- © 2005 American Society of Plant Biologists
Wall Shedding Turns On Genes
In the isogamous green alga Chlamydomonas reinhardtii, the initial contact between (+) and (−) gametes is mediated by mating type-specific agglutinins on the flagellar surface. The resulting flagellar adhesion turns on signaling pathways that prepare the way for gamete fusion. During this initial process, the activation of a flagellar adenylyl cyclase results in a nearly 10-fold increase of intracellular cAMP in the interacting gametes. This triggers dramatic alterations in the fusing cells, including the release of a cell wall-degrading protease and the erection of apically localized mating structures. In activated gametes, shedding of the cell wall exposes the naked protoplasts to osmotic shock. To further elucidate the molecular biology underlying this mating process, Hoffman and Beck (pp. 999–1014) have examined the regulation of three genes, GAS28, GAS30, and GAS31, that are expressed in the late phase of gametogenesis; all three encode for Hyp-rich glycoproteins that presumably serve as cell wall constituents. In addition, these genes are activated by zygote formation, cell wall removal, and osmotic stress. The induction by zygote formation could be traced to cell wall shedding prior to gamete fusion since it was seen in mutants defective in cell fusion. However, it was absent in mutants defective in the initial steps of mating, i.e. in flagellar agglutination and in accumulation of cAMP in response to this agglutination. Induction of the three GAS genes was also observed when cultures were exposed to hypo- or hyperosmotic stress. To address the question whether the induction seen upon cell wall removal from both gametes and vegetative cells was elicited by osmotic stress, cell wall removal was performed under isosmotic conditions. Since the activation of the GAS genes was still observed under such conditions, it appears that the signaling pathway(s) is (are) activated by wall removal itself.
Rhizoid Gravisensing under Conditions of Microgravity
The single-celled, tip-growing rhizoids of characean green algae show positive (downward-growing) gravitropism. Gravity susception is accomplished by the gravity-induced sedimentation of statoliths—small vacuoles containing BaSO4 crystals—onto the lower subapical cell flank. Actomyosin precisely controls statolith positioning and directs sedimenting statoliths to the confined gravisensitive region of the plasma membrane 10 to 30 μm behind the tip. For gravity perception to occur statoliths have to settle on this narrow belt-like membrane area and interact with membrane-bound receptors. When statoliths are displaced toward the cell flank without reaching the plasma membrane, positively gravitropic reorientation of the cell tip is not initiated. Limbach et al. (pp. 1030–1040) have examined the gravitropism of Chara globularis rhizoids under the microgravity conditions of sounding rocket (MAXUS) and parabolic plane flights in the European Space Agency's A3-Zero G Airbus (Fig. 1). Gravity perception was not interrupted by microgravity. Thus, even weightless statoliths were capable of activating the gravireceptor. Taking into account that increasing the weight of statoliths by centrifugation also did not affect gravitropic curvature, the idea that the gravireceptor in characean rhizoids is a mechanoreceptor activated by tension or pressure exerted by sedimented statoliths, can be excluded. Rather, it seems that it is the close contact of statoliths with the gravisensitive plasma membrane that is the determinant of graviperception. As such, the gravireceptor in characean rhizoids should be referred to as a contact receptor.
Gravisensing in Chara rhizoids was studied under the near-weightless conditions provided during parabolic flights of the European Space Agency's A300 Zero G Airbus.
H2O2 and Anthocyanin Production
Reactive oxygen species (ROS) play an important role in the defense of plants against biotic and abiotic stresses and in the induction of plant cell death. However, our understanding of the downstream responses of ROS in plants is far from complete. Following early reports of massive changes in plant gene expression following oxidative stress, it remains of interest to dissect, within these massive changes, functional entities that will shed more light on the physiological changes induced by oxidative stress. In plants, ROS and, more particularly, hydrogen peroxide (H2O2) play a dual role as toxic by-products of normal cell metabolism and as regulatory molecules in stress perception and signal transduction. Catalases are key players within the reactive oxygen-scavenging networks that modulate the steady-state levels of ROS in the different subcellular compartments of plants. In the peroxisomes, catalase is the principal scavenging enzyme, acting as an important sink for photorespiratory H2O2. Using microarrays, Vanderauwera et al. (pp. 806–821) have compared expression profiles between control and catalase-deficient Arabidopsis (Arabidopsis thaliana) plants exposed to high-light (HL) conditions. In total, 349 transcripts were significantly up-regulated by HL in catalase-deficient plants and 88 were down-regulated. From this data, they conclude that H2O2 plays a key role in the transcriptional up-regulation of small heat-shock proteins during HL stress. In addition, by combining a genetic (catalase deficiency) and an environmental (HL) perturbation, the authors were able to identify a transcriptional cluster out of the general light stress response that is involved in anthocyanin biosynthesis. This transcriptional cluster is strongly and rapidly induced by HL in control plants, but impaired in catalase-deficient plants. Control plants accumulated much more anthocyanins than catalase-deficient plants after exposure to HL. In contrast, catalase-deficient plants developed cell death within 8 h of HL, suggesting a protective role of “sun-blocking” anthocyanins against HL stress.
A Novel Root-Specific Auxin Antagonist
In their ongoing efforts to identify specific inhibitors of auxin signaling, Yamazoe et al. (pp. 779–789) have found a novel and specific auxin signaling inhibitor named terfestatin A (TrfA). This molecule, a terphenyl-β-glucoside, was isolated from a strain of Streptomyces in a forward screen for compounds that inhibit the expression of auxin-inducible genes in Arabidopsis. TrfA has the unusual property of antagonizing auxin-induced processes only in the roots. Indeed, TrfA, at concentrations in the tens of micromolar, potently antagonizes every known auxin response in Arabidopsis roots, including primary root inhibition, lateral root initiation, root hair promotion, and root gravitropism, but had only limited effects on shoot auxin responses. Previous studies have revealed that an important mechanism by which auxin induces specific gene expression is by promoting the degradation of short-lived nuclear proteins called Aux/IAA that repress auxin-responsive gene expression via the ubiquitin-proteasome pathway. During this process, the auxin receptor TIR1 interacts with Skp1 and Cullin proteins to form an E3 ubiquitin-ligase complex called SCFTIR1. SCFTIR1 assembly plays an essential role in the proteolytic pathway regulating auxin-dependent degradation of Aux/IAA repressors. The authors report that TrfA blocks the auxin-enhanced degradation of Aux/IAA repressor proteins without affecting the auxin-stimulated interaction between Aux/IAAs and TIR1. Thus, TrfA does not compete directly with auxin for binding to TIR1. Although the exact mechanism by which TrfA modulates Aux/IAA stability remains to be elucidated, TrfA should prove to be a most useful tool for studying auxin signaling.
A Gene Involved in Homologous Recombination
Agents such as ionizing radiation, DNA cross-linking reagents, and oxygen free radicals, produce double-strand DNA breaks (DSBs) in chromosomes. Homologous recombination (HR) is a major pathway by which DSBs are repaired. Rad5, a homolog of bacterial RecA recombinase, plays a key role in HR in eukaryotes by promoting homologous pairing and strand exchange reactions. Rad51 paralogs, which have been identified from yeast (Saccharomyces cerevisiae) to vertebrates, are thought to play an important role in the assembly or stabilization of Rad51. Previously, two RAD51 paralogous genes in Arabidopsis, named AtRAD51C and AtXRCC3, have been characterized: they are homologs of human RAD51C and XRCC3, respectively. To determine the role of RAD51C in meiotic and mitotic recombination in higher plants, Abe et al. (pp. 896–908) have characterized a T-DNA insertion mutant of AtRAD51C. Although the atrad51C mutant grew normally during vegetative developmental stage, the mutant produced aborted siliques and its anthers did not contain mature pollen grains. Crossing of the mutant with wild-type plant yielded defective male and female gametogeneses as evident by the lack of seed production. Furthermore, meiosis was severely disturbed in the mutant. The atrad51C mutant also showed increased sensitivity to γ-irradiation and cisplatin, agents known to induce DSBs. As expected, the efficiency of HR in somatic cells in the mutant was markedly reduced relative to that in wild-type plants.
Molecular Root Responses to Beneficial Soil Microbes
Several genera of soil bacteria, including Pseudomonas species, stimulate root proliferation or have antagonistic effects on pathogens in the rhizosphere. Unlike arbuscular mycorrhizal and nodule symbioses, however, few investigations have focused on the molecular bases of plant responses to beneficial rhizobacteria such as Pseudomonas. Sanchez et al. (pp. 1065–1077) have characterized plant genes induced during early root colonization of Medicago truncatula by a growth-promoting strain of Pseudomonas fluorescens by suppressive subtractive hybridization. Ten M. truncatula genes, coding proteins associated with a putative signal transduction pathway, showed an early and transient activation during initial interactions between wild-type M. truncatula and P. fluorescens. In P. fluorescens-inoculated roots of a Myc−Nod− genotype (TRV25) of M. truncatula, gene expression was not significantly enhanced, except for one gene encoding a Ca2+ and calmodulin-dependent protein kinase: This finding indicates a possible role of Ca2+ in the cellular interactions between M. truncatula and P. fluorescens. When expression of the 10 plant genes was compared in early stages of root colonization by mycorrhizal and rhizobial microsymbionts, Glomus mosseae activated all 10 genes whereas Sinorhizobium meliloti only activated one and inhibited four others. None of the genes responded to inoculation by either microsymbiont in roots of the TRV25 mutant. The similar response of the M. truncatula genes to P. fluorescens and G. mosseae points to common molecular pathways in the perception of the microbial signals by plant roots.
