This centrality-driven division between frontal and temporal semantic processing regions can be seen in the BOLD signal, with left IFG and left anterior MTG initially responding generally to the switch from non-word to word stimulus, before demonstrating clear correlation with central and peripheral HRF prediction peaks, respectively . While both temporal and frontal regions are implicated in semantic cognition, it has been suggested that left posterior MTG acts as a general interface between lexical and conceptual knowledge, anterior MTG is involved in specific semantic associations, while left IFG is more context specific, activating for conceptual knowledge that is cued by the preceding text . Consequently, for central textual ideas, which are more semantically-dependent on previous ideas, the IFG is increasingly involved in making appropriate semantic connections to the established context. On the other hand, processing peripheral ideas, or ideas which have looser semantic connections to the preceding text, would rely more heavily on regions that support general semantic knowledge to contextualize the present text. This suggests that within the fron to-temporal semantic control network, there is a functional divide between frontal and temporal contributions related to perception of textual centrality. Decreased activation over time for both central and peripheral ideas was similar to the patterns of temporal activations associated with passages—as language regions increased over time, activation of the visuospatial attention system decreased. This pattern is also apparent in the BOLD signal,chicken fodder system and appears to be anti-correlated with both central and peripheral phrases .
However, the extent and strength of the right IPS cluster in central ideas was significantly greater than peripheral. This difference can be explained by right IPS involvement in situation model construction—because central ideas contribute more to the situation model, they would consequently be more sensitive to the decreasing need of construction regions .Our temporal analyses assumed a linear relationship between time and neural activation of text processing; however, nonlinear temporal relationships may exist, and future studies should explore such non-linear changes. A second limitation is that our models assume that neural activation builds not only as the reader progresses through the paragraphs, but also during the baseline condition between the two paragraphs. Future work should compare whether removing this baseline assessment changes the patterns of temporal activation change. One methodological consideration is that our participants were skilled adult readers, and our passages were written at a fourth grade reading level. Future studies should manipulate the reading level of the passages and examine how this manipulation influences the neural correlates of expository comprehension, particularly regions associated with EF. Future studies should also consider the important interaction between text and reader by considering the background knowledge that the readers hold about each passage topic. Background knowledge plays an important role in building a coherent representation of the text and allows the reader to form a more meaningful representation that goes beyond the text-based ideas . A reader’s existing knowledge base is especially important to consider with respect to expository texts because they often use topic-relevant vocabulary that builds upon the reader’s assumed knowledge base.
Finally, future work should examine the neural correlates of building a coherent text representation among groups of readers known to be less sensitive to structural centrality, such as individuals with reading disability, individuals with ADHD, and foreign language learners. Comparing the patterns of activation associated with skilled and less skilled comprehension could help identify the comprehension processes that are disrupted and the underlying source of their comprehension difficulties. This insight could perhaps be employed to inform and improve reading comprehension instruction and interventions.Plasma membrane–localized receptors are critical components of the innate immune responses of animals and plants . Many of these receptors recognize and respond to the presence of conserved microbial molecules and are often referred to as pattern recognition receptors . In animals, this recognition is carried out, in part, by Toll-like receptors . Humans have 10 characterized TLRs that recognize conserved microbial molecules such as lipopolysaccharide or flagellin. In plants, cell surface receptor kinases and receptor-like proteins recognize microbial molecules in the apoplast . Well-characterized leucine-rich repeat –RKs include Arabidopsis FLS2 that detects flagellin, or its peptide epitope flg22, and the elongation factor Tu receptor that detects the bacterial elongation factor Tu, or its peptide epitope elf18 . Lacking an adaptive immune system, plants have an extended array of innate immune receptors encoded in their genome. Rice, for example, has more than 300 RKs predicted to serve as innate immune receptors based on the presence of a “non-RD” kinase domain, which lack the arginine-aspartate motif characteristic of most kinases . Of the few non-RD RKs characterized to date, all have a role in innate immunity or symbiosis . The rice XA21 RK, one of the first innate immune receptors isolated, mediates recognition of the Gram-negative bacterium Xanthomonas oryzae pv. oryzae , the causal agent of an agronomically important disease of rice .
Previous efforts to identify the microbial molecule that activates the XA21-mediated immune response led to the identification of a number of Xoo genes that are required for activation of XA21 . These genes encode a tyrosine sulfotransferase, RaxST, and three components of a predicted type 1 secretion system : a membrane fusion protein, RaxA; an adenosine triphosphate –binding cassette transporter, RaxB; and an outer membrane protein, RaxC. raxST, raxA, and raxB are located in a single operon . On the basis of these findings, we hypothesized that the activator of XA21-mediated immunity is a tyrosine-sulfated, type 1–secreted protein . However, the identity of this molecule has remained elusive . Here, we report the identification of the tyrosinesulfated protein RaxX as the activator of XA21-mediated immunity.In addition to raxST, we have previously identified two other rax genes involved in microbial sulfation. These genes, raxP and raxQ, encode an ATP sulfurylase and an adenosine 5′-phosphosulfate kinase, and work in concert to produce the universal sulfuryl group donor 3′-phosphoadenosine 5′-phosphosulfate . The requirement of these three genes for activation of XA21-mediated immunity by Xoo suggests that tyrosine sulfation plays a key functional role in this process. To further investigate this possibility, we transformed a raxST mutant strain , which forms long lesions on XA21-TP309, with a plasmid expressing raxST under control of its native promoter . PXO99DraxSTSp regained the ability to activate XA21-mediated immunity . RaxST carries a predicted PAPS binding motif conserved in mammalian sulfotransferases including the human tyrosine sulfotransferases TPST1 and TPST2 . In TPST2, mutation of the conserved arginine in the PAPS binding motif impairs enzymatic activity . We generated a similar mutation in raxST and tested if the expression of this mutant variant on a plasmid could complement the PXO99DraxSTSp infection phenotype on XA21-expressing rice plants. The strain PXO99DraxSTSp failed to activate XA21-mediated immunity , indicating that the sulfotransferase activity of RaxST is critical for its function. On the basis of the genetic association of raxX with the raxSTAB operon, the importance of tyrosine sulfation for activation of the XA21- mediated immune response, and the presence of a single tyrosine residue in PXO99 RaxX that is conserved among all available RaxX sequences , we hypothesized that RaxX Y41 is sulfated by RaxST and that this sulfation is required for RaxX function. To test this hypothesis, we transformed PXO99DraxX with a plasmid carrying a derivative of RaxX with tyrosine 41 mutated to phenylalanine [PXO99DraxX]. PXO99DraxX failed to activate XA21-mediated immunity in XA21-TP309 . We also demonstrated that sulfated RaxX peptides,fodder systems for cattle but not peptides carrying an Y41 to F substitution, are immunogenic on XA21-expressing rice plants . These results support the hypothesis that sulfation of RaxX Y41 is required for its activation of XA21-mediated immunity. To determine whether RaxX Y41 is sulfated by RaxST in vitro, we incubated a chemically synthesized peptide covering the C-terminal region of RaxX with purified His-RaxST in the presence of PAPS. Trypsin-digested peptides were analyzed by liquid chromatography–tandem mass spectrometry in both negative and positive nanoelectrospray modes with ultraviolet photodissociation to generate informative a, b, c, x, y, and z product ions from cleavage of the peptide backbone.
This method has previously been shown to facilitate the characterization of sulfated tyrosine residues within peptides by MS/MS.In negative ion mode, the sulfate group is retained on all product ions, thus allowing the sulfate modification to be unequivocally localized to Y41 of RaxX. MS/MS data showed fragment ions that account for 93% sequence coverage of peptide HVGGGDsYPPPGANPK . The high-resolution verification of the peptide mass in the negative mode MS1 is displayed in fig. S7. The extracted ion chromatograms of the peptides of interest and positive mode UVPD mass spectrum are shown in figs. S8 and S9, respectively. We next tested if RaxX is sulfated in vivo. Using selected reaction monitoring-MS , we observed sulfation on tryptic peptides covering Y41 derived from RaxX-His purified from PXO99 . The sulfated version of the tryptic peptide covering Y41 of RaxX-His purified from PXO99DraxST was not detectable with multiple SRM transitions above the background noise level . In contrast, we did detect the corresponding non-sulfated peptide covering Y41 at high levels and with high confidence for RaxX-His isolated from both PXO99 and PXO99DraxST . In combination, these results demonstrate that RaxST sulfates RaxX on Y41 in vivo and that sulfation of RaxX is required for its immunogenic activity on XA21-expressing rice plants.Infection assays using bacterial mutants clearly demonstrate that RaxX is required for activation of XA21-mediated immunity. We next sought to determine whether sulfated RaxX can trigger XA21-mediated defense responses in the absence of Xoo. For this purpose, we produced full length sulfated recombinant RaxX using an expanded genetic code approach . We heterologously expressed RaxX in E. coli together with an engineered aminoacyltRNA synthetase specific for sulfotyrosine, a cognate engineered amber suppressor tRNA, and the nonstandard amino acid sulfotyrosine . Nonsulfated RaxX was also expressed in E. coli without sulfotyrosine. We confirmed purity and tyrosine sulfation status of RaxX60 by gel-based assays and SRM-MS analysis . We tested if the resulting highly purified, full-length, sulfated RaxX60-sY and nonsulfated RaxX60-Y proteins could trigger defense gene expression in leaves of rice plants over expressing XA21 . As shown in fig. S13, sulfated RaxX60-sY, but not the nonsulfated form RaxX60-Y, triggered strong up-regulation of defense marker genes in detached leaves of Ubi::XA21. Leaves from Kitaake rice plants, which lack the XA21 immune receptor, are insensitive to RaxX60-Y and RaxX60-sY. These results demonstrate that sulfated RaxX60-sY is sufficient to activate XA21-mediated defense gene expression in rice. To identify a “minimal” epitope of RaxX that is sufficient to trigger these responses, we took biochemical and rational design approaches. We first tested whether chemically synthesized RaxX39 is sufficient to trigger XA21-dependent defense gene expression . We found that sulfated RaxX39-sY triggers defense gene expression in an XA21-dependent manner, whereas non-sulfated RaxX39 does not . To further narrow down the active region, we subjected RaxX39 to digestion with four site-specific proteases . The predicted digestion patterns were confirmed by gel-based assays and by SRM-MS analysis for ArgC, AspN, and trypsin digests . We tested the resulting RaxX fragments for their ability to activate XA21-dependent signaling . Only RaxX39-sY digestion products resulting from GluC and ArgC treatments retained activity on Ubi::XA21 plants. The ability of the ArgC fragment to activate XA21-mediated immunity was confirmed with chemically synthesized RaxX24-sY peptides . Next, we tested N- and C-terminaltruncated versions of RaxX24-sY peptides. Chemically synthesized RaxX21-sY retained the ability to induce XA21-dependent signaling, whereas RaxX18-sY was compromised in this activity . These results indicate that a chemically synthesized tyrosine-sulfated 21–amino acid derivative of RaxX, named RaxX21-sY, is sufficient to activate XA21- mediated defense responses. In addition to activation of defense marker gene expression, the activation of PRR-triggered immunity in plants often involves the production of ethylene and reactive oxygen species . These responses are known to contribute to the final disease outcome in many plant pathogen interactions . We therefore tested if RaxX21-sY can trigger these hallmarks of plant innate immune signaling in XA21- expressing rice leaves . As shown in Fig. 3 , only sulfated RaxX21-sY, but not RaxX21-Y, was able to activate defense marker gene expression and the production of ROS and ethylene in an XA21-dependent manner. These responses were most pronounced in rice plants overexpressing XA21 .