Hidehiro Fukaki

ARF7 and ARF19 Regulate Lateral Root Formation via Direct Activation of LBD/ASL Genes in Arabidopsis

Lateral root formation in Arabidopsis thaliana is regulated by two related AUXIN RESPONSE FACTORs, ARF7 and ARF19, which are transcriptional activators of early auxin response genes. The arf7 arf19 double knockout mutant is severely impaired in lateral root formation. Target-gene analysis in arf7 arf19 transgenic plants harboring inducible forms of ARF7 and ARF19 revealed that ARF7 and ARF19 directly regulate the auxin-mediated transcription of LATERAL ORGAN BOUNDARIES-DOMAIN16/ASYMMETRIC LEAVES2-LIKE18 (LBD16/ASL18) and/or LBD29/ASL16 in roots. Overexpression of LBD16/ASL18 and LBD29/ASL16 induces lateral root formation in the absence of ARF7 and ARF19. These LBD/ASL proteins are localized in the nucleus, and dominant repression of LBD16/ASL18 activity inhibits lateral root formation and auxin-mediated gene expression, strongly suggesting that these LBD/ASLs function downstream of ARF7- and ARF19-dependent auxin signaling in lateral root formation. Our results reveal that ARFs regulate lateral root formation via direct activation of LBD/ASLs in Arabidopsis.

Auxin Control of Root Development

A plants roots system determines both the capacity of a sessile organism to acquire nutrients and water, as well as providing a means to monitor the soil for a range of environmental conditions. Since auxins were first described, there has been a tight connection between this class of hormones and root development. Here we review some of the latest genetic, molecular, and cellular experiments that demonstrate the importance of generating and maintaining auxin gradients during root development. Refinements in the ability to monitor and measure auxin levels in root cells coupled with advances in our understanding of the sources of auxin that contribute to these pools represent important contributions to our understanding of how this class of hormones participates in the control of root development. In addition, we review the role of identified molecular components that convert auxin gradients into local differentiation events, which ultimately defines the root architecture.

Hormone interactions during lateral root formation

Lateral root (LR) formation, the production of new roots from parent roots, is a hormone- and environmentally-regulated developmental process in higher plants. Physiological and genetic studies using Arabidopsisthaliana and other plant species have revealed the roles of several plant hormones in LR formation, particularly the role of auxin in LR initiation and primordium development, resulting in much progress toward understanding the mechanisms of auxin-mediated LR formation. However, hormone interactions during LR formation have been relatively underexamined. Recent studies have shown that the plant hormones, cytokinin and abscisic acid negatively regulate LR formation whereas brassinosteroids positively regulate LR formation. On the other hand, ethylene has positive and negative roles during LR formation. This review summarizes recent findings on hormone-regulated LR formation in higher plants, focusing on auxin as a trigger and on the other hormones in LR formation, and discusses the possible interactions among plant hormones in this developmental process.

Cell Cycle Progression in the Pericycle Is Not Sufficient for SOLITARY ROOT/IAA14-Mediated Lateral Root Initiation in Arabidopsis thaliana

To study the mechanisms behind auxin-induced cell division, lateral root initiation was used as a model system. By means of microarray analysis, genome-wide transcriptional changes were monitored during the early steps of lateral root initiation. Inclusion of the dominant auxin signaling mutant solitary root1 (slr1) identified genes involved in lateral root initiation that act downstream of the auxin/indole-3-acetic acid (AUX/IAA) signaling pathway. Interestingly, key components of the cell cycle machinery were strongly defective in slr1, suggesting a direct link between AUX/IAA signaling and core cell cycle regulation. However, induction of the cell cycle in the mutant background by overexpression of the D-type cyclin (CYCD3;1) was able to trigger complete rounds of cell division in the pericycle that did not result in lateral root formation. Therefore, lateral root initiation can only take place when cell cycle activation is accompanied by cell fate respecification of pericycle cells. The microarray data also yielded evidence for the existence of both negative and positive feedback mechanisms that regulate auxin homeostasis and signal transduction in the pericycle, thereby fine-tuning the process of lateral root initiation.

Lateral root development in Arabidopsis: fifty shades of auxin

The developmental plasticity of the root system represents a key adaptive trait enabling plants to cope with abiotic stresses such as drought and is therefore important in the current context of global changes. Root branching through lateral root formation is an important component of the adaptability of the root system to its environment. Our understanding of the mechanisms controlling lateral root development has progressed tremendously in recent years through research in the model plant Arabidopsis thaliana (Arabidopsis). These studies have revealed that the phytohormone auxin acts as a common integrator to many endogenous and environmental signals regulating lateral root formation. Here, we review what has been learnt about the myriad roles of auxin during lateral root formation in Arabidopsis.

SGR2, a Phospholipase-Like Protein, and ZIG/SGR4, a SNARE, Are Involved in the Shoot Gravitropism of Arabidopsis

In higher plants, the shoot and the root generally show negative and positive gravitropism, respectively. To elucidate the molecular mechanisms involved in gravitropism, we have isolated many shoot gravitropism mutants in Arabidopsis. The sgr2 and zig/sgr4 mutants exhibited abnormal gravitropism in both inflorescence stems and hypocotyls. These genes probably are involved in the early step(s) of the gravitropic response. The sgr2 mutants also had misshapen seed and seedlings, whereas the stem of the zig/sgr4 mutants elongated in a zigzag fashion. The SGR2 gene encodes a novel protein that may be part of a gene family represented by bovine phosphatidic acid–preferring phospholipase A1 containing a putative transmembrane domain. This gene family has been reported only in eukaryotes. The ZIG gene was found to encode AtVTI11, a protein that is homologous with yeast VTI1 and is involved in vesicle transport. Our observations suggest that the two genes may be involved in a vacuolar membrane system that affects shoot gravitropism.

Auxin-mediated lateral root formation in higher plants.

Lateral root (LR) formation is an important organogenetic process that contributes to the establishment of root architecture in higher plants. In the angiosperms, LRs are initiated from the pericycle, an inner cell layer of the parent roots. Auxin is a key plant hormone that promotes LR formation, but the molecular mechanisms of auxin-mediated LR formation remain unknown. Molecular genetic studies using Arabidopsis mutants have revealed that the auxin transport system with a balance of influx and efflux is important for LR initiation and subsequent LR primordium development. In addition, normal auxin signaling mediated by two families of transcriptional regulators, Aux/IAAs and ARFs, is necessary for LR formation. This article is an update on the mechanisms of auxin-mediated LR formation in higher plants, particularly in Arabidopsis.

Gravitropic response of inflorescence stems in Arabidopsis thaliana.

We have characterized the gravitropic response of inflorescence stems in Arabidopsis thaliana. When the inflorescence stems were placed horizontally, they curved upward about 90[deg] within 90 min in darkness at 23[deg]C, exhibiting strong negative gravitropism. Decapitated stem segments (without all flowers, flower buds, and apical apices) also showed gravitropic responses when they included the elongation zone. This result indicates that the minimum elements needed for the gravitropic response exist in the decapitated inflorescence stem segments. At least the 3-min gravistimulation time was sufficient to induce the initial curvature at 23[deg]C after a lag time of about 30 min. In the gravitropic response of inflorescence stems, (a) the gravity perception site exists through the elongating zone, (b) auxin is involved in this response, (c) the gravitropic curvature was inhibited at 4[deg]C but at least the gravity perception step could occur, and (d) two curvatures could be induced in sequence at 23[deg]C by two opposite directional horizontal gravistimulations at 4[deg]C.

The endodermis and shoot gravitropism

Shoots and roots of higher plants exhibit negative and positive gravitropism, respectively. A variety of gravitropic mutants have recently been isolated from Arabidopsis, the characterization of which demonstrates that the molecular mechanisms of the gravitropic responses in roots, hypocotyls and inflorescence stems are different. The cytological and molecular analysis of two mutants, shoot gravitropism 1 (sgrl), which is allelic to scarecrow (scr), and sgr7, which is allelic to short-root(shr), indicate that the endodermis is the site of gravity perception in shoots. These data suggest a new model for shoot gravitropism.

The Auxin-Regulated AP2/EREBP Gene PUCHI Is Required for Morphogenesis in the Early Lateral Root Primordium of Arabidopsis

Organ primordia develop from founder cells into organs due to coordinated patterns of cell division. How patterned cell division is regulated during organ formation, however, is not well understood. Here, we show that the PUCHI gene, which encodes a putative APETALA2/ethylene-responsive element binding protein transcription factor, is required for the coordinated pattern of cell divisions during lateral root formation in Arabidopsis thaliana. Recessive mutations in PUCHI disturbed cell division patterns in the lateral root primordium, resulting in swelling of the proximal region of lateral roots. PUCHI expression was initially detected in all of the cells in early lateral root primordia, and later it was restricted to the proximal region of the primordia. Stable expression of PUCHI required auxin-responsive elements in its promoter region, and exogenous auxin increased the level of PUCHI mRNA accumulation. These results suggest that PUCHI acts downstream of auxin signaling and that this gene contributes to lateral root morphogenesis through affecting the pattern of cell divisions during the early stages of primordium development.