Nitric Oxide as a signal molecule in intra- and extra-cellular bacteria-plant interactions

Abstract

All plants live in intimate association with many microorganisms. These microorganisms colonize the surfaces, the intercellular spaces within tissues or even the inside of plant cells (Brencic and Winans, 2005). Some of the most complex interactions that terrestrial plants experience occur between the roots and their surrounding environment (Bais et al., 2006). It is precisely in the soil where these interactions are subjected to a plethora of conditions that determine the success or failure of root colonization, leading to effects on plant developmental processes. Soil has been divided into three main zones according proximity to the root: (1) rhizoplane or the root surface, (2) rhizosphere or the soil under root “influence,” and (3) bulk soil (Manthey et al., 1994). Bacteria that inhabit soils can affect plant growth and development. The interactions they establish with roots vary from beneficial, deleterious, or neutral effects on plants (Hirsch et al., 2003). The distribution of microorganisms in the rhizosphere can be classified into four main categories related to root proximity and intimacy: (1) bacteria living in the soil near roots, using metabolites “leaked” from roots as carbon (C) and N sources, (2) bacteria colonizing the rhizoplane, (3) bacteria residing in root tissue, inhabiting spaces between cortical cells, and (4) bacteria living inside cells in specialized root structures or nodules (Gray and Smith, 2005). The latter are generally represented by two groups: the legume-rhizobia and the woody plant- Frankia associations (Hallman et al., 1997; Gray et al., 2005). A more simple and convenient classification based on the preferred colonizing site is proposed by Gray and Smith (2005) dividing plant growth-promoting rhizobacteria (PGPR) into extracellular PGPR (ePGPR), and intracellular PGPR (iPGPR), the latter existing inside root cells generally in specialized nodular structures. classification based on the preferred colonizing site is proposed by Gray and Smith (2005) dividing plant growth-promoting rhizobacteria (PGPR) into extracellular PGPR (ePGPR), and intracellular PGPR (iPGPR), the latter existing inside root cells generally in specialized nodular structures. associations (Hallman et al., 1997; Gray et al., 2005). A more simple and convenient classification based on the preferred colonizing site is proposed by Gray and Smith (2005) dividing plant growth-promoting rhizobacteria (PGPR) into extracellular PGPR (ePGPR), and intracellular PGPR (iPGPR), the latter existing inside root cells generally in specialized nodular structures.