s over lapped in two of the six studies varied dramatically. Comparative studies reveal commonly regulated and stage specifically regulated genes by HLB Despite our finding that only a small proportion of Pro besets are significantly regulated in any of two studies, we reasoned that those Probesets commonly regulated in all of the studies may rep resent either a common core pathway or default pathway in response to the Las infection. We first found a total of 13 Probesets that are commonly up regulated in all of the six studies, representing only 0. 4% of the HLB up regulated genes. However, the number of Probesets significantly regulated in any of five studies increased to 42.

It is possible that in the ab sence of the HLB bacterial challenge some of the HLB up regulated genes already had higher transcript levels in the relatively Cilengitide resistant germplasm US 897 compared to the relatively susceptible mandarin Cleopatra and thus they were not up regulated any more in US 897 in re sponse to the Las infection, however, they could be sig nificantly regulated in all other four studies. We did identify a total of eight Probesets for this type of expres sion pattern and consequently they were also added to the list of the HLB commonly regulated genes. Surprisingly, there was no Probeset commonly down regulated in all of the six studies and only one Probeset that is significantly down regulated in five stud ies. This Probeset, Cit. 18719. 1. S1 at, is annotated to en code a gene similar to Arabidopsis AT5G18600 encoded glutaredoxin family protein involved in cell redox homeostasis.

Gene Ontology analysis of the subset of 21 commonly up regulated Probesets indicates that metabolism, transport, hor mone responses and unknown processes are the largest groups. The three Probe sets representing the genes involved in hormone re sponse indicate that gibberelic acid, abscisic acid, auxin, ethylene and jasmonic acid may have certain role in mediating the citrus response to HLB. Interestingly, three Probesets belonging to the category of unknown process might also be involved in ethylene response as they exhibit the high est homology to genes that are associated with ethylene response using the manual BLAST search. In addition, there is a transcription factor gene represented by the Probeset, Cit. 12214. 1. S1 s at and another putative RAP2.

4 like ethylene transcription factor repre sented by Cit. 3534. 1. S1 s at. Taken together, the exist ence of these commonly up regulated genes strongly indicates that metabolism, transport, hormone response and transcriptional regulation play a critical role and may define the default or basal pathways in citrus dur ing the whole process of the Las infection. In contrast to the commonly regulated genes in HLB response, we found various numbers of stage specifically regulated genes as this group of genes were only regu lated at a particular stage. There are 27 and 7 Probesets that are respectively up regulated and down regulated o