Cytokinins regulate spatially specific ethylene production to control root growth in Arabidopsis

Two principal growth regulators,cytokinins and ethylene,are known to interact in the regulation of plant growth.However,information about the underlying molecular mechanism and positional specificity of cytokinin/ethylene crosstalk in the control of root growth is scarce.We have identified the spatial specificity of cytokinin-regulated root elongation and root apical meristem(RAM)size,both of which we demonstrate to be dependent on ethylene biosynthesis.Upregulation of the cytokinin biosynthetic gene ISOPENTENYLTRANSFERASE(IPT)in proximal and peripheral tissues leads to both root and RAM shortening.By contrast,IPT activation in distal and inner tissues reduces RAM size while leaving the root length comparable to that of mock-treated controls.We show that cytokinins regulate two steps specific to ethylene biosynthesis:production of the ethylene precursor 1-aminocyclopropane-1-carboxylate(ACC)by ACC SYNTHASEs(ACSs)and its conversion to ethylene by ACC OXIDASEs(ACOs).We describe cytokinin-and ethylene-specific regulation controlling the activity of ACSs and ACOs that are spatially discrete along both proximo/distal and radial root axes.Using direct ethylene measurements,we identify ACO2,ACO3,and ACO4 as being responsible for ethylene biosynthesis and ethylene-regulated root and RAM shortening in cytokinin-treated Arabidopsis.Direct interaction between ARABIDOPSIS RESPONSE REGULATOR 2(ARR2),a member of the multistep phosphorelay cascade,and the C-terminal portion of ETHYLENE INSENSITIVE 2(EIN2-C),a key regulator of canonical ethylene signaling,is involved in the cytokinin-induced,ethylene-mediated control of ACO4.We propose tight cooperation between cytokinin and ethylene signaling in the spatially specific regulation of ethylene biosynthesis as a key aspect of the hormonal control of root growth.

Structural Aspects of Multistep Phosphorelay- Mediated Signaling in Plants

The multistep phosphorelay （MSP） is a central signaling pathway in plants integrating a wide spectrum of hormonal and environmental inputs and controlling numerous developmental adaptations. For the thor- ough comprehension of the molecular mechanisms underlying the MSP-mediated signal recognition and transduction, the detailed structural characterization of individual members of the pathway is critical. In this review we describe and discuss the recently known crystal and nuclear magnetic resonance structures of proteins acting in MSP signaling in higher plants, focusing particularly on cytokinin and ethylene signaling in Arabidopsis thaliana. We discuss the range of functional aspects of available structural infor- mation including determination of ligand specificity, activation of the receptor via its autophosphorylaUon, and downstream signal transduction through the phosphorelay. We compare the plant structures with their bacterial counterparts and show that although the overall similarity is high, the differences in structural de- tails are frequent and functionally important. Finally, we discuss emerging knowledge on molecular recog- nition mechanisms in the MSP, and mention the latest findings regarding structural determinants of signaling specificity in the Arabidopsis MSP that could serve as a general model of this pathway in all higher plants.

Putative Nitrogen Sensing Systems in Higher Plants

Nitrogen （N） metabolism is essential for the biosynthesis of vital biomolecules. N status thus exerts profound effects on plant growth and development, and must be closely monitored. In bacteria and fungi, a few sophisticated N sensing systems have been extensively studied. In animals, the ability to receive amino acid signals has evolved to become an integral part of the nervous coordination system. In this review, we will summarize recent developments in the search for putative N sensing systems in higher plants based on homologous systems in bacteria, fungi, and animals. Apparently, although plants have separated and diversified from other organisms during the evolution process, striking similarities can be found in their N sensing systems compared with those of their counterparts; however, our understanding of these systems is still incomplete. Significant modifications of the N sensing systems （including cross-talk with other signal transduction pathways） in higher plants may be a strategy of adaptation to their unique mode of life.

Structure-Function Analysis of Arabidopsis thaliana Histidine Kinase AHK5 Bound to Its Cognate Phosphotransfer Protein AHP1

The multi-step phosphorelay （MSP） system defines a key signal transduction pathway in plants and many eukaryotes. In this system, external stimuli first lead to the activation of a histidine kinase, followed by transfer of a phosphoryl group from the receiver domain of the kinase （HKRD） to downstream, cytosolic phosphotransfer proteins （HPs）. In order to establish the determinants of specificity for this signaling relay system, we have solved the first crystal structure of a plant HKRD, AHK5RD, in complex with one of its cognate HPs, AHP1. AHP1 binds AHK5RD via a prominent hydrogen bond docking ridge and a hydrophobic patch. These features are conserved among all AHP proteins, but differ significantly from other structurally characterized prokaryotic and eukaryotic HPs. Surface plasmon resonance experiments show that AHK5RD binds to AHP1-3 with similar, micromolar affinity, consistent with the transient nature of this signaling complex. Our correlation of structural and functional data provide the first insight, at the atomic level as well as with quantitative affinity data, into the molecular recognition events governing the MSP in plants.

小肠结肠炎耶尔森菌对多粘菌素B的压力应答

【背景】小肠结肠炎耶尔森菌(Yersinia enterocolitica)是重要的人畜共患食源性病原菌。由于其生存环境与传染性生活方式,小肠结肠炎耶尔森菌暴露在各种生存压力中,而胞膜压力应答能力对维持其环境耐受性和毒力发挥着重要作用。【目的】探究小肠结肠炎耶尔森菌在胞膜压力应答中的调节机制。【方法】通过使用多粘菌素B破坏小肠结肠炎耶尔森菌细胞膜的稳定性,并从生长能力、运动能力、生物被膜形成能力以及相关基因表达的变化探讨Rcs (Regulator of Capsule Synthesis)系统对多粘菌素B产生的胞膜压力的应答。【结果】多粘菌素B引起的细胞胞膜压力抑制了小肠结肠炎耶尔森菌的运动和生物被膜形成能力;而阻断Rcs信号途径后,小肠结肠炎耶尔森菌的运动和生物被膜形成能力有所恢复。对flhC、hmsS、hmsT等关键下游表型基因的表达水平的分析结果表明Rcs双组分系统对由多粘菌素B诱导的胞膜压力作出响应,通过感知胞膜胁迫向胞内传递信号,积极地调控细菌增强对抗菌肽的抗性。【结论】明确了Rcs双组分系统在响应多粘菌素B压力胁迫中的特异性调控作用,加深了对小肠结肠炎耶尔森菌环境应答机制的认识。