A nuclear-targeted cameleon demonstrates intranuclear Ca2+ spiking in Medicago truncatula root hairs in response to rhizobial nodulation factors

Arbuscular mycorrhizal hyphopodia and germinated spore exudates trigger Ca2+ spiking in the legume and nonlegume root epidermis

• The aim of this study was to investigate Ca(2+) responses to endosymbiotic arbuscular mycorrhizal (AM) fungi in the host root epidermis following pre-infection hyphopodium formation in both legumes and nonlegumes, and to determine to what extent these responses could be mimicked by germinated fungal spore exudate. • Root organ cultures of both Medicago truncatula and Daucus carota, expressing the nuclear-localized cameleon reporter NupYC2.1, were used to monitor AM-elicited Ca(2+) responses in host root tissues. • Ca(2+) spiking was observed in cells contacted by AM hyphopodia for both hosts, with highest frequencies correlating with the epidermal nucleus positioned facing the fungal contact site. Treatment with AM spore exudate also elicited Ca(2+) spiking within the AM-responsive zone of the root and, in both cases, spiking was dependent on the M. truncatula common SYM genes DMI1/2, but not on the rhizobial Nod factor perception gene NFP. • These findings support the conclusion that AM fungal root penetration is preceded by a SYM pathway-dependent oscillatory Ca(2+) response, whose evolutionary origin predates the divergence between asterid and rosid clades. Our results further show that fungal symbiotic signals are already generated during spore germination, and that cameleon-expressing root organ cultures represent a novel AM-specific bio-assay for such signals.

In vivo imaging of Ca2+, pH, and reactive oxygen species using fluorescent probes in plants

Changes in the levels of Ca(2+), pH, and reactive oxygen species (ROS) are recognized as key cellular regulators involved in diverse physiological and developmental processes in plants. Critical to understanding how they exert such widespread control is an appreciation of their spatial and temporal dynamics at levels from organ to organelle and from seconds to many hours. With appropriate controls, fluorescent sensors can provide a robust approach with which to quantify such changes in Ca(2+), pH, and ROS in real time, in vivo. The fluorescent cellular probes available for visualization split into two broad classes: (a) dyes and (b) an increasingly diverse set of genetically encoded sensors based around green fluorescent proteins (GFPs). The GFP probes in particular can be targeted to well-defined subcellular locales, offering the possibility of high-resolution mapping of these signals within the cell.

Biosensors in plants

Biosensors come in an increasing array of forms and their development is defining the rate of advance for our understanding of many natural processes. Developmental biology is increasingly using mathematical models and yet few of these models are based on quantitative recordings. In particular, we know comparatively little about the endogenous concentrations or fluxes of signalling molecules such as the phytohormones, an area of great potential for new biosensors. There are extremely useful biosensors for some signals, but most remain qualitative. Other qualities sought in biosensors are temporal and spatial resolution and, usually, an ability to use them without significantly perturbing the system. Currently, the biosensors with the best properties are the genetically encoded optical biosensors based on FRET, but each sensor needs extensive specific effort to develop. Sensor technologies using antibodies as the recognition domain are more generic, but these tend to be more invasive and there are few examples of their use in plant biology. By capturing some of the opportunities appearing with advances in platform technologies it is hoped that more biosensors will become available to plant scientists.

Flavonoid-induced calcium signalling in Rhizobium leguminosarum bv. viciae

• Legume-rhizobium symbiosis requires a complex dialogue based on the exchange of diffusible signals between the partners. Compatible rhizobia express key nodulation (nod) genes in response to plant signals - flavonoids - before infection. Host plants sense counterpart rhizobial signalling molecules - Nod factors - through transient changes in intracellular free-calcium. Here we investigate the potential involvement of Ca(2+) in the symbiotic signalling pathway activated by flavonoids in Rhizobium leguminosarum bv. viciae. • By using aequorin-expressing rhizobial strains, we monitored intracellular Ca(2+) dynamics and the Ca(2+) dependence of nod gene transcriptional activation. • Flavonoid inducers triggered, in R. leguminosarum, transient increases in the concentration of intracellular Ca(2+) that were essential for the induction of nod genes. Signalling molecules not specifically related to rhizobia, such as strigolactones, were not perceived by rhizobia through Ca(2+) variations. A Rhizobium strain cured of the symbiotic plasmid responded to inducers with an unchanged Ca(2+) signature, showing that the transcriptional regulator NodD is not directly involved in this stage of flavonoid perception and plays its role downstream of the Ca(2+) signalling event. • These findings demonstrate a key role played by Ca(2+) in sensing and transducing plant-specific flavonoid signals in rhizobia and open up a new perspective in the flavonoid-NodD paradigm of nod gene regulation.

Root hair systems biology

Plant functional genomic studies have largely measured the response of whole plants, organs and tissues, resulting in the dilution of the signal from individual cells. Methods are needed where the full repertoire of functional genomic tools can be applied to a single plant cell. Root hair cells are an attractive model to study the biology of a single, differentiated cell type because of their ease of isolation, polar growth, and role in water and nutrient uptake, as well as being the site of infection by nitrogen-fixing bacteria. This review highlights the recent advances in our understanding of plant root hair biology and examines whether the root hair has potential as a model for plant cell systems biology.

Ion channels at the nucleus: electrophysiology meets the genome

The nuclear envelope is increasingly viewed from an electrophysiological perspective by researchers interested in signal transduction pathways that influence gene transcription and other processes in the nucleus. Here, we describe evidence for ion channels and transporters in the nuclear membranes and for possible ion gating by the nuclear pores. We argue that a systems-level understanding of cellular regulation is likely to require the assimilation of nuclear electrophysiology into molecular and biochemical signaling pathways.

Cross-talk between ROS and calcium in regulation of nuclear activities

Calcium and Reactive Oxygen Species (ROS) are acknowledged as crucial second messengers involved in the response to various biotic and abiotic stresses. However, it is still not clear how these two compounds can play a role in different signaling pathways leading the plant to a variety of processes such as root development or defense against pathogens. Recently, it has been shown that the concept of calcium and ROS signatures, initially discovered in the cytoplasm, can also be extended to the nucleus of plant cells. In addition, it has been clearly proved that both ROS and calcium signals are intimately interconnected. How this cross-talk can finally modulate the translocation and/or the activity of nuclear proteins leading to the control of specific genes expression is the main focus of this review. We will especially focus on how calcium and ROS interact at the molecular level to modify their targets.

NENA, a Lotus japonicus homolog of Sec13, is required for rhizodermal infection by arbuscular mycorrhiza fungi and rhizobia but dispensable for cortical endosymbiotic development

Legumes form symbioses with arbuscular mycorrhiza (AM) fungi and nitrogen fixing root nodule bacteria. Intracellular root infection by either endosymbiont is controlled by the activation of the calcium and calmodulin-dependent kinase (CCaMK), a central regulatory component of the plant's common symbiosis signaling network. We performed a microscopy screen for Lotus japonicus mutants defective in AM development and isolated a mutant, nena, that aborted fungal infection in the rhizodermis. NENA encodes a WD40 repeat protein related to the nucleoporins Sec13 and Seh1. Localization of NENA to the nuclear rim and yeast two-hybrid experiments indicated a role for NENA in a conserved subcomplex of the nuclear pore scaffold. Although nena mutants were able to form pink nodules in symbiosis with Mesorhizobium loti, root hair infection was not observed. Moreover, Nod factor induction of the symbiotic genes NIN, SbtM4, and SbtS, as well as perinuclear calcium spiking, were impaired. Detailed phenotypic analyses of nena mutants revealed a rhizobial infection mode that overcame the lack of rhizodermal responsiveness and carried the hallmarks of crack entry, including a requirement for ethylene. CCaMK-dependent processes were only abolished in the rhizodermis but not in the cortex of nena mutants. These data support the concept of tissue-specific components for the activation of CCaMK.

The molecular network governing nodule organogenesis and infection in the model legume Lotus japonicus

Bacterial infection of interior tissues of legume root nodules is controlled at the epidermal cell layer and is closely coordinated with progressing organ development. Using spontaneous nodulating Lotus japonicus plant mutants to uncouple nodule organogenesis from infection, we have determined the role of 16 genes in these two developmental processes. We show that host-encoded mechanisms control three alternative entry processes operating in the epidermis, the root cortex and at the single cell level. Single cell infection did not involve the formation of trans-cellular infection threads and was independent of host Nod-factor receptors and bacterial Nod-factor signals. In contrast, Nod-factor perception was required for epidermal root hair infection threads, whereas primary signal transduction genes preceding the secondary Ca2+ oscillations have an indirect role. We provide support for the origin of rhizobial infection through direct intercellular epidermal invasion and subsequent evolution of crack entry and root hair invasions observed in most extant legumes.

Nuclear calcium controls the apoptotic-like cell death induced by d-erythro-sphinganine in tobacco cells

Studies performed in animals have highlighted the major role of sphingolipids in regulating the balance between cell proliferation and cell death. Sphingolipids have also been shown to induce cell death in plants via calcium-based signalling pathways but the contribution of free cytosolic and/or nuclear calcium in the overall process has never been evaluated. Here, we show that increase in tobacco BY-2 cells of the endogenous content of Long Chain Bases (LCBs) caused by external application of d-erythro-sphinganine (DHS) is followed by immediate dose-dependent elevations of cellular free calcium concentration within the first minute in the cytosol and 10min later in the nucleus. Cells challenged with DHS enter a death process through apoptotic-like mechanisms. Lanthanum chloride, a general blocker of calcium entry, suppresses the cellular calcium variations and the PCD induced by DHS. Interestingly, dl-2-amino-5-phosphopentanoic acid (AP5) and [(+)-dizocilpine] (MK801), two inhibitors of animal and plant ionotropic glutamate receptors, suppress DHS-induced cell death symptoms by selectively inhibiting the variations of nuclear calcium concentration. The selective action of these compounds demonstrates the crucial role of nuclear calcium signature in controlling DHS-induced cell death in tobacco cells.