This special issue of “Plant Hormones” by Chinese Bulletin of Botany is dedicated to the “Xiangshan Conference—Plant Hormones and Green Revolution”, which will be held in October 2006 in Beijing. Here, we present an overview on the plant hormone research in China for readers of interest. Until 1980s, research on plant hormones in China mainly focused on physiological studies. With rapid growth of the Chinese economy, a rising generation of young scientists has been able to make significant progress in elucidating the regulatory mechanisms of plant hormone metabolism, transport and signal transduction. In particular, Chinese scientists have made substantial contributions to our current understanding to the field by the identification of hormone receptors, the hormone-regulated plant architecture formation as well as interactions among different hormone signaling pathways. Finally, based on the trends of the field as well as the potentials of Chinese scientists, we offer a prospective view on the future studies on plant hormones in China.

Plants modulate their shoot architecture by regulating activities of the shoot apical meristem and axillary meristems. Meristem activities are regulated by a network of environmental information, developmental stage and genetic makeup of the plant. The fate of signal integration, to a large extent, depends on the action of plant hormones. Auxin plays an essential role in the establishment and maintenance of the apical dominance, which is a central issue in regulating shoot branching. This review focuses on recent advances in the study of auxin biosynthesis, metabolism, polar transport, and signaling pathway, as well as their involvements in the control of the architecture of aerial parts. Prospects in the field are also briefly discussed.

Leaf is the first lateral organ produced by the activity of the shoot apical meristem (SAM). Early leaf development is artificially divided into three main stages: the initiation of leaf primordium, the establishment of leaf adaxial-abaxial polarity, and the expansion of leaf blade. Numerous of evidence indicates that leaf development is regulated by inner genetic mechanism and outer environmental cues. Phytohormones, especially auxin, play essential roles in coordination of these two regulation mechanisms. Auxin homeostasis, polar transport and signaling affect the whole progress of leaf development. In this mini-review, we will summarize recent progress of auxin regulation in leaf development and morphogenesis, and try to understand the complex regulation network of leaf development.

Polar auxin transport (PAT), a unique process in plant modulates organogenesis, development and tropic response. Here we reviewed the regulation mechanism of PAT based on recent research progresses. Evidences on molecular genetics and physiology support a hypothesis that the process is depended on activities of auxin influx and efflux facilitators. Asymmetric distribution and activities of the efflux facilitator PIN1 and the influx facilitator AUX1 are impacted by a guanine-nucleotide exchange factor (GEF) for the ADPribosylation factor (ARF) and ARF-GTPase activating protein (ARF-GAP), respectively, which are involved in Golgi stacks mediated vesicle trafficking. The activity of efflux facilitator PIN2 is modulated also by ROP,a small G protein in Arabidopsis. A hypothesis about the regulation of polar auxin transportation by ARF is suggested.

The plant phytohormone cytokinin regulates numerous growth and developmental processes by regulating cell division and cell differentiation. During the past ten years, remarkable progress has been made to our understanding on the cytokinin metabolism, transport and signaling, mainly using Arabidopsis thaliana as a model system. In addition, substantial attentions have also been paid to crosstalks between cytokinin and other signaling pathways. According to our current understanding, cytokinin signaling is mediated by sequentially transferring a phosphoryl group in different members of a two-component system, referred to as phosphorelay. The two-component system may also act as a module linking cytokinin and other signaling pathways. Most known members of the two-component system are functionally redundant, but also show distinctive function in the regulation of a variety of developmental processes. In this review, we summarize our current understanding on cytokinin metabolism and transport, and discuss in detail on recent advances in cytokinin signaling as well as its cross-talks with other signaling pathways. We also make prospects on possible improvement of important agricultural traits by manipulating the cytokinin network.

The phytohormone gibberellins (GA) play an important role in controlling and modulating diverse developmental processes, such as seed germination, hypocotyls elongation, leaf expansion and flowering time.In recent years, there are significant progresses in understanding of GA biosynthesis pathway and GA signaling in Arabidopsis and rice. This review highlights GA biosynthesis pathway and their regulation, including a new pathway identified in rice, and molecular model of“ De-repress” in GA signaling. GA promotes plant growth via 26S proteasome-dependent proteolysis of DELLA proteins repressors, one of the key component in GA signaling, and also depends on GA-mediated interaction between GA receptor and DELLA proteins. Finally, this paper discussed the cross-talking between GA and other hormones, and modulation of adaptation to environments.

ABA, an important phytohormone induced by biotic stress and abiotic stress, plays great roles in plant tolerance to stresses. In this review, we summarize the regulation of ABA biosynthesis and metabolism by plant stresses, the roles of ABA in regulating stomatal closure and gene expression, and the crosstalk between plant stress signaling.

The gaseous hormone ethylene has numerous effects during plant growth and development. It is important to know how ethylene is synthesized and how the signal is transduced. During the past twenty years, the isolation and characterization of various mutants that show an altered triple-response phenotype has uncovered a largely linear ethylene signaling pathway with branches in the downstream response pathway. In Arabidopsis, perception of ethylene is performed by five receptors, ETR1, ERS1, ETR2, ERS2, EIN4, which exhibit structural and functional redundancy and are negative regulators of ethylene signaling. The receptors are homodimer in vivo. The membrane-bound N-terminal of ETR1 binds ethylene with the assistance of a copper cofactor Cu (Ⅰ). Although ETR1 was reported to possess histidine kinase activity whereas other receptors have serine/theronine kinase activity, the mechanism of ethylene receptors signaling is largely unclear. The receptors interact with a Raf-like protein kinase CTR1, which is a negative regulator in the ethylene response. Inactivation of CTR1 leads to activation of EIN2, which consists of a novel C-terminal signaling domain, and a N-terminal transmembrane domain with sequence similarity to the Nramp family of metal ion transporters.Downstream of EIN2, EIN3 and EILs function as primary transcription factors that can induce expression of ERF1 and other secondary transcription factors, which in turn regulate a large number of ethylene response genes. EIN3 is regulated by a proteasome-mediated protein degradation pathway. As ethylene is a versatile phytohormone, its response pathway has multiple interactions (crosstalk) with other signaling pathway.

The gaseous plant hormone ethylene regulates a variety of developmental process and biotic and abiotic stress response in plant. During the past decade, molecular genetic studies on the model plant Arabidopsis have established a linear signal transduction pathway from signal perception on the endoplasmic reticulum membrane to transcriptional regulation in the nucleus. Ethylene receptor family in Arabidopsis consists of five components, including ETR1, ERS1, ETR2, ERS2 and EIN4, at least one of which, ETR1, was reported to localize on endoplasmic reticulum, and negatively regulates ethylene response by forming a complex with Raflike kinase CTR1. Downstream of CTR1 are EIN2 and EIN3/EILs, which positively regulate ethylene response.In the absence of ethylene signal, EIN3 is quickly degraded through an ubiquitin/26S proteasome pathway mediated by two F-box proteins, EBF1 and EBF2. EIN5, a 5’→3’ exoribonuclease, antagonizes the negative feedback regulation on EIN3 by promoting EBF1 and EBF2 mRNA decay. Despite the recent advances on the understanding of plant response to ethylene, many aspects of the ethylene signaling pathway remain unknown, especially the biochemical nature and the regulatory mechanisms of key pathway components, as well as the molecular basis of various interactions between the ethylene response pathway and other signaling pathways. In the coming years the research in this fast advancing field might provide much of the needed answers to these problems.

Plant hormone brassinosteroid (BR) acts as an important regulator in plant growth and development,and responses to environmental stimuli. BR also regulates the agritraits of many crops. Analyses on mutant phenotypes and gene functions provide the information on BR biosynthesis and physiological roles. This review focuses on the BR recent progresses of BR biosynthesis and metabolism, the underlying signaling pathways, and further the interplay with other hormones.

Brassinosteroids (BRs) signals were perceived by the membrane receptor complex BRI1/BAK1 and transferred to nucleus through series proteins, resulting in the regulation of down-stream responsive genes. This review focuses on recent progresses in the studies of the BR signal transduction of Arabidopsis, and discusses the unsolved issues and key points of future studies.

The ubiquitin/26S proteosome pathway mainly consists of ubiquitin activating enzyme (E1),ubiquitin conjugating enzyme (E2), ubiquitin protein ligase (E3), and 26S proteasome. In the initial reaction, E1 activates the Ub by coupling ATP hydrolysis, the activated Ub is then transferred to an E2. The E2 either transfers ubiquitin directly to the E3 in the case of HECT E3s or binds the E3 and transfers the ubiquitin to the substrate. Polyubiquinated proteins are eventually degraded by the 26S proteasome. In plants, regulated protein degradation by the ubiquitin/26S proteosome contributes significantly to development by affecting a wide range of processes, including hormone signaling, photomorphogenesis, and flower development.

Apoplast is an important compartment outside of plasma membrane of the plant cell and is the source of various primary signals. Biotic or abiotic stimuli can first cause the alteration in apoplastic signaling system; on the other hand, apoplast provides a convenient pathway for cell-cell communication, and therefore plays a key role in regulation of cell differentiation, organ genesis, and growth development of plants. This review summarized recent advance in the research of apoplastic polypeptides and proteins in our lab.

Although the first peptide hormone (systemin) in plants has been discovered for more than ten years, till now only four peptides have been accepted as peptide hormones. They are systemin, PSK, CLV3 and SCR, involved in wounding response, cell division, stem cell maintenance in the shoot apical meristem, and the interaction between pollen and stigma, respectively. These small peptides act as ligands to interact with receptor kinases located on the surface of the cytoplasma membrane, triggering downstream signal transductions.In this paper we outline the progresses made in this area, with emphasis on CLV3 peptides. We also provide our prospective vision in this field.

Plant organ regeneration has an important role in plant reproduction and genetic engineering, and also provides an experimental system to study molecular mechanism of plant development. So far, almost all of vegetative and reproductive organs have been regenerated in vitro, which might be helpful to further understand the molecular mechanism of plant organogenesis. In this review, regulation of gene expression and functions of a few genes involved in the regeneration of vegetative organs are introduced. Further, roles of organ identity genes, such as HAG1 and HoMADS1, are evaluated in reproductive organ development. Finally, we discuss the questions which may be focused in the future study.

Bestatin, an inhibitor of some aminopeptidases, potently activates jasmonic acid-induced defense genes expression in the model systems of tomato and Arabidopsis. We have employed bestatin as a small molecular chemical tool to dissect jasmonic acid signaling in Arabidopsis and identified a collection of bestatin response (ber) mutants. ber15 is one of these mutants that are resistant to the inhibitory effect of bestatin on root elongation. Subsequent investigation revealed that ber15 is less sensitive to applied jasmonic acid in term of root growth inhibition, suggesting that the BER15 gene play a role in JA signaling. Map-based cloning of BER15 demonstrated BER15 encodes a cytochrome P450 monooxygenase which acts as a key enzyme in brassinolide synthesis. Further functional analysis of BER15 will broaden our knowledge of the interaction mechanism between brassinolide biosynthesis and JA signaling.