ИСТИНА

Pectin methylesterase (PME) is a ubiquitous cell wall (CW) enzyme that plays an important role in plant morphogenesis and immunity (1). Synthesis of PME is one of the aspects of plant growth that leads to the demethylesterification of pectins, a process that accompanies methanol (MeOH) generation as part of the maturation of the plant cell. Here, we provide experimental evidence that the sink-source transition and leaf maturation are accompanied by changes in the accumulation of PME mRNA and MeOH content in tobacco plants. On the other hand, mechanical damage promotes the activation of PME, which leads to the emission of MeOH by the damaged plant. Methanol, in its turn, influences on intact leaves of the same plant and on neighbouring plants. We previously showed that an effective innate immune strategy used by plants against pathogen attack involves cell-to-cell communication, synthesis of PME and emission of MeOH. MeOH release leads to the accumulation of mRNAs from MeOH-induced genes (MIGs), including non-cell-autonomous pathway protein (NCAPP) encoding mRNA (2). PME mRNA synthesis is highly regulated during plant growth and in response to stress. We suggest that there is a mechanism(s) that returns the system to the state prior to pathogen attack. Here, we provide experimental evidence that NCAPP regulates PME synthesis. Using a transient system of expression as well as stably transformed plants, we obtained experimental evidence in favour of the existence of a mechanism by which increased NCAPP synthesis suppresses the formation of enzymatically active PME and hence suppresses the emission of MeOH. In Nicotiana benthamiana leaves agroinjected with binary vectors encoding NCAPP, deletion mutants of NCAPP and the proPME leader sequence fused with GFP, NCAPP acted as a competitive inhibitor of proPME intracellular trafficking. We also demonstrated that the formation of mature PME in the CW is suppressed by NCAPP. Conversely, transgenic N. benthamiana plants in which the NCAPP gene had been silenced manifested diminished NCAPP mRNA stability, high levels of PME mRNA accumulation, increased PME protein content of the CW and elevated MeOH emission. We hypothesise that NCAPP is a protein factor that regulates PME synthesis. According to our model, plant leaf damage and pathogen attack lead to the following cyclic events: (a) increased PME gene expression results in the release of gaseous MeOH; (b) emission of MeOH by the damaged plant causes induction of several MeOH-induced genes, including NCAPP, in the intact leaves of the infected plant and in the leaves of neighbouring plants; and (c) up-regulation of NCAPP leads to antibacterial resistance accompanied by increased plasmodesmatal gating and increased spreading of plant viruses. The cycle ends when increased NCAPP synthesis suppresses the formation of enzymatically active PME, suppressing the emission of MeOH and returning the system to its original state.