Jasmonate signaling: toward an integrated view

Linking development to defense: auxin in plant-pathogen interactions

Although the plant growth hormone auxin has long been recognized as a regulator of plant defense, the molecular mechanisms involved are still largely unknown. Recent studies reviewed here reveal new insights into the role of auxin in plant defense. Similar to the signaling pathways of the defense-associated plant hormones salicylic acid (SA) and jasmonic acid (JA), auxin signaling differentially affects resistance to separate pathogen groups. Recent evidence suggests that the auxin and SA pathways act in a mutually antagonistic manner during plant defense, whereas auxin and JA signaling share many commonalities. Auxin also affects disease outcomes indirectly through effects on development. Here, we discuss the multiple ways in which auxin regulation of plant growth and development might be intimately linked to plant defense.

Fusarium oxysporum hijacks COI1-mediated jasmonate signaling to promote disease development in Arabidopsis

Although defense responses mediated by the plant oxylipin jasmonic acid (JA) are often necessary for resistance against pathogens with necrotrophic lifestyles, in this report we demonstrate that jasmonate signaling mediated through COI1 in Arabidopsis thaliana is responsible for susceptibility to wilt disease caused by the root-infecting fungal pathogen Fusarium oxysporum. Despite compromised JA-dependent defense responses, the JA perception mutant coronatine insensitive 1 (coi1), but not JA biosynthesis mutants, exhibited a high level of resistance to wilt disease caused by F. oxysporum. This response was independent from salicylic acid-dependent defenses, as coi1/NahG plants showed similar disease resistance to coi1 plants. Inoculation of reciprocal grafts made between coi1 and wild-type plants revealed that coi1-mediated resistance occurred primarily through the coi1 rootstock tissues. Furthermore, microscopy and quantification of fungal DNA during infection indicated that coi1-mediated resistance was not associated with reduced fungal penetration and colonization until a late stage of infection, when leaf necrosis was highly developed in wild-type plants. In contrast to wild-type leaves, coi1 leaves showed no necrosis following the application of F. oxysporum culture filtrate, and showed reduced expression of senescence-associated genes during disease development, suggesting that coi1 resistance is most likely achieved through the inhibition of F. oxysporum-incited lesion development and plant senescence. Together, our results indicate that F. oxysporum hijacks non-defensive aspects of the JA-signaling pathway to cause wilt-disease symptoms that lead to plant death in Arabidopsis.

Hormone (dis)harmony moulds plant health and disease

Diseased plants often display phenotypes consistent with hormone perturbations. We review recent data that have revealed roles in plant-microbe interactions for cellular components and signaling molecules that previously were associated only with hormone signaling. A better understanding of cross-talk between hormonal and defense signaling pathways should reveal new potential targets for microbial effectors that attenuate host resistance mechanisms.

Networking by small-molecule hormones in plant immunity

Plants live in complex environments in which they intimately interact with a broad range of microbial pathogens with different lifestyles and infection strategies. The evolutionary arms race between plants and their attackers provided plants with a highly sophisticated defense system that, like the animal innate immune system, recognizes pathogen molecules and responds by activating specific defenses that are directed against the invader. Recent advances in plant immunity research have provided exciting new insights into the underlying defense signaling network. Diverse small-molecule hormones play pivotal roles in the regulation of this network. Their signaling pathways cross-communicate in an antagonistic or synergistic manner, providing the plant with a powerful capacity to finely regulate its immune response. Pathogens, on the other hand, can manipulate the plant's defense signaling network for their own benefit by affecting phytohormone homeostasis to antagonize the host immune response.

Jasmonate- and salicylate-mediated plant defense responses to insect herbivores, pathogens and parasitic plants

While extensive research has examined plant defense responses to attack by herbivores and pathogens, plant responses to parasitism by other plants are not well characterized. The expression of induced plant defenses is mediated by complex signaling networks in which the plant hormones jasmonic acid (JA) and salicylic acid (SA) play key roles. In general, JA-mediated signaling pathways are implicated in the regulation of antiherbivore defenses, while the SA pathway is associated with defense responses against pathogens. However, there are many exceptions to this basic framework, and recent work suggests that interactions between the JA and SA pathways may play important roles in fine-tuning defense responses. The few studies that have addressed plant responses to parasitism by other plants suggest that both salicylates and jasmonates can mediate effective defenses. Here, a review is given of recent work on these defense signaling molecules with relevance for understanding their role in plant responses to parasitism.

Methyl jasmonate triggers loss of grain yield under drought stress

Drought is one of the major constraints to rice production worldwide. The development of rice panicle and spikelet meristem is repressed under the drought conditions, resulting in a reduction in the numbers of panicles and spikelets. In our recent report, we demonstrated that methyl jasmonate (MeJA) plays an important role in drought-induced loss of grain yield. Transgenic overexpression of the Arabidopsis gene jasmonic acid carboxyl methyltransferase (AtJMT) in rice resulted in a large reduction in grain yield through increased MeJA and ABA levels in young panicles. Exposure of nontransgenic plants to drought conditions also increased MeJA and ABA levels in young panicles and significantly reduced grain yield. In both cases, the reduction in grain yield was due to lower numbers of spikelets and lower filling rates than were observed for nontransgenic (NT) controls. The ABA increase in AtJMT transgenic panicles grown in non-drought conditions suggests that MeJA, rather than drought stress, induces ABA biosynthesis under drought conditions. These results led us postulate that plants produce MeJA during drought stress, which in turn stimulates the production of ABA, together leading to a loss of grain yield.

Jasmonate-inducible gene: What does it mean?

The diverse functions of jasmonates (JAs) in plant cells are reflected by the extensive reprogramming of gene expression after JA perception. Here, we review the transcriptome signatures associated with JA signalling in Arabidopsis thaliana and other plant species. Transcript profiling studies clearly highlight the renowned capacity of JAs to elicit evolutionarily distant metabolic pathways across the plant kingdom. However, JA-related transcriptomes show limited overlap and, for most cellular processes, the context in which the JA signal is perceived is crucial in shaping the response. This emphasizes that JA signal transduction is integrated into an elaborate signalling network.

The chromatin remodeler SPLAYED regulates specific stress signaling pathways

Organisms are continuously exposed to a myriad of environmental stresses. Central to an organism's survival is the ability to mount a robust transcriptional response to the imposed stress. An emerging mechanism of transcriptional control involves dynamic changes in chromatin structure. Alterations in chromatin structure are brought about by a number of different mechanisms, including chromatin modifications, which covalently modify histone proteins; incorporation of histone variants; and chromatin remodeling, which utilizes ATP hydrolysis to alter histone-DNA contacts. While considerable insight into the mechanisms of chromatin remodeling has been gained, the biological role of chromatin remodeling complexes beyond their function as regulators of cellular differentiation and development has remained poorly understood. Here, we provide genetic, biochemical, and biological evidence for the critical role of chromatin remodeling in mediating plant defense against specific biotic stresses. We found that the Arabidopsis SWI/SNF class chromatin remodeling ATPase SPLAYED (SYD) is required for the expression of selected genes downstream of the jasmonate (JA) and ethylene (ET) signaling pathways. SYD is also directly recruited to the promoters of several of these genes. Furthermore, we show that SYD is required for resistance against the necrotrophic pathogen Botrytis cinerea but not the biotrophic pathogen Pseudomonas syringae. These findings demonstrate not only that chromatin remodeling is required for selective pathogen resistance, but also that chromatin remodelers such as SYD can regulate specific pathways within biotic stress signaling networks.

Jasmonate signaling: a conserved mechanism of hormone sensing

The lipid-derived hormone jasmonate (JA) regulates diverse aspects of plant immunity and development. Among the central components of the JA signaling cascade are the E3 ubiquitin ligase SCFCOI1 and Jasmonate ZIM-domain (JAZ) proteins that repress transcription of JA-responsive genes. Recent studies provide evidence that amino acid-conjugated forms of JA initiate signal transduction upon formation of a coronatine-insensitive1 (COI1)-JA-JAZ ternary complex in which JAZs are ubiquitinated and subsequently degraded. Coronatine, a virulence factor produced by the plant pathogen Pseudomonas syringae, is a potent agonist of this hormone receptor system. Coronatine-induced targeting of JAZs to COI1 obstructs host immune responses to P. syrinage, providing a striking example of how pathogens exploit hormone signaling pathways in the host to promote disease. These findings, together with homology between COI1 and the auxin receptor, TIR1, extend the paradigm of F-box proteins as intracellular sensors of small molecules, and suggest a common evolutionary origin of the auxin and JA response pathways.