Jasmonates (JAs) are a family of oxylipin phytohormones regulating plant development and growth and mediating ‘defense versus growth’ responses. The upstream JA biosynthetic precursor cis-(+)-12-oxo-phytodienoic acid (cis-OPDA) acts independently of CORONATIVE INSENSITIVE 1 (COI1)-mediated JA signaling in several stress-induced and developmental processes. However, its perception and metabolism are only partially understood. A few years ago, a low abundant isoleucine analog of the biologically active JA-Ile, OPDA-Ile, was detected years ago in wounded leaves of flowering plants, opening up the possibility that conjugation of cis-OPDA to amino acids might be a relevant mechanism for cis-OPDA regulation. Here, we extended the analysis of amino acid conjugates of cis-OPDA and identified naturally occurring OPDA-Val, OPDA-Phe, OPDA-Ala, OPDA-Glu, and OPDA-Asp accumulating in response to biotic and abiotic stress in Arabidopsis (Arabidopsis thaliana). The OPDA-amino acid conjugates displayed cis-OPDA-related plant responses in a JA-Ile-dependent manner. We also showed that the synthesis and hydrolysis of cis-OPDA amino acid conjugates are mediated by members of the amidosynthetase GRETCHEN HAGEN 3 (GH3) and the amidohydrolase INDOLE-3-ACETYL-LEUCINE RESISTANT 1 (ILR1)/ILR1-like (ILL) families. Thus, OPDA amino acid conjugates function in the catabolism or temporary storage of cis-OPDA in stress responses instead of acting as chemical signals per se.

The roles of mitogen-activated protein kinases (MAPKs) in plant–fungal pathogenic interactions are poorly understood in crops. Here, microscopic, phenotypic, proteomic, and biochemical analyses revealed that roots of independent transcription activator-like effector nuclease (TALEN)-based knockout lines of barley (Hordeum vulgare L.) MAPK 3 (HvMPK3 KO) were resistant against Fusarium graminearum infection. When co-cultured with roots of the HvMPK3 KO lines, F. graminearum hyphae were excluded to the extracellular space, the growth pattern of extracellular hyphae was considerably deregulated, mycelia development was less efficient, and number of appressoria-like structures and their penetration potential were substantially reduced. Intracellular penetration of hyphae was preceded by the massive production of reactive oxygen species (ROS) in attacked cells of the wild-type (WT), but ROS production was mitigated in the HvMPK3 KO lines. Suppression of ROS production in these lines coincided with elevated abundance of catalase (CAT) and ascorbate peroxidase (APX). Moreover, differential proteomic analysis revealed downregulation of several defense-related proteins in WT, and the upregulation of pathogenesis-related protein 1 (PR-1) and cysteine proteases in HvMPK3 KO lines. Proteins involved in suberin formation, such as peroxidases, lipid transfer proteins (LTPs), and the GDSL esterase/lipase (containing “GDSL” aminosequence motif) were differentially regulated in HvMPK3 KO lines after F. graminearum inoculation. Consistent with proteomic analysis, microscopic observations showed enhanced suberin accumulation in roots of HvMPK3 KO lines, most likely contributing to the arrested infection by F. graminearum. These results suggest that TALEN-based knockout of HvMPK3 leads to barley root resistance against Fusarium root rot.

The development of above-ground lateral organs is initiated at the peripheral zone of the shoot apical meristem (SAM). The coordination of cell fate determination and the maintenance of stem cells are achieved through a complex regulatory network comprised of transcription factors. Two AP2/ERF transcription factor family genes, ESR1/DRN and ESR2/DRNL/SOB/BOL, regulate cotyledon and flower formation and de novo organogenesis in tissue culture. However, their roles in post-embryonic lateral organ development remain elusive. In this study, we analyzed the genetic interactions among SAM-related genes, WUS and STM, two ESR genes, and one of the HD-ZIP III members, REV, whose protein product interacts with ESR1 in planta. We found that esr1 mutations substantially enhanced the wus and stm phenotypes, which bear a striking resemblance to those of the wus rev and stm rev double mutants, respectively. Aberrant adaxial–abaxial polarity is observed in wus esr1 at relatively low penetrance. On the contrary, the esr2 mutation partially suppressed stm phenotypes in the later vegetative phase. Such complex genetic interactions appear to be attributed to the distinct expression pattern of two ESR genes because the ESR1 promoter-driving ESR2 is capable of rescuing phenotypes caused by the esr1 mutation. Our results pose the unique genetic relevance of ESR1 and the SAM-related gene interactions in the development of rosette leaves.

Hormone perception and signaling pathways play a fundamental regulatory function in cell growth, developmental, and physiological processes in both plant and animal systems. Those pathways are activated by hormone binding to the receptor to trigger cellular responses. Equally important are mechanisms that suppress activated transduction cascades to reset the system. Different mechanisms at the level of hormone biosynthesis and deactivation through degradation, conjugation, and production of repressors that attenuate transduction cascades downstream of receptors are known. In animal systems, decoy receptors have been identified as another important mechanism for fine-tuning the activity of the signaling pathways in processes like inflammatory responses, apoptosis, and blood vessel formation. Decoy receptors recognize and bind specific signaling molecules, but they cannot activate downstream signaling pathways thus providing competitive inhibition. Here we describe the first decoy receptor in plants. We show that the splicing variant of CRE1/AHK4 receptor of cytokinin, a hormone with a key role in the regulation of cell division and meristem maintenance in plants, acts as a decoy receptor to attenuate cytokinin signaling. We propose that this novel mechanism of signaling control applies in processes when modulation of CK signaling is needed.

The roles of mitogen-activated protein kinases (MAPKs) in plant-fungal pathogenic interactions are less understood in crops. Here, microscopic, phenotyping, proteomic and biochemical analyses revealed that independent TALEN-based knockout lines of Hordeum vulgare MITOGEN-ACTIVATED PROTEIN KINASE 3 ( HvMPK3 KO) were resistant against Fusarium graminearum infection. When co-cultured with roots of the HvMPK3 KO lines, F. graminearum hyphae were excluded to the extracellular space, the growth pattern of hyphae was considerably deregulated, mycelia development was less efficient and number of appressoria and their penetration potential were significantly reduced. Intracellular penetration of hyphae was preceded by the massive production of reactive oxygen species (ROS) in attacked cells of the wild type, but it was mitigated in the HvMPK3 KO lines. Suppression of ROS production in these lines coincided with the elevated abundances of catalase and ascorbate peroxidase. Moreover, differential proteomic analysis revealed downregulation of defense-related proteins in wild type, and the upregulation of peroxidases, lipid transfer proteins, and cysteine proteases in HvMPK3 KO lines after 24h of F. graminearum inoculation. Consistently with proteomic analysis, microscopic observations showed an enhanced suberin accumulation in roots of HvMPK3 KO lines, most likely contributing to the arrested infection by F. graminearum . These results suggest that TALEN-based knockout of HvMPK3 leads to the barley root resistance against Fusarium root rot.