Integrating important environmental signals with intrinsic developmental programmes is a crucial adaptive requirement for plant growth, survival, and reproduction. Key environmental cues include changes in several light variables, while important intrinsic (and highly interactive) regulators of many developmental processes include the phytohormones cytokinins (CKs) and ethylene. Here, we discuss the latest discoveries regarding the molecular mechanisms mediating CK/ethylene crosstalk at diverse levels of biosynthetic and metabolic pathways and their complex interactions with light. Furthermore, we summarize evidence indicating that multiple hormonal and light signals are integrated in the multistep phosphorelay (MSP) pathway, a backbone signalling pathway in plants. Inter alia, there are strong overlaps in subcellular localizations and functional similarities in components of these pathways, including receptors and various downstream agents. We highlight recent research demonstrating the importance of CK/ethylene/light crosstalk in selected aspects of plant development, particularly seed germination and early seedling development. The findings clearly demonstrate the crucial integration of plant responses to phytohormones and adaptive responses to environmental cues. Finally, we tentatively identify key future challenges to refine our understanding of the molecular mechanisms mediating crosstalk between light and hormonal signals, and their integration during plant life cycles.

Cytokinins and ethylene control plant development via sensors from the histidine kinase (HK) family. However, downstream signaling pathways for the key phytohormones are distinct. Here we report that not only cytokinin but also ethylene is able to control root apical meristem (RAM) size through activation of the multistep phosphorelay (MSP) pathway. We found that both cytokinin and ethylene-dependent RAM shortening requires ethylene binding to ETR1 and the HK activity of ETR1. The receiver domain of ETR1 interacts with MSP signaling intermediates acting downstream of cytokinin receptors, further substantiating the role of ETR1 in MSP signaling. We revealed that both cytokinin and ethylene induce the MSP in similar and distinct cell types with ETR1-mediated ethylene signaling controlling MSP output specifically in the root transition zone. We identified members of the MSP pathway specific and common to both hormones and showed that ETR1-regulated ARR3 controls RAM size. ETR1-mediated MSP spatially differs from canonical CTR1/EIN2/ EIN3 ethylene signaling and is independent of EIN2, indicating that both pathways can be spatially and functionally separated. Furthermore, we demonstrated that canonical ethylene signaling controls MSP responsiveness to cytokinin specifically in the root transition zone, presumably via regulation of ARR10, one of the positive regulators of MSP signaling in Arabidopsis.

Cryptogein is a proteinaceous elicitor secreted by Phytophthora cryptogea that can induce resistance to P. parasitica in tobacco plants. On the basis of previous computer modelling experiments, by site-directed mutagenesis a series of cryptogein variants was prepared with altered abilities to bind sterols, phospholipids or both. The sterol binding and phospholipid transfer activities corresponded well with the previously reported structural data. Induction of the synthesis of reactive oxygen species (ROS) in tobacco cells in suspension and proteomic analysis of intercellular fluid changes in tobacco leaves triggered by these mutant proteins were not proportional to their ability to bind or transfer sterols and phospholipids. However, changes in the intercellular proteome corresponded to transcription levels of defence genes and resistance to P. parasitica and structure-prediction of mutants did not reveal any significant changes in protein structure. These results suggest, contrary to previous proposals, that the sterol-binding ability of cryptogein and its mutants, and the associated conformational change in the ω-loop, might not be principal factors in either ROS production or resistance induction. Nevertheless, the results support the importance of the ω-loop for the interaction of the protein with the high affinity binding site on the plasma membrane.

Republic, CZ-165 02 Prague, Czech Republic (B.P., J.M.)The plant hormones cytokinins (CKs) regulate multiple developmental and physiological processes in Arabidopsis (Arabidopsis thaliana). Responses to CKs vary in different organs and tissues (e.g. the response to CKs has been shown to be opposite in shoot and root samples). However, the tissue-specific targets of CKs and the mechanisms underlying such specificity remain largely unclear. Here, we show that the Arabidopsis proteome responds with strong tissue and time specificity to the aromatic CK 6-benzylaminopurine (BAP) and that fast posttranscriptional and/or posttranslational regulation of protein abundance is involved in the contrasting shoot and root proteome responses to BAP. We demonstrate that BAP predominantly regulates proteins involved in carbohydrate and energy metabolism in the shoot as well as protein synthesis and destination in the root. Furthermore, we found that BAP treatment affects endogenous hormonal homeostasis, again with strong tissue specificity. In the shoot, BAP up-regulates the abundance of proteins involved in abscisic acid (ABA) biosynthesis and the ABA response, whereas in the root, BAP rapidly and strongly up-regulates the majority of proteins in the ethylene biosynthetic pathway. This was further corroborated by direct measurements of hormone metabolites, showing that BAP increases ABA levels in the shoot and 1-aminocyclopropane-1-carboxylic acid, the rate-limiting precursor of ethylene biosynthesis, in the root. In support of the physiological importance of these findings, we identified the role of proteins mediating BAP-induced ethylene production, METHIONINE SYNTHASE1 and ACC OXIDASE2, in the early root growth response to BAP.

Plant hormones are key regulators of a number of developmental and adaptive responses in plants, integrating the control of intrinsic developmental regulatory circuits with environmental inputs. Here we provide an overview of the molecular mechanisms underlying hormonal regulation of root development. We focus on key events during both embryonic and postembryonic development including specification of the hypophysis as a future organizer of the root apical meristem (RAM), hypophysis asymmetric division, specification of the quiescent centre (QC) and the stem cell niche (SCN), RAM maturation and maintenance of QC/SCN activity and RAM size. We address both well-established as well as newly-proposed concepts, highlight potential ambiguities in recent terminology and classification criteria of longitudinal root zonation and point to contrasting results and alternative scenarios for recent models. In the concluding remarks, we summarise the common principles of hormonal control during root development, the mechanisms potentially explaining often antagonistic outputs of hormone action and propose possible future research directions on hormones in the root.

Plants, like other sessile organisms, need to sense many different signals, and in response to them, modify their developmental programs to be able to survive in a highly changing environment. The multistep phosphorelay (MSP) in plants is a good candidate for a response mechanism that integrates multiple signal types both environmental and intrinsic in origin. Recently, ethylene was shown to control MSP activity via the histidine kinase (HK) activity of ETHYLENE RESPONSE 1 (ETR1)1,2, but the underlying molecular mechanism still remains unclear. Here we show that although ETR1 is an active HK, its receiver domain (ETR1RD) is structurally and functionally unable to accept the phosphate from the phosphorylated His in the ETR1 HK domain (ETR1HK) to initiate the phosphorelay to ARABIDOPSIS HISTIDINE-CONTAINING PHOSPHOTRANSMITTERs (AHPs), the next link downstream members in MSP signaling. Instead, ETR1 interacts with another HK ARABIDOPSIS HISTIDINE KINASE 5 (AHK5) and transfers the phosphate from ETR1HK through the receiver domain of AHK5 (AHK5RD), and subsequently to AHP1, AHP2 and AHP3, independently of the HK activity of AHK5. We show that AHK5 is necessary for ethylene-initiated, but not cytokinin-initiated, MSP signaling in planta and that it thus mediates hormonal control of root growth.

The two principal growth regulators cytokinins and ethylene are known to interact in the regulation of plant growth. However, information about underlying molecular mechanism and positional specificity of the cytokinin/ethylene crosstalk in root growth control is scarce. We have identified spatial specificity of cytokinin-regulated root elongation and root apical meristem (RAM) size, both of which we demonstrate to be ethylene biosynthesis-dependent. Upregulation of the cytokinin biosynthetic gene ISOPENTENYLTRANSFERASE (IPT) in proximal and peripheral tissues leads to both root and RAM shortening. In contrast, IPT activation in distal and inner tissues reduces RAM size while leaving the root length comparable to mock-treated controls. We show that cytokinins regulate two steps specific to ethylene biosynthesis, the production of 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 the ethylene-regulated root and RAM shortening in cytokinin-treated Arabidopsis. Finally, we describe the tight cooperation between cytokinin and ethylene signaling in cytokinin-induced, ethylene-regulated control of ACO4 due to the 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.