Brassinosteroids act as a positive regulator for resistance against root-knot nematode involving RESPIRATORY BURST OXIDASE HOMOLOG-dependent activation of MAPKs in tomato

Interplay of hormones with reactive oxygen species (ROS) fine-tunes the response of plants to stress; however, the crosstalk between brassinosteroids (BRs) and ROS in nematode resistance is unclear. In this study, we found that low BR biosynthesis or lack of BR receptor increased, whilst exogenous BR decreased the susceptibility of tomato plants to Meloidogyne incognita. Hormone quantification coupled with hormone mutant complementation experiments revealed that BR did not induce the defence response by triggering salicylic acid (SA), jasmonic acid/ethylene (JA/ET) or abscisic acid (ABA) signalling pathway. Notably, roots of BR-deficient plants had decreased apoplastic ROS accumulation, transcript of RESPIRATORY BURST OXIDASE HOMOLOG1 (RBOH1) and WHITEFLY INDUCED1 (WFI1), and reduced activation of mitogen-activated protein kinase 1/2 (MPK1/2) and MPK3. Silencing of RBOH1, WFI1, MPK1, MPK2 and MPK3 all increased the root susceptibility to nematode and attenuated BR-induced resistance against the nematode. Significantly, suppressed transcript of RBOH1 compromised BR-induced activation of MPK1/2 and MPK3. These results strongly suggest that RBOH-dependent MPK activation is involved in the BR-induced systemic resistance against the nematode.