Genome-wide identification of PTI1 family in Setaria italica and salinity-responsive functional analysis of SiPTI1-5.

BACKGROUND: PTI1 (Pto-interacting 1) protein kinase belongs to the receptor-like cytoplasmic kinase (RLCK) group of receptor-like protein kinases (RLK), but lack extracellular and transmembrane domains. PTI1 was first identified in tomato (Solanum lycopersicum) and named SlPTI1, which has been reported to interact with bacterial effector Pto, a serine/threonine protein kinase involved in plant resistance to bacterial disease. Briefly, the host PTI1 specifically recognizes and interacts with the bacterial effector AvrPto, which triggers hypersensitive cell death to inhibit the pathogen growth in the local infection site. Previous studies have demonstrated that PTI1 is associated with oxidative stress and hypersensitivity. RESULTS: We identified 12 putative PTI1 genes from the genome of foxtail millet (Setaria italica) in this study. Gene replication analysis indicated that both segmental replication events played an important role in the expansion of PTI1 gene family in foxtail millet. The PTI1 family members of model plants, i.e. S. italica, Arabidopsis (Arabidopsis thaliana), rice (Oryza sativa), maize (Zea mays), S. lycopersicum, and soybean (Glycine max), were classified into six major categories according to the phylogenetic analysis, among which the PTI1 family members in foxtail millet showed higher degree of homology with those of rice and maize. The analysis of a complete set of SiPTI1 genes/proteins including classification, chromosomal location, orthologous relationships and duplication. The tissue expression characteristics revealed that SiPTI1 genes are mainly expressed in stems and leaves. Experimental qRT-PCR results demonstrated that 12 SiPTI1 genes were induced by multiple stresses. Subcellular localization visualized that all of foxtail millet SiPTI1s were localized to the plasma membrane. Additionally, heterologous expression of SiPTI1-5 in yeast and E. coli enhanced their tolerance to salt stress. CONCLUSIONS: Our results contribute to a more comprehensive understanding of the roles of PTI1 protein kinases and will be useful in prioritizing particular PTI1 for future functional validation studies in foxtail millet.

Trichoderma harzianum enhances antioxidant defense of tomato seedlings and resistance to water deficit.

Some plant-symbiotic strains of the genus Trichoderma colonize roots and induce profound changes in plant gene expression that lead to enhanced growth, especially under biotic and abiotic stresses. In this study, we tested the hypothesis that one of the protective mechanisms enhanced by T. harzianum T22 colonization is the antioxidant defense mechanism. Having established that strain T22 modulates the expression of the genes encoding antioxidant enzymes, the status of antioxidant defense of tomato seedlings in response to colonization by T22 and water deficit was investigated. Total ascorbate or glutathione levels were not affected by either stimuli, but under water deficit, antioxidant pools became more oxidized (lower ratios of reduced to oxidized forms), whereas colonized plants maintained redox state as high as or higher than unstressed and untreated plants. The enhanced redox state of colonized plants could be explained by their higher activity of ascorbate and glutathione-recycling enzymes, higher activity of superoxide dismutase, catalase, and ascorbate peroxidase, in both root and shoot throughout the experiment. Similar enzymes were induced in uncolonized plants in response to water-deficit stress but to a lower extent when compared with colonized plants. This orchestrated enhancement in activity of reactive oxygen species (ROS)-scavenging pathways in colonized plants in response to stress supports the hypothesis that enhanced resistance of colonized plants to water deficit is at least partly due to higher capacity to scavenge ROS and recycle oxidized ascorbate and glutathione, a mechanism that is expected to enhance tolerance to abiotic and biotic stresses.

Seed treatment with Trichoderma harzianum alleviates biotic, abiotic, and physiological stresses in germinating seeds and seedlings.

Trichoderma spp. are endophytic plant symbionts that are widely used as seed treatments to control diseases and to enhance plant growth and yield. Although some recent work has been published on their abilities to alleviate abiotic stresses, specific knowledge of mechanisms, abilities to control multiple plant stress factors, their effects on seed and seedlings is lacking. We examined the effects of seed treatment with T. harzianum strain T22 on germination of seed exposed to biotic stress (seed and seedling disease caused by Pythium ultimum) and abiotic stresses (osmotic, salinity, chilling, or heat stress). We also evaluated the ability of the beneficial fungus to overcome physiological stress (poor seed quality induced by seed aging). If seed were not under any of the stresses noted above, T22 generally had little effect upon seedling performance. However, under stress, treated seed germinated consistently faster and more uniformly than untreated seeds whether the stress was osmotic, salt, or suboptimal temperatures. The consistent response to varying stresses suggests a common mechanism through which the plant-fungus association enhances tolerance to a wide range of abiotic stresses as well as biotic stress. A common factor that negatively affects plants under these stress conditions is accumulation of toxic reactive oxygen species (ROS), and we tested the hypothesis that T22 reduced damages resulting from accumulation of ROS in stressed plants. Treatment of seeds reduced accumulation of lipid peroxides in seedlings under osmotic stress or in aged seeds. In addition, we showed that the effect of exogenous application of an antioxidant, glutathione, or application of T22, resulted in a similar positive effect on seed germination under osmotic stress or in aged seed. This evidence supports the model that T. harzianum strain T22 increases seedling vigor and ameliorates stress by inducing physiological protection in plants against oxidative damage.

Root-expressed maize lipoxygenase 3 negatively regulates induced systemic resistance to Colletotrichum graminicola in shoots.

We have previously reported that disruption of a maize root-expressed 9-lipoxygenase (9-LOX) gene, ZmLOX3, results in dramatic increase in resistance to diverse leaf and stalk pathogens. Despite evident economic significance of these findings, the mechanism behind this increased resistance remained elusive. In this study, we found that increased resistance of the lox3-4 mutants is due to constitutive activation of induced systemic resistance (ISR) signaling. We showed that ZmLOX3 lacked expression in leaves in response to anthracnose leaf blight pathogen Colletotrichum graminicola, but was expressed constitutively in the roots, thus, prompting our hypothesis: the roots of lox3-4 mutants are the source of increased resistance in leaves. Supporting this hypothesis, treatment of wild-type plants (WT) with xylem sap of lox3-4 mutant induced resistance to C. graminicola to the levels comparable to those observed in lox3-4 mutant. Moreover, treating mutants with the sap collected from WT plants partially restored the susceptibility to C. graminicola. lox3-4 mutants showed primed defense responses upon infection, which included earlier and greater induction of defense-related PAL and GST genes compared to WT. In addition to the greater expression of the octadecanoid pathway genes, lox3-4 mutant responded earlier and with a greater accumulation of H2O2 in response to C. graminicola infection or treatment with alamethicin. These findings suggest that lox3-4 mutants display constitutive ISR-like signaling. In support of this idea, root colonization by Trichoderma virens strain GV29-8 induced the same level of disease resistance in WT as the treatment with the mutant sap, but had no additional resistance effect in lox3-4 mutant. While treatment with T. virens GV29 strongly and rapidly suppressed ZmLOX3 expression in hydroponically grown WT roots, T. virens Δsml mutant, which is deficient in ISR induction, was unable to suppress expression of ZmLOX3, thus, providing genetic evidence that SM1 function in ISR, at least in part, by suppressing host ZmLOX3 gene. This study and the genetic tools generated herein will allow the identification of the signals regulating the induction of resistance to aboveground attackers by beneficial soil microorganisms in the future.

Induced systemic resistance and plant responses to fungal biocontrol agents.

Biocontrol fungi (BCF) are agents that control plant diseases. These include the well-known Trichoderma spp. and the recently described Sebacinales spp. They have the ability to control numerous foliar, root, and fruit pathogens and even invertebrates such as nematodes. However, this is only a subset of their abilities. We now know that they also have the ability to ameliorate a wide range of abiotic stresses, and some of them can also alleviate physiological stresses such as seed aging. They can also enhance nutrient uptake in plants and can substantially increase nitrogen use efficiency in crops. These abilities may be more important to agriculture than disease control. Some strains also have abilities to improve photosynthetic efficiency and probably respiratory activities of plants. All of these capabilities are a consequence of their abilities to reprogram plant gene expression, probably through activation of a limited number of general plant pathways.