Controlling hormone signaling is a plant and pathogen challenge for growth and survival
Introduction
Plants have evolved an immune system to recognize and respond to pathogen attack. In recent years there has been much progress in understanding this plant defense strategy. Numerous components involved in pathogen perception, signal generation and transmission, and activation of defense products restricting pathogen invasion, have been identified. Compelling evidence demonstrates that hormones such as salicylic acid (SA), jasmonates (JAs) and ethylene (ET) are the primary signals inducing defense responses through recognized defense hormone signaling pathways. Significant insight has also been gained into the machinery used by pathogens to invade plant tissues, including their strategies to interfere with the activation of plant defense.
Appropriate re-allocation of limiting resources to energy-costly induced defense responses, as opposed to demands for growth and reproduction, is critical to triggering immunity in plants. Emerging data is starting to unravel the mechanisms used by plants to maintain an equilibrium between these two processes, which is fundamental for their survival and effective competition with other plant species. Recent studies demonstrate that, in addition to known defense pathways (signaled by SA, JAs and ET), oxylipins other than JA, and hormones such as brassinosteroids (BL), auxins, gibberellins (GA), cytokinins (CK), and abscisic acid (ABA), play roles in plant responses to pathogen assault. Upon microbial attack, plants modify the relative abundance of these hormones, and the expression of their responsive genes, as an instrument to activate an efficient defense response allowing plant survival. Importantly, pathogens can counteract this strategy by interfering with these plant hormonal changes and also by producing plant hormones themselves as a component of their invading strategy [1•]. Here, we outline the latest discoveries showing the involvement of multiple hormone signaling pathways in plant defense, and report the most striking illustrations of the interaction between distinct pathways and how pathogens interfere with these signaling processes to colonize plants.
Section snippets
Activation of immunity and interactions between defense signaling pathways
Our understanding of the plant immune system has increased enormously in the last decade. A major achievement has been the demonstration that plants use two generalized modes of recognition to sense a harmful microbe. First, perception of conserved microbial molecules (generally designated as microbe associated molecular patterns or MAMPs) by pattern recognition receptors (PRRs) which are generally localized at the cell surface [2, 3, 4]. This initial recognition induces basal resistance or
Plant and pathogen oxylipins modulate defense and susceptibility
JA is an essential hormone for the regulation of defense and developmental responses. It belongs to a family of active molecules, the oxylipins, that originate in the oxidation of various fatty acids usually by the activities of lipoxygenases (9-lipoxygenases and 13-lipoxygenases) and α-dioxygenases. Production of oxylipins and phytoprostanes, a group of non-enzymatically formed oxylipins [20, 21], is a universal response of plants to pathogen attack. Given the importance of the
Changes in growth promoting hormones in plants during pathogen infection
Important growth and developmental processes are executed through signaling pathways governed by hormones such as BL, auxins, CK and GA. In addition, there is increasing evidence that regulation of these signaling pathways helps determine the outcome of a plant–pathogen interaction.
BLs are essential hormones for plant growth and development. Genetic and molecular analyses have defined key components of the BL signaling pathway, including a cell surface leucine repeat-like kinase receptor BRI1,
Modulation of ABA content and signaling during plant–pathogen interactions
A number of recent publications have described altered ABA levels during the interaction of plants with invading pathogens. Distinct actions of this hormone depend upon the infection stage and the specific host pathogen interaction. At a pre-invasion phase, plants enhanced resistance to application of P. syringae pv. tomato DC3000 by inducing stomatal closure and restricting pathogen entry [32•]. That the ABA-deficient mutant aba3-1 was more susceptible to P. syringae pv. tomato DC3000 when it
Conclusions and future prospects
Recent studies of plant–pathogen interactions have provided compelling evidence to support the importance of plant hormone signaling in determining the outcome of the interaction. By modulating hormone signaling pathways plants contribute not only directly to defense but also appear to control vital processes for resistance such as distribution of resources between defense and requirements for growth, cell death, water, and plant architecture. Pathogens on the other hand, have evolved invading
References and recommended reading
Papers of particular interest, published within the period of the review, have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgements
We thank J Paz-Ares and R Solano for constructive comments and stimulating discussions. This work was supported by Grant BIO2006-08581 from the Ministry of Education and Science to C.C. MAL was supported by fellowship from the Ministry of Education and Science, GB is a Ramón y Cajal fellow supported by the Spanish Ministry of Education and Science, Consejo Superior de Investigaciones Científicas, Madrid, Spain.
References (58)
- et al.
Pathological hormone imbalances
Curr Opin Plant Biol
(2007) - et al.
Perception of the bacterial PAMP EF-Tu by the receptor EFR restricts Agrobacterium-mediated transformation
Cell
(2006) - et al.
CERK1, a LysM receptor kinase, is essential for chitin elicitor signaling in Arabidopsis
Proc Natl Acad Sci U S A
(2007) - et al.
The plant immune system
Nature
(2006) Contrasting mechanisms of defense against biotrophic and necrotrophic pathogens
Annu Rev Phytopathol
(2005)- et al.
Integrated regulation of the type III secretion system and other virulence determinants in Ralstonia solanacearum
PLoS Pathog
(2006) - et al.
Reactive electrophile species
Curr Opin Plant Biol
(2007) - et al.
Oxylipins produced by the 9-lipoxygenase pathway in Arabidopsis regulate lateral root development and defense responses through a specific signaling cascade
Plant Cell
(2007) - et al.
Isolation of a sexual sporulation hormone from Aspergillus nidulans
J Bacteriol
(1989) - et al.
A flagellin-induced complex of the receptor FLS2 and BAK1 initiates plant defence
Nature
(2007)