We report the genome-wide analysis of direct target genes of SND1 and VND7, two Arabidopsis thaliana NAC domain transcription factors that are master regulators of secondary wall biosynthesis in ...fibers and vessels, respectively. Systematic mapping of the SND1 binding sequence using electrophoretic mobility shift assay and transactivation analysis demonstrated that SND1 together with other secondary wall NACs （SWNs）, including VND6, VND7, NST1, and NST2, bind to an imperfect palindromic 19-bp consensus sequence designated as secondary wall NAC binding element （SNBE）, （T/A）NN（C/T） （TICIG）TNNNNNNNA（AIC）GN（AJCIT） （A/T）, in the promoters of their direct targets. Genome-wide analysis of direct targets of SND1 and VND7 revealed that they directly activate the expression of not only downstream transcription factors, but also a number of non-transcription factor genes involved in secondary wall biosynthesis, cell wall modification, and programmed cell death, the promoters of which all contain multiple SNBE sites. SND1 and VND7 directly regulate the expression of a set of common targets but each of them also preferentially induces a distinct set of direct targets, which is likely attributed to their differential activation strength toward SNBE sites. Complementation study showed that the SWNs were able to rescue the secondary wall defect in the sndl nstl mutant, indicating that they are functionally interchangeable. Together, our results provide important insight into the complex transcriptional program and the evolutionary mechanism underlying secondary wall biosynthesis, cell wall modification, and programmed cell death in secondary wall-containing cell types.
The ability to make secondary cell walls was a pivotal step for vascular plants in their conquest of dry land. Here, we review recent molecular and genetic studies that reveal that a group of
...Arabidopsis (
Arabidopsis thaliana) secondary wall-associated NAC domain transcription factors are master switches regulating a cascade of downstream transcription factors, leading to activation of the secondary wall biosynthetic program. Close homologs of the
Arabidopsis secondary wall NACs and their downstream transcription factors exist in diverse taxa of vascular plants and some are functional orthologs of their
Arabidopsis counterparts. There is evidence to suggest that the secondary wall NAC-mediated transcriptional regulation of secondary wall biosynthesis is a conserved mechanism throughout vascular plants.
It has previously been shown that SECONDARY WALL-ASSOCIATED NAC DOMAIN PROTEIN1 (SND1) is a key transcription factor regulating secondary cell wall formation, including the biosynthesis of cellulose, ...xylan, and lignin. In this study, we show that two closely related SND1-regulated MYB transcription factors, MYB58 and MYB63, are transcriptional regulators specifically activating lignin biosynthetic genes during secondary wall formation in Arabidopsis thaliana. MYB58 and MYB63 are phylogenetically distinct from previously characterized MYBs shown to be associated with secondary wall formation or phenylpropanoid metabolism. Expression studies showed that MYB58 and MYB63 are specifically expressed in fibers and vessels undergoing secondary wall thickening. Dominant repression of their functions led to a reduction in secondary wall thickening and lignin content. Overexpression of MYB58 and MYB63 resulted in specific activation of lignin biosynthetic genes and concomitant ectopic deposition of lignin in cells that are normally unlignified. MYB58 was able to activate directly the expression of lignin biosynthetic genes and a secondary wall-associated laccase (LAC4) gene. Furthermore, the expression of MYB58 and MYB63 was shown to be regulated by the SND1 close homologs NST1, NST2, VND6, and VND7 and their downstream target MYB46. Together, our results indicate that MYB58 and MYB63 are specific transcriptional activators of lignin biosynthesis in the SND1-mediated transcriptional network regulating secondary wall formation.
Wood is the most abundant biomass produced by land plants. Dissection of the molecular mechanisms underlying the transcriptional regulation of wood formation is a fundamental issue in plant biology ...and has important implications in tree biotechnology. Although a number of transcription factors in tree species have been shown to be associated with wood formation and some of them are implicated in lignin biosynthesis, none of them have been demonstrated to be key regulators of the biosynthesis of all three major components of wood. In this report, we have identified a group of NAC domain transcription factors, PtrWNDs, that are preferentially expressed in developing wood of poplar (Populus trichocarpa). Expression of PtrWNDs in the Arabidopsis (Arabidopsis thaliana) snd1 nst1 double mutant effectively complemented the secondary wall defects in fibers, indicating that PtrWNDs are capable of activating the entire secondary wall biosynthetic program. Overexpression of PtrWND2B and PtrWND6B in Arabidopsis induced the expression of secondary wall-associated transcription factors and secondary wall biosynthetic genes and, concomitantly, the ectopic deposition of cellulose, xylan, and lignin. Furthermore, PtrWND2B and PtrWND6B were able to activate the promoter activities of a number of poplar wood-associated transcription factors and wood biosynthetic genes. Together, these results demonstrate that PtrWNDs are functional orthologs of SND1 and suggest that PtrWNDs together with their downstream transcription factors form a transcriptional network involved in the regulation of wood formation in poplar.
SECONDARY WALL-ASSOCIATED NAC DOMAIN PROTEIN1 (SND1) is a master transcriptional switch activating the developmental program of secondary wall biosynthesis. Here, we demonstrate that a battery of ...SND1-regulated transcription factors is required for normal secondary wall biosynthesis in Arabidopsis thaliana. The expression of 11 SND1-regulated transcription factors, namely, SND2, SND3, MYB103, MYB85, MYB52, MYB54, MYB69, MYB42, MYB43, MYB20, and KNAT7 (a Knotted1-like homeodomain protein), was developmentally associated with cells undergoing secondary wall thickening. Of these, dominant repression of SND2, SND3, MYB103, MYB85, MYB52, MYB54, and KNAT7 significantly reduced secondary wall thickening in fiber cells. Overexpression of SND2, SND3, and MYB103 increased secondary wall thickening in fibers, and overexpression of MYB85 led to ectopic deposition of lignin in epidermal and cortical cells in stems. Furthermore, SND2, SND3, MYB103, MYB85, MYB52, and MYB54 were able to induce secondary wall biosynthetic genes. Direct target analysis using the estrogen-inducible system revealed that MYB46, SND3, MYB103, and KNAT7 were direct targets of SND1 and also of its close homologs, NST1, NST2, and vessel-specific VND6 and VND7. Together, these results demonstrate that a transcriptional network consisting of SND1 and its downstream targets is involved in regulating secondary wall biosynthesis in fibers and that NST1, NST2, VND6, and VND7 are functional homologs of SND1 that regulate the same downstream targets in different cell types.
Wood biomass is mainly made of secondary cell walls; hence, elucidation of the molecular mechanisms underlying the transcriptional regulation of secondary wall biosynthesis during wood formation will ...be instrumental to design strategies for genetic improvement of wood biomass. Here, we provide direct evidence demonstrating that the poplar (Populus trichocarpa) wood-associated NAC domain transcription factors (PtrWNDs) are master switches activating a suite of downstream transcription factors, and together, they are involved in the coordinated regulation of secondary wall biosynthesis during wood formation. We show that transgenic poplar plants with dominant repression of PtrWNDs functions exhibit a drastic reduction in secondary wall thickening in woody cells, and those with PtrWND overexpression result in ectopic deposition of secondary walls. Analysis of PtrWND2B overexpressors revealed up-regulation of the expression of a number of wood-associated transcription factors, the promoters of which were also activated by PtrWND6B and the Eucalyptus EgWND1. Transactivation analysis and electrophoretic mobility shift assay demonstrated that PtrWNDs and EgWND1 activated gene expression through direct binding to the secondary wall NAC-binding elements, which are present in the promoters of several wood-associated transcription factors and a number of genes involved in secondary wall biosynthesis and modification. The WND-regulated transcription factors PtrNAC150, PtrNAC156, PtrNAC157, PtrMYB18, PtrMYB74, PtrMYB75, PtrMYB121, PtrMYB128, PtrZF1, and PtrGATA8 were able to activate the promoter activities of the biosynthetic genes for all three major wood components. Our study has uncovered that the WND master switches together with a battery of their downstream transcription factors form a transcriptional network controlling secondary wall biosynthesis during wood formation.
Plant growth under low water availability adversely affects many key processes with morphological, physiological, biochemical and molecular consequences. Here, we found that a rice gene, OsCTR1, ...encoding the RING Ub E3 ligase plays an important role in drought tolerance. OsCTR1 was highly expressed in response to dehydration treatment and defense‐related phytohormones, and its encoded protein was localized in both the chloroplasts and the cytosol. Intriguingly, the OsCTR1 protein was found predominantly targeted to the cytosol when rice protoplasts transfected with OsCTR1 were treated with abscisic acid (ABA). Several interacting partners were identified, which were mainly targeted to the chloroplasts, and interactions with OsCTR1 were confirmed by using biomolecular fluorescence complementation (BiFC). Interestingly, two chloroplast‐localized proteins (OsCP12 and OsRP1) interacted with OsCTR1 in the cytosol, and ubiquitination by OsCTR1 led to protein degradation via the Ub 26S proteasome. Heterogeneous overexpression of OsCTR1 in Arabidopsis exhibited hypersensitive phenotypes with respect to ABA‐responsive seed germination, seedling growth and stomatal closure. The ABA‐sensitive transgenic plants also showed improvement in their tolerance against severe water deficits. Taken together, our findings lend support to the hypothesis that the molecular functions of OsCTR1 are related to tolerance to water‐deficit stress via ABA‐dependent regulation and related systems.
Device engineering based on computer-aided simulations is essential to make silicon (Si) quantum bits (qubits) be competitive to commercial platforms based on superconductors and trapped ions. ...Combining device simulations with the Bayesian optimization (BO), here we propose a systematic design approach that is quite useful to procure fast and precise entangling operations of qubits encoded to electron spins in electrode-driven Si quantum dot (QD) systems. For a target problem of the controlled-X (CNOT) logic operation, we employ BO with the Gaussian process regression to evolve design factors of a Si double QD system to the ones that are optimal in terms of speed and fidelity of a CNOT logic driven by a single microwave pulse. The design framework not only clearly contributes to cost-efficient securing of solutions that enhance performance of the target quantum operation, but can be extended to implement more complicated logics with Si QD structures in experimentally unprecedented ways.
Harpins are glycine-rich and heat-stable proteins that are secreted through type III secretion system in gram-negative plant-pathogenic bacteria. Many studies show that these proteins are mostly ...targeted to the extracellular space of plant tissues, unlike bacterial effector proteins that act inside the plant cells. Over the two decades since the first harpin of pathogen origin, HrpN of Erwinia amylovora, was reported in 1992 as a cell-free elicitor of hypersensitive response (HR), diverse functional aspects of harpins have been determined. Some harpins were shown to have virulence activity, probably because of their involvement in the translocation of effector proteins into plant cytoplasm. Based on this function, harpins are now considered to be translocators. Their abilities of pore formation in the artificial membrane, binding to lipid components, and oligomerization are consistent with this idea. When harpins are applied to plants directly or expressed in plant cells, these proteins trigger diverse beneficial responses such as induction of defense responses against diverse pathogens and insects and enhancement of plant growth. Therefore, in this review, we will summarize the functions of harpins as virulence factors (or translocators) of bacterial pathogens, elicitors of HR and immune responses, and plant growth enhancers.
Xylan, the second most abundant cell wall polysaccharide, is composed of a linear backbone of β-(1,4)-linked xylosyl residues that are often substituted with sugar side chains, such as glucuronic ...acid (GlcA) and methylglucuronic acid (MeGlcA). It has recently been shown that mutations of two Arabidopsis family GT8 genes, GUX1 and GUX2, affect the addition of GlcA and MeGlcA to xylan, but it is not known whether they encode glucuronyltransferases (GlcATs) or indirectly regulate the GlcAT activity. In this study, we performed biochemical and genetic analyses of three Arabidopsis GUX genes to determine their roles in the GlcA substitution of xylan and secondary wall deposition. The GUX1/2/3 genes were found to be expressed in interfascicular fibers and xylem cells, the two major types of secondary wall-containing cells that have abundant xylan. When expressed in tobacco BY2 cells, the GUX1/2/3 proteins exhibited an activity capable of transferring GlcA residues from the UDP-GlcA donor onto xylooligomer acceptors, demonstrating that these GUX proteins possess xylan GlcAT activity. Analyses of the single, double and triple gux mutants revealed that simultaneous mutations of all three GUX genes led to a complete loss of GlcA and MeGlcA side chains on xylan, indicating that all three GUX proteins are involved in the GlcA substitution of xylan. Furthermore, a complete loss of GlcA and MeGlcA side chains in the gux1/2/3 triple mutant resulted in reduced secondary wall thickening, collapsed vessel morphology and reduced plant growth. Together, our results provide biochemical and genetic evidence that GUX1/2/3 are GlcATs responsible for the GlcA substitution of xylan, which is essential for normal secondary wall deposition and plant development.
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