The NGATHA genes direct style development in the Arabidopsis gynoecium

Auxin biosynthesis and its role in plant development

Indole-3-acetic acid (IAA), the main auxin in higher plants, has profound effects on plant growth and development. Both plants and some plant pathogens can produce IAA to modulate plant growth. Although the genes and biochemical reactions for auxin biosynthesis in some plant pathogens are well understood, elucidation of the mechanisms by which plants produce auxin has proven to be difficult. So far, no single complete pathway of de novo auxin biosynthesis in plants has been firmly established. However, recent studies have led to the discoveries of several genes in tryptophan-dependent auxin biosynthesis pathways. Recent findings have also determined that local auxin biosynthesis plays essential roles in many developmental processes including gametogenesis, embryogenesis, seedling growth, vascular patterning, and flower development. In this review, I summarize the recent advances in dissecting auxin biosynthetic pathways and how the understanding of auxin biosynthesis provides a crucial angle for analyzing the mechanisms of plant development.

Auxin biosynthesis and its role in plant development

Indole-3-acetic acid (IAA), the main auxin in higher plants, has profound effects on plant growth and development. Both plants and some plant pathogens can produce IAA to modulate plant growth. Although the genes and biochemical reactions for auxin biosynthesis in some plant pathogens are well understood, elucidation of the mechanisms by which plants produce auxin has proven to be difficult. So far, no single complete pathway of de novo auxin biosynthesis in plants has been firmly established. However, recent studies have led to the discoveries of several genes in tryptophan-dependent auxin biosynthesis pathways. Recent findings have also determined that local auxin biosynthesis plays essential roles in many developmental processes including gametogenesis, embryogenesis, seedling growth, vascular patterning, and flower development. In this review, I summarize the recent advances in dissecting auxin biosynthetic pathways and how the understanding of auxin biosynthesis provides a crucial angle for analyzing the mechanisms of plant development.

Control of tissue and organ growth in plants

Plant organs grow to characteristic, species-specific sizes and shapes. At the cellular level, organ growth is initially characterized by cell proliferation, which gives way to cell expansion at later stages. Using mainly Arabidopsis thaliana as a model species, a number of factors have been isolated in recent years that promote or restrict organ growth, with the altered organ size being associated with changes in cell number, in cell size, or in both. However, cells in an organ do not appear to follow a strictly autonomous program of proliferation and expansion, and their behavior is coordinated in at least three different respects: normally sized organs can be formed consisting of altered numbers of cells with compensatory changes in the size of the individual cells, suggesting that cellular behavior is subject to organ-wide control; the growth of cells derived from more than one clonal origin is coordinated within a plant lateral organ with its different histological layers; and growth of cells in different regions of an organ is coordinated to generate a reasonably flat leaf or floral organ. Organ growth is strongly modulated by environmental factors, and the molecular basis for this regulation is beginning to be understood. Given the complexity of organ growth as a dynamic four-dimensional process, precise quantification of growth parameters and mathematical modeling are increasingly used to understand this fascinating problem of plant biology.

Distinct and dynamic auxin activities during reproductive development

Flowering plants have evolved sophisticated and complicated reproductive structures to ensure optimal conditions for the next generation. Successful reproduction relies on careful timing and coordination of tissue development, which requires constant communication between these tissues. Work on flower and fruit development over the last decade places the phytohormone auxin in a key role as a master of patterning and tissue specification of reproductive organs. Although many questions still remain, it is now clear that auxin mediates its function in flowers and fruits through an integrated process of biosynthesis, transport, and signaling, as well as interaction with other hormonal pathways. In addition, the knowledge obtained so far about auxin function already allows researchers to develop tools for crop improvement and precision agriculture.

REVEILLE1, a Myb-like transcription factor, integrates the circadian clock and auxin pathways

The circadian clock modulates expression of a large fraction of the Arabidopsis genome and affects many aspects of plant growth and development. We have discovered one way in which the circadian system regulates hormone signaling, identifying a node that links the clock and auxin networks. Auxin plays key roles in development and responses to environmental cues, in part through regulation of plant growth. We have characterized REVEILLE1 (RVE1), a Myb-like, clock-regulated transcription factor that is homologous to the central clock genes CIRCADIAN CLOCK ASSOCIATED 1 (CCA1) and LATE ELONGATED HYPOCOTYL (LHY). Despite this homology, inactivation of RVE1 does not affect circadian rhythmicity but instead causes a growth phenotype, indicating this factor is a clock output affecting plant development. CCA1 regulates growth via the bHLH transcription factors PHYTOCHROME INTERACTING FACTOR4 (PIF4) and PIF5, but RVE1 acts independently of these genes. RVE1 instead controls auxin levels, promoting free auxin production during the day but having no effect during the night. RVE1 positively regulates the expression of the auxin biosynthetic gene YUCCA8 (YUC8), providing a mechanism for its growth-promoting effects. RVE1 is therefore a node that connects two important signaling networks that coordinate plant growth with rhythmic changes in the environment.

Dynamic, auxin-responsive plasma membrane-to-nucleus movement of Arabidopsis BRX

In Arabidopsis, interplay between nuclear auxin perception and trans-cellular polar auxin transport determines the transcriptional auxin response. In brevis radix (brx) mutants, this response is impaired, probably indirectly because of disturbed crosstalk between the auxin and brassinosteroid pathways. Here we provide evidence that BRX protein is plasma membrane-associated, but translocates to the nucleus upon auxin treatment to modulate cellular growth, possibly in conjunction with NGATHA class B3 domain-type transcription factors. Application of the polar auxin transport inhibitor naphthalene phthalamic acid (NPA) resulted in increased BRX abundance at the plasma membrane. Thus, nuclear translocation of BRX could depend on cellular auxin concentration or on auxin flux. Supporting this idea,NPA treatment of wild-type roots phenocopied the brx root meristem phenotype. Moreover, BRX is constitutively turned over by the proteasome pathway in the nucleus. However, a stabilized C-terminal BRX fragment significantly rescued the brx root growth phenotype and triggered a hypocotyl gain-of-function phenotype, similar to strong overexpressors of full length BRX. Therefore, although BRX activity is required in the nucleus,excess activity interferes with normal development. Finally, similar to the PIN-FORMED 1 (PIN1) auxin efflux carrier, BRX is polarly localized in vascular cells and subject to endocytic recycling. Expression of BRX under control of the PIN1 promoter fully rescued the brx short root phenotype, suggesting that the two genes act in the same tissues. Collectively, our results suggest that BRX might provide a contextual readout to synchronize cellular growth with the auxin concentration gradient across the root tip.