater than that from control plants (1.9 nmol per plant). Both growth and P accumulation were greater in transgenic plants with high CS activity than that in control plants when they were grown on an acid soil where the availability of phosphate was low due to the formation of Al-phosphate. It appears that the overexpression of CS in A. thaliana improves the growth in phosphorous limited soil as a result of enhanced citrate excretion from the roots.

Leaf trichomes in Arabidopsis thaliana develop through several distinct cellular processes, such as patterning, differentiation, and growth. Although recent studies have identified several key transcription factors as regulating early patterning and differentiation steps, it is still largely unknown how these regulatory proteins mediate subsequent trichome development, which is accompanied by rapid cell growth and branching. Here, we report a novel trichome mutation in Arabidopsis, which in contrast with previously identified mutants, increases trichome cell size without altering its overall patterning or branching. We show that the corresponding gene encodes a GT-2-LIKE1 (GTL1) protein, a member of the trihelix transcription factor family. GTL1 is present within the nucleus during the postbranching stages of trichome development, and its loss of function leads to an increase in the nuclear DNA content only in trichomes that have completed branching. Our data further demonstrate that the gtl1 mutation modifies the expression of several cell cycle genes and partially rescues the ploidy defects in the cyclin-dependent kinase inhibitor mutant siamese. Taken together, this study provides the genetic evidence for the requirement of transcriptional regulation in the repression of ploidy-dependent plant cell growth as well as for an involvement of GTL trihelix proteins in this regulation.

How plants determine the final size of growing cells is an important, yet unresolved, issue. Root hairs provide an excellent model system with which to study this as their final cell size is remarkably constant under constant environmental conditions. Previous studies have demonstrated that a basic helix-loop helix transcription factor ROOT HAIR DEFECTIVE 6-LIKE 4 (RSL4) promotes root hair growth, but how hair growth is terminated is not known. In this study, we demonstrate that a trihelix transcription factor GT-2-LIKE1 (GTL1) and its homolog DF1 repress root hair growth in Arabidopsis. Our transcriptional data, combined with genome-wide chromatin-binding data, show that GTL1 and DF1 directly bind the RSL4 promoter and regulate its expression to repress root hair growth. Our data further show that GTL1 and RSL4 regulate each other, as well as a set of common downstream genes, many of which have previously been implicated in root hair growth. This study therefore uncovers a core regulatory module that fine-tunes the extent of root hair growth by the orchestrated actions of opposing transcription factors.

Spatial and temporal control of cell growth is central for the morphogenesis of multicellular organisms. For some cell types that undergo extensive post‐mitotic cell growth, such as neurons and hair cells, orchestrating the extent of post‐mitotic cell growth with development is vital for their physiology and function. Previous studies suggested that the extent of cell growth is linked with an increase in ploidy by endoreduplication but how developmental signals control endocycling and cell growth is not understood in both animals and plants. In this study we show that a trihelix transcription factor, GT2‐LIKE 1 (GTL1), actively terminates ploidy‐dependent cell growth and its developmentally regulated expression is one of the key determinants of cell size in Arabidopsis leaf hair cells (trichomes). Through genome‐wide chromatin‐binding studies (ChIP‐chip) coupled with transcriptional profiling, we further demonstrate that GTL1 directly represses the transcription of CDH1/FZR/CCS52, an activator of the anaphase‐promoting complex/cyclosome (APC/C), to stop the endocycle progression and ploidy‐dependent cell growth. Thus, our findings uncover a previously uncharacterised key molecular link between developmental programming and cell‐size control, highlighting the central role of APC/C in post‐mitotic cell growth.

Plants modify organ growth and tune morphogenesis in response to various endogenous and environmental cues. At the cellular level, organ growth is often adjusted by alterations in cell growth, but the molecular mechanisms underlying this control remain poorly understood. In this study, we identify the DNA BINDING WITH ONE FINGER (DOF)-type transcription regulator OBF BINDING PROTEIN4 (OBP4) as a repressor of cell growth. Ectopic expression of OBP4 in Arabidopsis (Arabidopsis thaliana) inhibits cell growth, resulting in severe dwarfism and the repression of genes involved in the regulation of water transport, root hair development, and stress responses. Among the basic helix-loop-helix transcription factors known to control root hair growth, OBP4 binds the ROOT HAIR DEFECTIVE6-LIKE2 (RSL2) promoter to repress its expression. The accumulation of OBP4 proteins is detected in expanding root epidermal cells, and its expression level is increased by the application of abscisic acid (ABA) at concentrations sufficient to inhibit root hair growth. ABA-dependent induction of OBP4 is associated with the reduced expression of RSL2. Furthermore, ectopic expression of OBP4 or loss of RSL2 function results in ABAinsensitive root hair growth. Taken together, our results suggest that OBP4-mediated transcriptional repression of RSL2 contributes to the ABA-dependent inhibition of root hair growth in Arabidopsis.