Studies on the resistance mechanism of rice to BPH have been based on physical interactions, and more recently, on gene expression levels
[9, 11, 46–48]. There is increasing evidence that TF genes play important roles in plant defense responses against phloem-feeding insects. TF genes represent a key element in the modification of gene expression during the plant defense reaction
[8, 17, 18, 49].
Here, we provide an overview of the expression profile of TF genes related to BPH resistance, based on comparisons of susceptible and resistant rice plants. Using TF gene probes, we detected 2038 TF genes showing differential expressions between the two rice varieties RHT and TN1 (Additional file
2). Of these, 442 TF genes were probably related to BPH-induced resistance, because their expressions were increased or decreased in response to BPH infestation (Additional file
3). All these induced TF genes showed an interesting expression pattern at two time points (8 or 24 h) after BPH infestation (Figure
2B). After BPH feeding, the susceptible strain TN1 had more up- or down-regulated TF genes than did the resistant strain RHT. According to the different patterns of gene expression profiles, more TF genes were induced by BPH in the susceptible rice variety TN1 than in the resistant rice variety RHT. For instance, as shown in Figure
2B, TF genes specially up-regulated at 8, and 24 hours in TN1 were 105 and 154, respectively; while the numbers in RHT were 28 and 31. And TF genes specially down-regulated at 8, and 24 hours in TN1 were 61 and 65, respectively; while the numbers in RHT were 39 and 20. Because induced resistance is defined from the point of view of the herbivore and it does not necessarily benefit the plant. This means that the induced resistance may render the plant more susceptible to other stresses
. This may be the one important reason why TN1 rice is more susceptible than RHT. Further analysis results also support this point of view. In TN1, there were more up-regulated than down-regulated TF genes in response to BPH. In contrast, in RHT, there were more down-regulated than up-regulated genes in response to BPH (Figures
2A). This finding suggested that the damage caused by BPH feeding to the susceptible variety TN1 triggered expressions of a series of genes. The products of these genes may play roles in repairing damage to the phloem to prevent the loss of phloem sap, and in defense against invasion of the pathogen and bacteria. In the resistant variety RHT, this reaction involved cessation of several metabolic pathways to prevent the loss of phloem sap, and metabolic activity was repressed throughout the entire plant. This is an efficient method of defense against BPH feeding, because it reduces the amount of substances supplied to the phloem, similar to the function of the resistance gene Bph14. Therefore, various TFs could have different functions in this reaction. This finding provided evidence at the molecular level for why the resistant cultivar, RHT, suffered less damage than the susceptible cultivar, TN1.
Studies on the physical test phenotype by EPG (electrical penetration graph) of these cultivars showed that TN1 had a greater damage score after exposure to BPH, while the phloem sap of RHT could not be extracted constantly by BPH stylets
[1, 45]. According to this study, the expressions of many TF genes were induced in TN1 by exposure to BPH. This would probably lead to metabolic disorders in TN1, which probably the reason why this rice variety is susceptible to BPH. However, in RHT, far fewer TF genes were induced after BPH exposure. Most of the TF genes showing altered expressions in RHT were down-regulated (Figure
2B), so we boldly deduced that certain metabolic pathways were turned off. This may explain why BPH was unable to extract nutrition from the RHT phloem sap.
Among the down-regulated TF genes, all members of FHA, Orphans, and Sigma70-like families, and most members of the MYB family were significantly down-regulated at 8 h after BPH infestation (Additional file
5). A similar pattern of regulation also occurred in Arabidopsis after aphid infestation
. Members of the MYB family have highly diversified biological functions, and the expressions of most Arabidopsis MYB genes were responsive to one or multiple types of hormone and stress treatments
. Studies of the Arabidopsis defense response to chewing insects showed that knockdown of a MYB gene, AtMYB102, enhanced susceptibility to white cabbage butterfly (Pieris rapae)
. In tobacco, transgenic plants overexpressing a wheat MYB gene, TaPIMP1, showed significantly enhanced resistance to the pathogen Ralstonia solanacearum, and increased tolerance to drought and salt stresses
. Another study on TFs suggested that several members of the MYB family play important roles in photosynthesis and metabolism
. During their physiological responses to insects, plants reallocate energy from photosynthesis to the defense response
. During BPH feeding, the chlorophyll level, net photosynthetic rate, stomatal conductance, and transpiration rates significantly decreased in the susceptible cultivar, MH63
. These patterns of decreased metabolic activity were consistent with the down-regulation of MYB family genes observed in BPH-infested rice plants in this study. The down-regulation of these genes may have played a role in reducing photosynthesis. Microarray analyses have shown that down-regulation of TF genes occurs during compatible plant-aphid interactions
In both rice varieties used in the present study, the up-regulated TF genes after 8-h of BPH infestation included most members of the AP2-EREBP, NAC, and WRKY families and all members of the ABI3VP1 family (Additional file
5). The AP2 TF family is one of several that are unique to the plant lineage. This family, whose members contain an EREBP (ethylene responsive element binding factor) domain, is involved in regulation of plant disease resistance
. Genes encoding members of the AP2-EREBP family of TFs are also involved in plant resistance to insects. The AP2-EREBP TF response to herbivore attack might depend on a hormone-dependent signaling pathway. Over-expression of OsERF3 positively regulated TrypPI activity and boosted the resistance of rice to the chewing herbivore SSB, whereas it negatively regulated resistance against BPH
. The expression pattern of up-regulated AP2-EREBP genes may reflect those induced by physical damage during BPH feeding, and the down-regulated AP2 TFs may be involved in the BPH resistance interaction. Previous studies on NAC, WRKY, and Zn-finger TF families mainly focused on responses to fungal and bacterial pathogens
[23, 25, 33]. According to those studies, their functions rely on JA and SA signaling pathways, which are involved in the BPH-resistance response.
We narrowed down the number of TF genes that were probably involved in BPH resistance by increasing the FC value to >10 (P<0.05). At this FC level, there were 37 TF genes that were BPH-induced and 26 associated with constitutive resistance in RHT. Further analysis indicated that there were 13 and 8 TF genes most probably involved in BPH-induced and constitutive resistance, respectively. Further research is underway to study these 21 genes in more detail. Most of the microarray expression profiles were consistent with the qPCR results. (Figure
4 & Additional file
7). However, only a few time points were analyzed in the microarray analyses, and so it is difficult to determine the exact time at which gene expression peaked after induction by BPH. To address this point, more time points should be included in qPCR analyses of gene expression.