The Effector HopQ1 Interacts with 14-3-3 Proteins
In order to cause disease and suppress host defense responses, gram-negative bacterial pathogens deliver effector proteins into host cells via the type III secretion system. Plant pathogenic bacteria typically deliver a large number (20–40) of effector types into host cells during infection. Progress in understanding individual effectors’ contributions to virulence has been made by generating transgenic plants that express effectors. HopQ1 (for Hrp outer protein Q), an effector from a virulent Pseudomonas syringae strain that causes bacterial speck on tomato (Solanum lycopersicum) and Arabidopsis (Arabidopsis thaliana) is the subject of two contributions in this issue. Li et al. (pp. 2062–2074) investigated the virulence function and host targets of HopQ1 in tomato. They demonstrate that HopQ1 enhances bacterial virulence and associates with tomato 14-3-3 proteins in a phosphorylation-dependent manner. 14-3-3 proteins are a family of conserved regulatory molecules that have the ability to bind a multitude of functionally diverse signaling proteins, including kinases, phosphatases, and transmembrane receptors. (The name “14-3-3” refers to the particular elution and migration pattern of these proteins on DEAE-cellulose chromatography and starch-gel electrophoresis.) The results of this study indicate that phosphorylation and subsequent interaction with tomato 14-3-3 proteins affects HopQ1’s virulence-promoting activities and subcellular localization. In an equally meritorious study, Giska et al. (pp. 2049–2061) draw similar conclusions based on their studies of another virulent P. syringae strain that infects tobacco (Nicotiana benthamiana).
Genomics of Combined Stresses
Most research concerning plant stress has typically focused on one type of stress in isolation even though plants are often exposed simultaneously to multiple biotic or abiotic stresses. Complex and often interconnected signaling pathways coordinate the responses of plants to different stresses. Whole genome expression profiling with microarrays is a useful tool to monitor changes in transcript levels, and thereby gene expression, in response to stresses and other factors. Many previous studies have examined the effects of a single stress upon plant genome expression, but Rasmussen et al. (pp. 1783–1794) have conducted a large-scale microarray experiment to analyze plant responses to multiple, concurrent stresses and to identify the level and functions of stress regulatory networks. To this end, they subjected 10 ecotypes of Arabidopsis to five individual stress treatments (cold, heat, high light, salt, and flagellin) and six combinations of these stress treatments under the same growth and experimental conditions. Although it might seem reasonable to speculate that the responses of plants to two stresses might be duplicative or perhaps additive, this is not the case. This analysis shows that when two stresses are combined, an average of 61% of the transcripts respond in modes that cannot be predicted from individual single stress treatments. In addition, only a minor fraction (6%) of the transcripts exhibit antagonistic responses to stress combinations under which the plants apparently must prioritize between the responses. Given the novelty of the responses uncovered, the authors explored the modular organization of transcription networks and identified stress-responsive modules and potentially key regulatory genes that may shed further light on plant responses to multiple stresses.
Autophagy as an Energy Source at Night
Under stressful conditions, plants can sometimes suffer energy deficits, particularly at night. Nocturnal energy deficits can also occur in certain mutants. For example, wild-type Arabidopsis plants accumulate starch during the day and degrade it for respiration at night. However, nocturnal energy availability is perturbed in starchless mutants, in which a lack of starch accumulation causes a transient sugar deficit at night. When energy deficits occur, a plant must produce alternative respiratory substrates as an energy source for survival and growth. Autophagy, an intracellular process leading to the vacuolar degradation of cytoplasmic components, is considered to play an important role in nutrient recycling under starvation conditions. Previous studies have shown that autophagy-deficient (atg) mutants show reduced growth under short-day conditions. This growth inhibition is largely relieved under continuous light or under short-day conditions combined with feeding of exogenous Suc, suggesting that autophagy is involved in energy production at night for growth. Izumi et al. (pp. 1682–1693) generated starchless and atg double mutants and grew them under different photoperiods. The double mutants showed more severe phenotypes than did atg or starchless single mutants. Transcript analyses of dark-inducible genes revealed that the sugar starvation symptoms observed in starchless mutants became more severe in starchless atg double mutants. The contents of free amino acids increased and the transcript levels of several genes involved in amino acid catabolism were elevated in starchless mutant leaves. Thus, it appears that autophagy can contribute to energy availability at night by providing a supply of alternative energy sources such as amino acids.
Redox Control of Chloroplast DNA Replication
Because chloroplasts possess their own genome, chloroplast DNA must be duplicated so that each daughter chloroplast inherits the requisite DNA after division. However, it is unclear how the replication of chloroplast DNA is regulated and whether the replication is coupled with the timing of chloroplast division. Most green algae have one or a few chloroplasts that divide once per cell cycle before the cell completes cytokinesis. In contrast, land plants and certain algal species contain dozens of chloroplasts per cell that divide nonsynchronously, even within the same cell. Because land plants evolved from algae, there is likely to have been a linkage between the cell cycle and chloroplast division in their algal ancestor that was subsequently lost during land plant evolution. A recent study showed that the timing of chloroplast division in algae is restricted temporally to the S-phase when the chloroplast division machinery forms. To examine how chloroplast DNA replication is regulated in the mixotrophic green alga Chlamydomonas reinhardtii, Kabeya and Miyagishima (pp. 2102–2112) examined whether DNA replication is regulated by the cell cycle, as is the case for chloroplast division. They addressed this issue using synchronous light-grown cultures as well as heterotrophic cultures. Their results indicate that chloroplast DNA replication occurs independently of either the cell cycle or the timing of chloroplast division. Instead, they found that chloroplast DNA replication occurs when light is available in photoautotrophic cultures. However, DNA replication can occur in darkness in heterotrophic cultures. When dimethylthiourea, a reactive oxygen species scavenger was added to the photoautotrophic culture, chloroplast DNA was replicated even in the dark. In contrast, when methylviologen, a reactive oxygen species inducer, was added, chloroplast DNA was not replicated in the light. Moreover, the chloroplast DNA replication activity in both the isolated chloroplasts and nucleoids was increased by dithiothreitol, while it was repressed by diamide, a specific thiol-oxidizing reagent. These results suggest that chloroplast DNA replication is regulated by the redox state in the cell, which is sensed by the chloroplast nucleoids.
Evolution of Strigolactone Signaling
Strigolactones (SLs) are carotenoid-derived phytohormones with diverse roles. They are secreted from plant roots and serve as attractants for arbuscular mycorrhizal fungi. They also have a wide range of endogenous functions, including the regulation of root and shoot system architecture. In support of an ancient origin for SL secretion, the liverwort Marchantia polymorpha and the moss Physcomitrella patens, have been shown to produce SLs. Furthermore, the presence of SLs in charophyte algae indicates that SL production may predate the emergence of land plants, and indeed Chara corallina responds to SL treatment by producing longer rhizoids. To date, six genes associated with SL synthesis and signaling have been identified using shoot branching mutants of Arabidopsis (more axillary growth [max] mutants) and rice (Oryza sativa; dwarf [d] mutants). MAX3/D17 and MAX4/D10 encode carotenoid cleavage dioxygenases (CCD7 and CCD8, respectively). These enzymes are capable of sequentially cleaving the carotenoid 9-cis-β-carotene to produce a novel compound, carlactone, which is a putative SL intermediate. Challis et al. (pp. 1885–1902) present a phylogenetic analysis of the MAX/D genes. The CCD enzymes, MAX3 and MAX4, are present in all land plant groups and are members of gene families represented across the major domains of life. A crucial innovation in SL signaling appears to have been the recruitment of an F-box protein, MAX2, to the pathway. This protein is the common downstream effector of at least two SL-like signals in Arabidopsis, namely SLs and karrikins. MAX1, which encodes a cytochrome p450, may have played a significant role in upstream SL signal diversity and its later refinement. Thus, the angiosperm pathway seems to have been defined by the rapid evolution of MAX2 in early land plants.
Endogenous Target Mimics for MicroRNAs in Plants
MicroRNAs (miRNAs) are endogenous small RNAs that have essential roles in plant development and physiology. In plants, miRNAs are short, single-stranded RNAs of about 21 nucleotides in length, which function mainly as posttranscriptional regulators. They work by targeting mRNAs via sequence complementarity, and usually result in the degradation of the mRNA targets. Target mimicry is a recently identified regulatory mechanism in plants in which decoy RNAs bind to miRNAs via complementary sequences, and block the interaction between miRNAs and their authentic targets. Both endogenous decoy RNAs (miRNA target mimics) and engineered artificial RNAs can induce target mimicry effects. To date, only the Induced by Phosphate Starvation1 RNA has been shown to function as an endogenous miRNA target mimic (eTM). Wu et al. (pp. 1875–1884) have developed a computational method and systematically identified intergenic or noncoding gene-originated eTMs for 20 conserved miRNAs in Arabidopsis and rice. The predicted miRNA binding sites were well conserved among eTMs of the same miRNA, whereas sequences outside of the binding sites showed considerable variation. Transgenic plants overexpressing a rice eTM for miR160-3 had severe developmental defects, with dwarf size and serrated leaves (as well as accelerated flowering time). These phenotypes resembled those of plants overexpressing artificial miR160 target mimics. Transgenic plants overexpressing an eTM for miR166 had spoon-shaped cotyledons and abnormal rosette leaf shapes. The effectiveness of eTMs for three other miRNAs was also confirmed by transient agroinfiltration assay. These results demonstrate that the eTMs of several miRNAs can effectively inhibit the functions of their corresponding miRNAs.
Glossary
- SL
strigolactones
- miRNA
microRNA
- eTM
endogenous miRNA target mimic