Chen Changbin, Dickman Martin B
Department of Plant Pathology, University of Nebraska-Lincoln, Nebraska, NE 68583, USA.
Mol Microbiol. 2005 Jan;55(1):299-311. doi: 10.1111/j.1365-2958.2004.04390.x.
Sclerotinia sclerotiorum is a filamentous ascomycete phytopathogen able to infect an extremely wide range of cultivated plants. Our previous studies have shown that increases in cAMP levels result in the impairment of the development of the sclerotium, a highly differentiated structure important in the disease cycle of this fungus. cAMP also inhibits the activation of a S. sclerotiorum mitogen-activated protein kinase (MAPK), which we have previously shown to be required for sclerotial maturation; thus cAMP-mediated sclerotial inhibition is modulated through MAPK. However, the mechanism(s) by which cAMP inhibits MAPK remains unclear. Here we demonstrate that a protein kinase A (PKA)-independent signalling pathway probably mediates MAPK inhibition by cAMP. Expression of a dominant negative form of Ras, an upstream activator of the MAPK pathway, also inhibited sclerotial development and MAPK activation, suggesting that a conserved Ras/MAPK pathway is required for sclerotial development. Evidence from bacterial toxins that specifically inhibit the activity of small GTPases, suggested that Rap-1 or Ras is involved in cAMP action. The Rap-1 inhibitor, GGTI-298, restored MAPK activation in the presence of cAMP, further suggesting that Rap-1 is responsible for cAMP-dependent MAPK inhibition. Importantly, inhibition of Rap-1 is able to restore sclerotial development blocked by cAMP. Our results suggest a novel mechanism involving the requirement of Ras/MAPK pathway for sclerotial development that is negatively regulated by a PKA-independent cAMP signalling pathway. Cross-talk between these two pathways is mediated by Rap-1.
核盘菌是一种丝状子囊菌植物病原体，能够感染极其广泛的栽培植物。我们之前的研究表明，环磷酸腺苷（cAMP）水平的升高会导致菌核发育受损，菌核是这种真菌病害循环中一种高度分化的重要结构。cAMP还会抑制核盘菌丝裂原活化蛋白激酶（MAPK）的激活，我们之前已证明该激酶是菌核成熟所必需的；因此，cAMP介导的菌核抑制是通过MAPK进行调节的。然而，cAMP抑制MAPK的机制仍不清楚。在这里，我们证明了一条不依赖蛋白激酶A（PKA）的信号通路可能介导了cAMP对MAPK的抑制作用。MAPK途径的上游激活剂Ras的显性负性形式的表达也抑制了菌核发育和MAPK激活，这表明保守的Ras/MAPK途径是菌核发育所必需的。来自特异性抑制小GTP酶活性的细菌毒素的证据表明，Rap-1或Ras参与了cAMP的作用。Rap-1抑制剂GGTI-298在有cAMP存在的情况下恢复了MAPK的激活，进一步表明Rap-1负责cAMP依赖性的MAPK抑制。重要的是，抑制Rap-1能够恢复被cAMP阻断的菌核发育。我们的结果提示了一种新机制，即菌核发育需要Ras/MAPK途径，而该途径受到一条不依赖PKA的cAMP信号通路的负调控。这两条途径之间的相互作用是由Rap-1介导的。
