False pop out.

A single, unique target often pops out quickly and efficiently from a field of homogenous distractors in visual search. Pop out has helped shape theories of visual attention and feature integration as well as to identify basic features in human vision. Here we report a new phenomenon, false pop out, wherein one of the homogenous distractors competes with the singleton target to pop out, perhaps by breaking an overall grouping or pattern emerging from the display. We show the effect occurs with more than 1 type of stimulus, and we discuss the implications of such a counterintuitive finding for theories of visual search.

Distinct and conserved transcriptomic changes during nematode-induced giant cell development in tomato compared with Arabidopsis: a functional role for gene repression.

Root-knot nematodes (RKNs) induce giant cells (GCs) from root vascular cells inside the galls. Accompanying molecular changes as a function of infection time and across different species, and their functional impact, are still poorly understood. Thus, the transcriptomes of tomato galls and laser capture microdissected (LCM) GCs over the course of parasitism were compared with those of Arabidopsis, and functional analysis of a repressed gene was performed. Microarray hybridization with RNA from galls and LCM GCs, infection-reproduction tests and quantitative reverse transcription-polymerase chain reaction (qRT-PCR) transcriptional profiles in susceptible and resistant (Mi-1) lines were performed in tomato. Tomato GC-induced genes include some possibly contributing to the epigenetic control of GC identity. GC-repressed genes are conserved between tomato and Arabidopsis, notably those involved in lignin deposition. However, genes related to the regulation of gene expression diverge, suggesting that diverse transcriptional regulators mediate common responses leading to GC formation in different plant species. TPX1, a cell wall peroxidase specifically involved in lignification, was strongly repressed in GCs/galls, but induced in a nearly isogenic Mi-1 resistant line on nematode infection. TPX1 overexpression in susceptible plants hindered nematode reproduction and GC expansion. Time-course and cross-species comparisons of gall and GC transcriptomes provide novel insights pointing to the relevance of gene repression during RKN establishment.

Grouping and emergent features in vision: toward a theory of basic Gestalts.

Gestalt phenomena are often so powerful that mere demonstrations can confirm their existence, but Gestalts have proven hard to define and measure. Here we outline a theory of basic Gestalts (TBG) that defines Gestalts as emergent features (EFs). The logic relies on discovering wholes that are more discriminable than are the parts from which they are built. These wholes contain EFs that can act as basic features in human vision. As context is added to a visual stimulus, a hierarchy of EFs appears. Starting with a single dot and adding a second yields the first two potential EFs: the proximity (distance) and orientation (angle) between the two dots. A third dot introduces two more potential EFs: symmetry and linearity; a fourth dot produces surroundedness. This hierarchy may extend to collinearity, parallelism, closure, and more. We use the magnitude of Configural Superiority Effects to measure the salience of EFs on a common scale, potentially letting us compare the strengths of various grouping principles. TBG appears promising, with our initial experiments establishing and quantifying at least three basic EFs in human vision.

Activation of geminivirus V-sense promoters in roots is restricted to nematode feeding sites.

Obligate sedentary endoparasitic nematodes, such as the root-knot and cyst nematodes, elicit the differentiation of specialized nematode nurse or feeding cells [nematode feeding sites (NFS), giant cells and syncytia, respectively]. During NFS differentiation, marked changes in cell cycle progression occur, partly similar to those induced by some geminiviruses. In this work, we describe the activation of V-sense promoters from the Maize streak virus (MSV) and Wheat dwarf virus (WDV) in NFS formed by root-knot and cyst nematodes. Both promoters were transiently active in microinjection experiments. In tobacco and Arabidopsis transgenic lines carrying promoter-beta-glucuronidase fusions, the MSV V-sense promoter was activated in the vascular tissues of aerial plant parts, primarily leaf and cotyledon phloem tissue and some floral structures. Interestingly, in roots, promoter activation was restricted to syncytia and giant cells tested with four different nematode populations, but undetectable in the rest of the root system. As the activity of the promoter in transgenic rootstocks should be restricted to NFS only, the MSV promoter may have utility in engineering grafted crops for nematode control. Therefore, this study represents a step in the provision of some of the much needed additional data on promoters with restricted activation in NFS useful in biotechnological nematode control strategies.

Isolation of RNA from laser-capture-microdissected giant cells at early differentiation stages suitable for differential transcriptome analysis.

Plant organ gene expression profile analyses are complicated by the various cell types, and therefore transcription patterns, present in each organ. For example, each gall formed in roots following root knot nematode infection contains between four and eight specialized feeding cells (giant cells, GCs) embedded within hypertrophied root tissues. A recent goal in plant science has been the isolation of nematode feeding cell mRNAs for subsequent gene expression analysis. By adapting current protocols for different plant species and cells, we have developed a simple and rapid method for obtaining GCs from frozen tissue sections of tomato with good morphology and preserved RNA. The tissue sections obtained were suitable for the laser capture microdissection of GCs 6-7 days post-infection, and even of very early developing GCs (48-72 h post-infection), by fixation of tissue with ethanol-acetic acid, infiltration with sucrose and freezing in isopentane with optimal cutting temperature medium. This process was also successful for obtaining control vascular cells from uninfected roots for direct comparison with GCs. A minimum of about 300 GCs and 600 control vascular cells was required for efficient linear RNA amplification through in vitro transcription. Laser capture microdissection-derived RNA, after two rounds of amplification, was successfully used for microarray hybridization and validated with several differentially expressed genes by quantitative polymerase chain reaction. Consistent with our results, 117 homologous genes were found to be co-regulated in a previous microarray analysis of Arabidopsis galls at the same developmental stage. Therefore, we conclude that our method allows the isolation of a sufficient quantity of RNA with a high quality/integrity, appropriate for differential transcriptome analysis.

Induction of the Hahsp17.7G4 promoter by root-knot nematodes: involvement of heat-shock elements in promoter activity in giant cells.

Root-knot nematodes feed from specialized giant cells induced in the plants that they parasitize. We found that the promoter of the Hahsp17.7G4 gene, which encodes a small heat-shock protein involved in embryogenesis and stress responses, directed GUS expression in tobacco galls induced by the root-knot nematode Meloidogyne incognita. In roots containing a GUS reporter fusion to the Hahsp17.7G4 promoter, 10% of the galls stained for GUS expression 1 to 3 days after infection and the fraction stained increased to 60 to 80% 17 to 20 days after infection. A DNA fragment from -83 to +163, which contains heat-shock element (HSE) core sequences, is sufficient to support a promoter activity largely restricted to giant cells within the galls. Two-point mutations in HSE cores, previously reported to abolish the heat-shock response and to strongly reduce the embryogenesis response of the same promoter, did not reduce expression in giant cells. This suggests a distinct regulation of the promoter by nematodes. However, additional point mutations located at positions crucial for binding of heat-shock transcription factors (HSFs) caused a severe decrease in the nematode response. These results demonstrate that HSEs are involved in the promoter activation in giant cells and suggest that HSFs may mediate this response.