OXIDATIVE SIGNAL-INDUCIBLE1 induces immunity by coordinating N-hydroxypipecolic acid, salicylic acid, and camalexin synthesis

Expression of OXIDATIVE SIGNAL-INDUCIBLE1 (OXI1) is induced by a number of stress conditions and regulates the interaction of plants with pathogenic and beneficial microbes. In this work, we generated Arabidopsis OXI1 knockout and genomic OXI1 overexpression lines and show by transcriptome, proteome, and metabolome analysis that OXI1 triggers ALD1, SARD4, and FMO1 expressions to promote the biosynthesis of pipecolic acid (Pip) and N-hydroxypipecolic acid (NHP). OXI1 contributes to enhanced immunity by induced SA biosynthesis via CBP60g-induced expression of SID2 and camalexin accumulation via WRKY33-targeted transcription of PAD3. OXI1 regulates genes involved in reactive oxygen species (ROS) generation such as RbohD and RbohF. OXI1 knock out plants show enhanced expression of nuclear and chloroplast genes of photosynthesis and enhanced growth under ambient conditions, while OXI1 overexpressing plants accumulate NHP, SA, camalexin, and ROS and show a gain-of-function (GOF) cell death phenotype and enhanced pathogen resistance. The OXI1 GOF phenotypes are completely suppressed when compromising N-hydroxypipecolic acid (NHP) synthesis in the fmo1 or ald1 background, showing that OXI1 regulation of immunity is mediated via the NHP pathway. Overall, these results show that OXI1 plays a key role in basal and effector-triggered plant immunity by regulating defense and programmed cell death via biosynthesis of salicylic acid, N-hydroxypipecolic acid, and camalexin.

AINTEGUMENTA and the D-type cyclin CYCD3;1 regulate root secondary growth and respond to cytokinins

Higher plant vasculature is characterized by two distinct developmental phases. Initially, a well-defined radial primary pattern is established. In eudicots, this is followed by secondary growth, which involves development of the cambium and is required for efficient water and nutrient transport and wood formation. Regulation of secondary growth involves several phytohormones, and cytokinins have been implicated as key players, particularly in the activation of cell proliferation, but the molecular mechanisms mediating this hormonal control remain unknown. Here we show that the genes encoding the transcription factor AINTEGUMENTA (ANT) and the D-type cyclin CYCD3;1 are expressed in the vascular cambium of Arabidopsis roots, respond to cytokinins and are both required for proper root secondary thickening. Cytokinin regulation of ANT and CYCD3 also occurs during secondary thickening of poplar stems, suggesting this represents a conserved regulatory mechanism.

WOX5 suppresses CYCLIN D activity to establish quiescence at the center of the root stem cell niche

In Arabidopsis, stem cells maintain the provision of new cells for root growth. They surround a group of slowly dividing cells named the quiescent center (QC), and, together, they form the stem cell niche (SCN). The QC acts as the signaling center of the SCN, repressing differentiation of the surrounding stem cells [1] and providing a pool of cells able to replace damaged stem cells [2, 3]. Maintenance of the stem cells depends on the transcription factor WUSCHEL-RELATED HOMEOBOX 5 (WOX5), which is specifically expressed in the QC [4]. However, the molecular mechanisms by which WOX5 promotes stem cell fate and whether WOX5 regulates proliferation of the QC are unknown. Here, we reveal a new role for WOX5 in restraining cell division in the cells of the QC, thereby establishing quiescence. In contrast, WOX5 and CYCD3;3/CYCD1;1 both promote cell proliferation in the nascent columella. The additional QC divisions occurring in wox5 mutants are suppressed in mutant combinations with the D type cyclins CYCD3;3 and CYCD1;1. Moreover, ectopic expression of CYCD3;3 in the QC is sufficient to induce cell division in the QC. WOX5 thus suppresses QC divisions that are otherwise promoted by CYCD3;3 and CYCD1;1, in part by interacting with the CYCD3;3 promoter to repress CYCD3;3 expression in the QC. Therefore, we propose a specific role for WOX5 in initiating and maintaining quiescence of the QC by excluding CYCD activity from the QC.

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WOX5 prevents divisions at the root stem cell niche center to initiate quiescence

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WOX5 suppresses CYCD expression in the quiescent center to restrict cell divisions

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WOX5 binds to the CYCD3;3 promoter

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CYCD3;3 and CYCD1;1 stimulate division during formation of the columella

The role of the kinase OXI1 in cadmium- and copper-induced molecular responses in Arabidopsis thaliana

The hypothesis that mitogen-activated protein kinase (MAPK) signalling is important in plant defences against metal stress has become accepted in recent years. To test the role of oxidative signal-inducible kinase (OXI1) in metal-induced oxidative signalling, the responses of oxi1 knockout lines to environmentally realistic cadmium (Cd) and copper (Cu) concentrations were compared with those of wild-type plants. A relationship between OXI1 and the activation of lipoxygenases and other initiators of oxylipin production was observed under these stress conditions, suggesting that lipoxygenase-1 may be a downstream component of OXI1 signalling. Metal-specific differences in OXI1 action were observed. For example, OXI1 was required for the up-regulation of antioxidative defences such as catalase in leaves and Fe-superoxide dismutase in roots, following exposure to Cu, processes that may involve the MEKK1-MKK2-WRKY25 cascade. Moreover, the induction of Cu/Zn superoxide dismutases in Cu-exposed leaves was regulated by OXI1 in a manner that involves fluctuations in the expression of miRNA398. These observations contrast markedly with the responses to Cd exposure, which also involves OXI1-independent pathways but rather involves changes in components mediating intracellular communication.

The Arabidopsis protein kinase Pto-interacting 1-4 is a common target of the oxidative signal-inducible 1 and mitogen-activated protein kinases

In Arabidopsis thaliana, the serine/threonine protein kinase oxidative signal-inducible 1 (OXI1), mediates oxidative stress signalling. Its activity is required for full activation of the mitogen-activated protein kinases (MAPKs), MPK3 and MPK6, in response to oxidative stress. In addition, the serine/threonine protein kinase Pto-interacting 1-2 (PTI1-2) has been positioned downstream from OXI1, but whether PTI1-2 signals through MAPK cascades is unclear. Using a yeast two-hybrid screen we show that OXI1 also interacts with PTI1-4. OXI1 and PTI1-4 are stress-responsive genes and are expressed in the same tissues. Therefore, studies were undertaken to determine whether PTI1-4 is positioned in the OXI1/MAPK signalling pathway. The interaction between OXI1 and PTI1-4 was confirmed by using in vivo co-immunoprecipitation experiments. OXI1 and PTI1-4 were substrates of MPK3 and MPK6 in vitro. Although no direct interaction was detected between OXI1 and MPK3 or MPK6, in vitro binding studies showed interactions between MPK3 or MPK6 with PTI1-4. In addition, PTI1-4 and MPK6 were found in vivo in the same protein complex. These results demonstrate that PTI1-4 signals via OXI1 and MPK6 signalling cascades.

The Arabidopsis D-type cyclin CYCD2;1 and the inhibitor ICK2/KRP2 modulate auxin-induced lateral root formation

The integration of cell division in root growth and development requires mediation of developmental and physiological signals through regulation of cyclin-dependent kinase activity. Cells within the pericycle form de novo lateral root meristems, and D-type cyclins (CYCD), as regulators of the G₁-to-S phase cell cycle transition, are anticipated to play a role. Here, we show that the D-type cyclin protein CYCD2;1 is nuclear in Arabidopsis thaliana root cells, with the highest concentration in apical and lateral meristems. Loss of CYCD2;1 has a marginal effect on unstimulated lateral root density, but CYCD2;1 is rate-limiting for the response to low levels of exogenous auxin. However, while CYCD2;1 expression requires sucrose, it does not respond to auxin. The protein Inhibitor-Interactor of CDK/Kip Related Protein2 (ICK2/KRP2), which interacts with CYCD2;1, inhibits lateral root formation, and ick2/krp2 mutants show increased lateral root density. ICK2/KRP2 can modulate the nuclear levels of CYCD2;1, and since auxin reduces ICK2/KRP2 protein levels, it affects both activity and cellular distribution of CYCD2;1. Hence, as ICK2/KRP2 levels decrease, the increase in lateral root density depends on CYCD2;1, irrespective of ICK2/CYCD2;1 nuclear localization. We propose that ICK2/KRP2 restrains root ramification by maintaining CYCD2;1 inactive and that this modulates pericycle responses to auxin fluctuations.

Site-specific phosphorylation profiling of Arabidopsis proteins by mass spectrometry and peptide chip analysis

An estimated one-third of all proteins in higher eukaryotes are regulated by phosphorylation by protein kinases (PKs). Although plant genomes encode more than 1000 PKs, the substrates of only a small fraction of these kinases are known. By mass spectrometry of peptides from cytoplasmic- and nuclear-enriched fractions, we determined 303 in vivo phosphorylation sites in Arabidopsis proteins. Among 21 different PKs, 12 were phosphorylated in their activation loops, suggesting that they were in their active state. Immunoblotting and mutational analysis confirmed a tyrosine phosphorylation site in the activation loop of a GSK3/shaggy-like kinase. Analysis of phosphorylation motifs in the substrates suggested links between several of these PKs and many target sites. To perform quantitative phosphorylation analysis, peptide arrays were generated with peptides corresponding to in vivo phosphorylation sites. These peptide chips were used for kinome profiling of subcellular fractions as well as H 2O 2-treated Arabidopsis cells. Different peptide phosphorylation profiles indicated the presence of overlapping but distinct PK activities in cytosolic and nuclear compartments. Among different H 2O 2-induced PK targets, a peptide of the serine/arginine-rich (SR) splicing factor SCL30 was most strongly affected. SRPK4 (SR protein-specific kinase 4) and MAPKs (mitogen-activated PKs) were found to phosphorylate this peptide, as well as full-length SCL30. However, whereas SRPK4 was constitutively active, MAPKs were activated by H 2O 2. These results suggest that SCL30 is targeted by different PKs. Together, our data demonstrate that a combination of mass spectrometry with peptide chip phosphorylation profiling has a great potential to unravel phosphoproteome dynamics and to identify PK substrates.

Reactive oxygen species signaling in plants

The evolution of aerobic metabolism such as respiration and photosynthesis resulted in the generation of reactive oxygen species (ROS). A common property of all ROS types is that they can cause oxidative damage to proteins, DNA, and lipids. This toxicity of ROS explains the evolution of complex arrays of nonenzymatic and enzymatic detoxification mechanisms in plants. However, increasing evidence indicates that plants also make use of ROS as signaling molecules for regulating development and various physiological responses. In this review, novel insights into the mechanisms of how plants sense and respond to ROS are discussed in the context of the biological effects and functions of ROS in plants.

Nickel resistance and chromatin condensation in Saccharomyces cerevisiae expressing a maize high mobility group I/Y protein

Expression of a maize cDNA encoding a high mobility group (HMG) I/Y protein enables growth of transformed yeast on a medium containing toxic nickel concentrations. No difference in the nickel content was measured between yeast cells expressing either the empty vector or the ZmHMG I/Y2 cDNA. The ZmHMG I/Y2 protein contains four AT hook motifs known to be involved in binding to the minor groove of AT-rich DNA regions. HMG I/Y proteins may act as architectural elements modifying chromatin structure. Indeed, a ZmHMG I/Y2-green fluorescent protein fusion protein was observed in yeast nuclei. Nickel toxicity has been suggested to occur through an epigenetic mechanism related to chromatin condensation and DNA methylation, leading to the silencing of neighboring genes. Therefore, the ZmHMG I/Y2 protein could prevent nickel toxicity by interfering with chromatin structure. Yeast cell growth in the presence of nickel and yeast cells expressing the ZmHMG I/Y2 cDNA increased telomeric URA3 gene silencing. Furthermore, ZmHMG I/Y2 restored a wild-type level of nickel sensitivity to the yeast (Delta)rpd3 mutant. Therefore, nickel resistance of yeast cells expressing the ZmHMG I/Y2 cDNA is likely achieved by chromatin structure modification, restricting nickel accessibility to DNA.