During the past 20 years, biochemical, genetic, and bioinformatic analyses have identified more than a dozen secreted peptide hormones in plants.　These peptide hormones have proven to be functionally more diverse than anticipated.　The number of functionally characterized peptide hormones now exceeds the number of classical plant hormones and constitute one of the largest research areas in plant biology.　Because a number of uncharacterized genes encoding small secreted peptides have been identified in Arabidopsis, unraveling the signal transduction pathways mediated by secreted peptides is currently one of the most exciting targets.　This review series covers recent topics in peptide hormone research in plants.

Nitrogen （N） is a critical nutrient for plants but is often distributed unevenly in the soil.　Plants therefore have evolved a systemic mechanism by which N starvation on one side of the root system leads to a compensatory upregulation of nitrate uptake on the other side.　However, the molecular components underlying this long-distance signaling, called systemic N-demand signaling, have long remained elusive.　We identified that N starvation-induced small 15-amino-acid secreted peptide, CEP （C-terminally encoded peptide）, acts as a root-derived ascending N-demand signal to the shoot via xylem.　CEP is recognized in the shoot by specific LRR-receptor kinase, CEPR （CEP receptor）, that leads to the production of non-secreted polypeptide, CEPD （CEP downstream）, as a secondary signal.　CEPD acts as a shoot-derived descending signal to the root via phloem and ultimately upregulates nitrate transporter NRT2.1 gene in the roots.　Thus, CEP family peptides induced on one side of the roots by local N starvation is able to mediate upregulation of NRT2.1 in the distant part of the roots.　Our findings provide new insights into the molecular basis of plant adaptation to a dynamically fluctuating N environment.

Peptide hormones are versatile regulators of plant growth and environmental responses.　To date, approximately 20 families of plant peptide hormones have been found.　Members in each family contain minor structural variations, which are in some cases implicated in functional differentiation in plants.　Based on the genetic diversity of CLE family peptides, we have recently found a new type of artificial peptides that can activate two distinct receptors.　In this review, we introduce the story of this finding and discuss the mechanisms of action of the artificial peptide ligands.

To adapt ever-changing situations, multicellular organisms have developed several types of extracellular barriers as a result of evolutionary convergence.　Plant roots establish two types of extracellular barriers, namely Casparian strip （CS） and suberin lamellae, inside of the roots, not on their surface.　The Casparian strips mainly consist of lignin, a highly hydrophobic chemical, which blocks free diffusion of ions or small molecules across endodermal cell layers.　Despite of the physiological importance, the mechanisms underlying functional Casparian strip formation are not yet uncovered.　Recently, two independent groups identified exactly the same peptide-receptor pair for Casparian strip maturation by completely different approaches.　Geldner’s group in Lausanne identified the pair by forward genetics and database search, while Matsu-bayashi’s group in Nagoya identified the pair by in silico and biochemical screens.　This review focuses on how these two groups identified the sulfated peptides, CIF1/2 （Casparian strip integrity factors 1/2）, and how these peptides function on the Casparian strips.　Furthermore, we discuss the mode of action of the peptides during CS maturation in developmental context and their roles under physiological or stress conditions.

Higher plants respond to various types of environmental stresses to adapt against those stressful conditions.　They receive environmental information from roots and leaves and then integrate the information as a whole because the environmental conditions of underground and aerial parts are very different.　To adapt to complex environmental conditions at whole plant level, long-distance signaling is important for plants for their survival.　Recent studies have elucidated that various mobile molecules transmit the extracellular stimuli from sensing tissues to target organs.　These mobile molecules include hydroxy pressure, calcium currents, peptides, phytohormones and so on.　These indicate that the plants have unique and complex mechanisms for connecting various organs precisely although they do not have nervous system.　In this review, we focused to summarize current knowledge of molecules in long distance signaling for abiotic stress responses, mainly drought stress response.

Purine catabolism is ubiquitous in organisms, serving for the maintenance of cellular purine homeostasis.　In plants, another important role played by this metabolism is attributed to the recycling of purine-derived ammonia to general nitrogen metabolism for sustainable growth and development.　It has also been suggested that certain purine metabolites and enzymes are involved in plant adaptation to environmental stress.　The recent completion of molecular identification of the enzymes in Arabidopsis has made it possible, by using molecular genetic approaches, to examine the physiological importance of the pathway in these contexts.　Notably, recent forward and reverse genetic studies have revealed the emerging role in stress tolerance, highlighting xanthine dehydrogenase as a modulator of reactive oxygen species homeostasis and the intermediate allantoin as a stress protectant.　This review summarizes up-to-date research progress in plant purine catabolism and associated metabolites, focusing on the current outline of the enzymatic route, transgenic/mutant phenotype characterization, and possible protective mechanisms against abiotic and biotic stress.

Most of the important fruit crops in Rosaceae exhibit S-RNase-based self-incompatibility （SI） that is gametophytically controlled by a multigene complex, the S-locus.　The S haplotype-specific recognition systems of the Rosaceae were investigated through genetic and molecular characterization of self-compatible （SC） mutants.　These analyses revealed that S-RNase genes were indispensable for a stylar self-incompatible response in all members of the Rosaceae studied to date.　In contrast, the pollen factors are currently classified into two different types.　In Prunus, such as apricot and sweet cherry, there is a single functional F-box protein called SLF or SFB that is predicted to recognize self S-RNase, because almost all pollen-part self-compatible mutants （PPMs） analyzed had lost SLF/SFB.　In the Pyrinae, including apple and pear, there are multiple S-related F-box proteins that are postulated to recognize non-self S-RNase, whereas no PPMs with an altered S haplotype had been discovered.　Recently, our group developed a PPM of Japanese pear, which was identified from among male-derived progeny of a gamma-irradiated tree.　Here, I summarize the theoretical and practical contributions of known SC mutants and introduce the genetic and phenotypic basis of self-compatibility in the newly obtained mutant that will be valuable for the breeding of SC cultivars and further investigation of the mechanisms of regulation and function of SI in Rosaceae.

Pollen tubes of flowering plants, which germinate from pollen grains on the stigma surface of the pistil, precisely arrive ovules to achieve fertilization.　Ovular attractant peptides, LUREs, were first reported in 2009 after long search for more than 140 years.　Since then, studies on pollen tube guidance have been accelerated.　Many key molecules for pollen tube guidance have been identified, including tip-localizing LURE1-receptor PRK6 in Arabidopsis and ovule-derived bioactive sugar chain AMOR for capacitation of pollen tubes in Torenia.　In this article, current understanding is reviewed how pollen tubes control growth direction by perception of LURE attractant peptides.

Plant metabolism is characterized by a wide diversity of metabolites, with systems far more complicated than those of microorganisms.　Therefore, plant metabolic engineering is still a challenge compared to microbial engineering.　Metabolic engineering requires system-level understanding of metabolism and mathematical modeling is useful for understanding dynamic behaviors of plant metabolic systems.　Time-series metabolome data has great potential for estimating kinetic model parameters to construct a genome-wide metabolic network model.　However, data obtained by current metabolomics techniques does not meet the requirement for constructing accurate models.　In this article, we highlight novel strategies and algorithms to handle the underlying difficulties and construct dynamic in vivo models for large-scale plant metabolic systems.　The coarse but efficient modeling enables predictive metabolic engineering and also the prediction of unknown mechanisms regulating plant metabolism.

Unlike animals, plants do not possess the central nervous system, but they can immediately sense local environmental stresses, such as mechanical wounding and herbivore attack, propagate this information throughout the plant body and activate systemic defence responses in distant organs.　However, the molecular machinery underlying such rapid sensory and systemic signal transduction is poorly understood.　Using genetically-encoded calcium ion （Ca²⁺） and glutamic acid （Glu） indicators and a highly-sensitive wide-field fluorescence microscope, we have visualized the plant-wide spatial and temporal dynamics of cytosolic Ca²⁺ and apoplastic Glu levels in response to wounding in Arabidopsis leaves.　Here, we show that glutamate is a wound signal in plants that is leaked to the apoplastic region from damaged cells/tissues.　The GLUTAMATE RECEPTOR LIKE （GLR） family of Ca²⁺-permeable channels act as sensors that convert this damage-associated signal into an increase in cytosolic Ca²⁺ concentration.　This Ca²⁺ signal propagates throughout the entire plant via the plant-specific tissue/structure, phloem and plasmodesmata, and preemptively activates resistance responses in the distant undamaged organs.

The application of livestock manure compost contaminated with the herbicide clopyralid originating from imported grass hay and grains can cause crop growth disorder.　On examining the susceptibility of ornamental crops, growth disorders were observed in chrysanthemum, cosmos, zinnia, sunflower, aster （Asteraceae）, and crimson clover （Leguminosae） plants.　Therefore, more simple assay methods for assessing clopyralid content and technology for reducing clopyralid amount in compost and soil are required.