A salivary endo-β-1, 4-glucanase acts as an effector that enables the brown planthopper to feed on rice

The rice brown planthopper (BPH) Nilaparvata lugens (Stål) is one of the most 77 destructive insect pests on rice in Asia. After landing on plants, BPH rapidly accesses 78 plant phloem and sucks the phloem sap through unknown mechanisms. We discovered a 79 salivary endo-β-1,4-glucanase (NlEG1) which has endoglucanase activity with a 80 maximal activity at pH 6 at 37˚C and is secreted into rice plants by BPH. NlEG1 is 81 highly expressed in the salivary glands and midgut. Silencing NlEG1 decreases the 82 capacity of BPH to reach the phloem and reduces its food intake, mass, survival, and 83 fecundity on rice plants. By contrast, NlEG1 silencing had only a small effect on the 84 survival rate of BPH raised on artificial diet. Moreover, NlEG1 secreted by BPH did not 85 elicit the production of defense-related signal molecules -- salicylic acid, jasmonic acid 86 (JA), and jasmonoyl-isoleucine (JA-Ile) -- in rice, although wounding plus the 87 application of the recombination protein NlEG1 did slightly enhance the levels of JA 88 and JA-Ile in plants compared to in the corresponding controls. The data suggest that 89 NlEG1 enables the BPH’s stylet to reach the phloem by degrading celluloses in plant 90 cell walls (PCWs), thereby functioning as an effector that overcomes the PCW defense 91 in rice.