Cytological indication of the involvement of calcium and calcium-related proteins in the early responses of Bryonia dioica to mechanical stimulus

Influence of cell wall polymers and their modifying enzymes during plant-aphid interactions

Aphids are a major issue for commercial crops. These pests drain phloem nutrients and transmit ~50% of the known insect-borne viral diseases. During aphid feeding, trophic structures called stylets advance toward the phloem intercellularly, disrupting cell wall polymers. It is thought that cell wall-modifying enzymes (CWMEs) present in aphid saliva facilitate stylet penetration through this intercellular polymer network. Additionally, different studies have demonstrated that host settling preference, feeding behavior, and colony performance of aphids are influenced by modulating the CWME expression levels in host plants. CWMEs have been described as critical defensive elements for plants, but also as a key virulence factor for plant pathogens. However, whether CWMEs are elements of the plant defense mechanisms or the aphid infestation process remains unclear. Therefore, in order to better consider the function of CWMEs and cell wall-derived damage-associated molecular patterns (DAMPs) during plant-aphid interactions, the present review integrates different hypotheses, perspectives, and experimental evidence in the field of plant-aphid interactions and discusses similarities to other well-characterized models such as the fungi-plant pathosystems from the host and the attacker perspectives.

Comparison of intracellular location and stimulus reaction times of forisomes in sieve tubes of four legume species

Forisomes in legumes are responsible for fast sieve-element occlusion in response to injury to the vascular system. This prevents uncontrolled leakage of phloem sap and protects against invasion of pathogens. Here we compared forisomes of four different legumes (Pisum sativum, Vicia faba, Trifolium pratense and Medicago sativa) by their location (basal, central, apical) in the sieve element and reactivity to a distant heat stimulus. In each species, the majority of forisomes was located basally. Yet, we found differences in intracellular location: forisomes are distributed more evenly in the sieve elements of Pisum. After burning, basally located forisomes of the four species reacted with dispersion, followed by a spontaneous recondensation with similar reaction times. The results suggest universal forisome behaviour in fabacean species.

Phytoplasma-triggered Ca(2+) influx is involved in sieve-tube blockage

Phytoplasmas are obligate, phloem-restricted phytopathogens that are disseminated by phloem-sap-sucking insects. Phytoplasma infection severely impairs assimilate translocation in host plants and might be responsible for massive changes in phloem physiology. Methods to study phytoplasma- induced changes thus far provoked massive, native occlusion artifacts in sieve tubes. Hence, phytoplasma-phloem relationships were investigated here in intact Vicia faba host plants using a set of vital fluorescent probes and confocal laser-scanning microscopy. We focused on the effects of phytoplasma infection on phloem mass-flow performance and evaluated whether phytoplasmas induce sieve-plate occlusion. Apparently, phytoplasma infection brings about Ca(2+) influx into sieve tubes, leading to sieve-plate occlusion by callose deposition or protein plugging. In addition, Ca(2+) influx may confer cell wall thickening of conducting elements. In conclusion, phytoplasma effectors may cause gating of sieve-element Ca(2+) channels leading to sieve-tube occlusion with presumptive dramatic effects on phytoplasma spread and photoassimilate distribution.

Compatible and Incompatible Interactions in Wheat Involving the Bt-10 Gene for Resistance to Tilletia tritici, the Common Bunt Pathogen

ABSTRACT The infection of wheat lines Neepawa (susceptible), and its sib BW553 that is nearly isogenic for the Bt-10 resistance gene by differentially virulent races T1 and T27 of common bunt (Tilletia tritici), was followed for 21 days following seeding (dfs) using fluorescence and confocal microscopy. Spore germination was nonsynchronous and all spore stages including germination were observed 5 to 21 dfs. Initial host perception of pathogen invasion, based on autofluorescence in epidermal cells adjacent to the appressoria, was similar in both compatible and incompatible interactions, and occurred as early as 5 to 6 dfs. The total number of sites on a 1-cm segment of coleoptile adjacent to the seed that exhibited autofluorescence was similar in both the compatible and incompatible interactions and rose to a maximum of 35 to 40 per 1 cm length of coleoptile following 17 dfs, although new infection events were observed as late as 21 dfs. In the compatible interaction, the autofluorescence became more diffuse 10 to 12 dfs, emanating in all directions in association with fungal spread. In the incompatible interaction, autofluorescence remained restricted to a small area surrounding the penetration site. Two different reaction zones that extended further in tissues surrounding the penetration point in the incompatible interaction compared with the compatible interaction were identified. The accumulation of callose around invading fungal hyphae was observed during both the compatible and incompatible interactions from 8 to 21 dfs. While callose accumulation was more extensive and widespread in the incompatible interaction, it was clearly present in compatible interactions, particularly in treatments involving BW553. These results were confirmed by expression of callose synthase transcripts that were more abundant in BW553 than in Neepawa and were upregulated during pathogen infection in both compatible and incompatible interactions.

Annexins: putative linkers in dynamic membrane-cytoskeleton interactions in plant cells

The plasma membrane, the most external cellular structure, is at the forefront between the plant cell and its environment. Hence, it is naturally adapted to function in detection of external signals, their transduction throughout the cell, and finally, in cell reactions. Membrane lipids and the cytoskeleton, once regarded as simple and static structures, have recently been recognized as significant players in signal transduction. Proteins involved in signal detection and transduction are organised in specific domains at the plasma membrane. Their aggregation allows to bring together and orient the downstream and upstream members of signalling pathways. The cortical cytoskeleton provides a structural framework for rapid signal transduction from the cell periphery into the nucleus. It leads to intracellular reorganisation and wide-scale modulation of cellular metabolism which results in accumulation of newly synthesised proteins and/or secondary metabolites which, in turn, have to be distributed to the appropriate cell compartments. And again, in plant cells, the secretory vesicles that govern polar cellular transport are delivered to their target membranes by interaction with actin microfilaments. In search for factors that could govern subsequent steps of the cell response delineated above we focused on an evolutionary conserved protein family, the annexins, that bind in a calcium-dependent manner to membrane phospholipids. Annexins were proposed to regulate dynamic changes in membrane architecture and to organise the interface between secretory vesicles and the membrane. Certain proteins from this family were also identified as actin binding, making them ideal mediators in cell membrane and cytoskeleton interactions.

Calcium: silver bullet in signaling

Accumulating evidence suggests that Ca(2+) serves as a messenger in many normal growth and developmental process and in plant responses to biotic and abiotic stresses. Numerous signals have been shown to induce transient elevation of [Ca(2+)](cyt) in plants. Genetic, biochemical, molecular and cell biological approaches in recent years have resulted in significant progress in identifying several Ca(2+)-sensing proteins in plants and in understanding the function of some of these Ca(2+)-regulated proteins at the cellular and whole plant level. As more and more Ca(2+)-sensing proteins are identified it is becoming apparent that plants have several unique Ca(2+)-sensing proteins and that the downstream components of Ca(2+) signaling in plants have novel features and regulatory mechanisms. Although the mechanisms by which Ca(2+) regulates diverse biochemical and molecular processes and eventually physiological processes in response to diverse signals are beginning to be understood, recent studies have raised many interesting questions. Despite the fact that Ca(2+) sensing proteins are being identified at a rapid pace, progress on the function(s) of many of them is limited. Studies on plant 'signalome' - the identification of all signaling components in all messengers mediated transduction pathways, analysis of their function and regulation, and cross talk among these components - should help in understanding the inner workings of plant cell responses to diverse signals. New functional genomics approaches such as reverse genetics, microarray analyses coupled with in vivo protein-protein interaction studies and proteomics should not only permit functional analysis of various components in Ca(2+) signaling but also enable identification of a complex network of interactions.

Calcium signaling during abiotic stress in plants

Plants experience a wide array of environmental stimuli, not all of which are favorable, and, unlike animals, are unable to move away from stressful environments. They therefore require a mechanism with which to recognize and respond to abiotic stresses of many different types. Frequently this mechanism involves intracellular calcium. Stress-induced changes in the cytosolic concentration of Ca2+ ([Ca2+]cyt) occur as a result of influx of Ca2+ from outside the cell, or release of Ca2+ from intracellular stores. These alterations in [Ca2+]cyt constitute a signal that is transduced via calmodulin, calcium-dependent protein kinases, and other Ca(2+)-controlled proteins to effect a wide array of downstream responses involved in the protection of the plant and adjustment to the new environmental conditions. Ca2+ signaling has been implicated in plant responses to a number of abiotic stresses including low temperature, osmotic stress, heat, oxidative stress, anoxia, and mechanical perturbation, which are reviewed in this article.

Changes in plasma membrane fluidity of Bryonia dioica internodes during thigmomorphogenesis

Fluidity changes in plasma membrane (PM) lipid extracts or native membranes isolated from Bryonia dioica internodes after a mechanical stimulation were monitored by steady-state fluorescence polarization with 1,6-diphenyl-1,3,5-hexatriene as a probe. The signal was shown to rapidly induce an increase in the bulk lipid fluidity. This event was closely related to a relative enrichment in some phospholipid species (PC, PG and PS) as well as a significant increase in the unsaturation index of total fatty acyl chains. Free sterols and protein content did not appear to be involved into this process. After 48 h, lipids from rubbed internodes became less fluid than PM lipids from control internodes.