Near-infrared light and PIF4 promote plant antiviral defense by enhancing RNA interference.

The molecular mechanism underlying phototherapy and light treatment, which utilize various wavelength spectra of light, including near-infrared (NIR), to cure human and plant diseases, is obscure. Here we revealed that NIR light confers antiviral immunity by positively regulating PHYTOCHROME-INTERACTING FACTOR 4 (PIF4)-activated RNA interference (RNAi) in plants. PIF4, a central transcription factor involved in light signaling, accumulates to high levels under NIR light in plants. PIF4 directly induces the transcription of two essential components of RNAi, RNA-DEPENDENT RNA POLYMERASE 6 (RDR6) and ARGONAUTE 1 (AGO1), which play important roles in resistance to both DNA and RNA viruses. Moreover, the pathogenic determinant ßC1 protein, which is evolutionarily conserved and encoded by betasatellites, interacts with PIF4 and inhibits its positive regulation of RNAi by disrupting PIF4 dimerization. These findings shed light on the molecular mechanism of PIF4-mediated plant defense and provide a new perspective for the exploration of NIR antiviral treatment.

OsTUB1 confers salt insensitivity by interacting with Kinesin13A to stabilize microtubules and ion transporters in rice.

Salt stress is one of the major environmental factors limiting plant growth and development. Although microtubule (MT) organization is known to be involved in response to salt stress, few tubulin genes have been identified that confer salt insensitivity in plants. In this study, we identified a MT encoding gene, OsTUB1, that increased the survival rate of rice plants under salt stress by stabilizing MT organization and ion transporters. We found that OsTUB1 interacted with Kinesin13A protein, which was essential for OsTUB1-regulated MT organization under salt stress. Further molecular evidence revealed that a OsTUB1-Kinesin13A complex protected rice from salt stress by sustaining membrane-localized Na+ transporter OsHKT1;5, a key regulator of ionic homeostasis. Our results shed light on the function of tubulin and kinesin in regulating MT organization and stabilizing Na+ transporters and Na+ flux at the plasma membrane in rice. The identification of the OsTUB1-Kinesin13A complex provides novel genes for salt insensitivity rice breeding in areas with high soil salinity.

Diverse Begomoviruses Evolutionarily Hijack Plant Terpenoid-Based Defense to Promote Whitefly Performance.

Arthropod-borne pathogens and parasites are major threats to human health and global agriculture. They may directly or indirectly manipulate behaviors of arthropod vector for rapid transmission between hosts. The largest genus of plant viruses, Begomovirus, is transmitted exclusively by whitefly (Bemisia tabaci), a complex of at least 34 morphologically indistinguishable species. We have previously shown that plants infected with the tomato yellowleaf curl China virus (TYLCCNV) and its associated betasatellite (TYLCCNB) attract their whitefly vectors by subverting plant MYC2-regulated terpenoid biosynthesis, therefore forming an indirect mutualism between virus and vector via plant. However, the evolutionary mechanism of interactions between begomoviruses and their whitefly vectors is still poorly understood. Here we present evidence to suggest that indirect mutualism may happen over a millennium ago and at present extensively prevails. Detailed bioinformatics and functional analysis identified the serine-33 as an evolutionary conserved phosphorylation site in 105 of 119 Betasatellite species-encoded ßC1 proteins, which are responsible for suppressing plant terpenoid-based defense by interfering with MYC2 dimerization and are essential to promote whitefly performance. The substitution of serine-33 of ßC1 proteins with either aspartate (phosphorylation mimic mutants) or cysteine, the amino acid in the non-functional sßC1 encoded by Siegesbeckia yellow vein betasatellite SiYVB) impaired the ability of ßC1 functions on suppression of MYC2 dimerization, whitefly attraction and fitness. Moreover the gain of function mutation of cysteine-31 to serine in sßC1 protein of SiYVB restored these functions of ßC1 protein. Thus, the dynamic phosphorylation of serine-33 in ßC1 proteins helps the virus to evade host defense against insect vectors with an evolutionarily conserved manner. Our data provide a mechanistic explanation of how arboviruses evolutionarily modulate host defenses for rapid transmission.

Red-light is an environmental effector for mutualism between begomovirus and its vector whitefly.

Environments such as light condition influence the spread of infectious diseases by affecting insect vector behavior. However, whether and how light affects the host defense which further affects insect preference and performance, remains unclear, nor has been demonstrated how pathogens co-adapt light condition to facilitate vector transmission. We previously showed that begomoviral ßC1 inhibits MYC2-mediated jasmonate signaling to establish plant-dependent mutualism with its insect vector. Here we show red-light as an environmental catalyzer to promote mutualism of whitefly-begomovirus by stabilizing ßC1, which interacts with PHYTOCHROME-INTERACTING FACTORS (PIFs) transcription factors. PIFs positively control plant defenses against whitefly by directly binding to the promoter of terpene synthase genes and promoting their transcription. Moreover, PIFs interact with MYC2 to integrate light and jasmonate signaling and regulate the transcription of terpene synthase genes. However, begomovirus encoded ßC1 inhibits PIFs' and MYC2' transcriptional activity via disturbing their dimerization, thereby impairing plant defenses against whitefly-transmitted begomoviruses. Our results thus describe how a viral pathogen hijacks host external and internal signaling to enhance the mutualistic relationship with its insect vector.

Two new steroidal alkaloids from the rhizomes of Veratrum nigrum L. and their anti-TYLCV activity.

Two new steroidal alkaloids (1-2), together with seven known related steroidal alkaloids (3-9), were isolated from the rhizomes of Veratrum nigrum L. Their structures were elucidated by extensive spectroscopic analysis, and by comparison with literature data. Compound 1 possessed a rare 1, 3-oxazolidine unit within varazine-type alkaloids, and 2 was a 9-hydroxy-4-one derivative of 3-veratroylgermine. All isolates were evaluated inhibit tomato yellow leaf curl virus (TYLCV) activity. Compounds 5 and 7 (40 µg/mL) showed a significant anti-TYLCV activity in the host Nicotiana benthamiana with inhibition rates 74.6% and 63.4%, respectively, which are higher than that of the positive control ningnanmycin (51.4%).

Genome-wide associated study identifies NAC42-activated nitrate transporter conferring high nitrogen use efficiency in rice.

Over-application of nitrogen fertilizer in fields has had a negative impact on both environment and human health. Domesticated rice varieties with high nitrogen use efficiency (NUE) reduce fertilizer for sustainable agriculture. Here, we perform genome-wide association analysis of a diverse rice population displaying extreme nitrogen-related phenotypes over three successive years in the field, and identify an elite haplotype of nitrate transporter OsNPF6.1HapB that enhances nitrate uptake and confers high NUE by increasing yield under low nitrogen supply. OsNPF6.1HapB differs in both the protein and promoter element with natural variations, which are differentially trans-activated by OsNAC42, a NUE-related transcription factor. The rare natural allele OsNPF6.1HapB, derived from variation in wild rice and selected for enhancing both NUE and yield, has been lost in 90.3% of rice varieties due to the increased application of fertilizer. Our discovery highlights this NAC42-NPF6.1 signaling cascade as a strategy for high NUE and yield breeding in rice.

Viruses mobilize plant immunity to deter nonvector insect herbivores.

A parasite-infected host may promote performance of associated insect vectors; but possible parasite effects on nonvector insects have been largely unexplored. Here, we show that Begomovirus, the largest genus of plant viruses and transmitted exclusively by whitefly, reprogram plant immunity to promote the fitness of the vector and suppress performance of nonvector insects (i.e., cotton bollworm and aphid). Infected plants accumulated begomoviral ßC1 proteins in the phloem where they were bound to the plant transcription factor WRKY20. This viral hijacking of WRKY20 spatiotemporally redeployed plant chemical immunity within the leaf and had the asymmetrical benefiting effects on the begomoviruses and its whitefly vectors while negatively affecting two nonvector competitors. This type of interaction between a parasite and two types of herbivores, i.e., vectors and nonvectors, occurs widely in various natural and agricultural ecosystems; thus, our results have broad implications for the ecological significance of parasite-vector-host tripartite interactions.

The Orthotospovirus nonstructural protein NSs suppresses plant MYC-regulated jasmonate signaling leading to enhanced vector attraction and performance.

Pandemics of vector-borne human and plant diseases often depend on the behaviors of their arthropod vectors. Arboviruses, including many bunyaviruses, manipulate vector behavior to accelerate their own transmission to vertebrates, birds, insects, and plants. However, the molecular mechanism underlying this manipulation remains elusive. Here, we report that the non-structural protein NSs of Tomato spotted wilt orthotospovirus, a prototype of the Tospoviridae family and the Orthotospovirus genus, is a key viral factor that indirectly modifies vector preference and increases vector performance. NSs suppresses the biosynthesis of plant volatile monoterpenes, which serve as repellents of the vector western flower thrips (WFT, Frankliniella occidentalis). NSs directly interacts with MYC2, the jasmonate (JA) signaling master regulator and its two close homologs MYC3 and MYC4, to disable JA-mediated activation of terpene synthase genes. The dysfunction of the MYCs subsequently attenuates host defenses, increases the attraction of thrips, and improves thrips fitness. Moreover, MYC2 associated with NSs of Tomato zonate spot orthotospovirus, another Euro/Asian-type orthotospovirus, suggesting that MYC2 is an evolutionarily conserved target of Orthotospovirus species for suppression of terpene-based resistance to promote vector performance. These findings elucidate the molecular mechanism through which an orthotospovirus indirectly manipulates vector behaviors and therefore facilitates pathogen transmission. Our results provide insights into the molecular mechanisms by which Orthotospovirus NSs counteracts plant immunity for pathogen transmission.

A whitefly effector Bsp9 targets host immunity regulator WRKY33 to promote performance.

Whiteflies, Bemisia tabaci (Hemiptera), are pests causing economic damage to many crops, capable of transmitting hundreds of plant vector-borne viruses. They are believed to secrete salivary protein effectors that can improve vector colonization and reproductive fitness in host plants. However, little is known about effector biology and the precise mechanism of action of whitefly effectors. Here, we report a functional screening of B. tabaci salivary effector proteins (Bsp) capable of modulating plant innate immunity triggered by plant endogenous pattern peptide Pep1. Four immunity suppressors and two elicitors were identified. Bsp9, the most effective immunity suppressor, was further identified to directly interact with an immunity regulator WRKY33. We provide evidence that Bsp9 may suppress plant immune signalling by interfering with the interaction between WRKY33 and a central regulator in the MAPK cascade. The interference by Bsp9 therefore reduces plant resistance to whitefly by inhibiting activation of WRKY33-regulated immunity-related genes. Further detailed analysis based on transgenic plants found that whitefly effector Bsp9 could promote whitefly preference and performance, increasing virus transmission. This study enriches our knowledge on insect effector biology. This article is part of the theme issue 'Biotic signalling sheds light on smart pest management'.

The LIKE SEX FOUR2 regulates root development by modulating reactive oxygen species homeostasis in Arabidopsis.

Maintaining reactive oxygen species (ROS) homeostasis plays a central role in plants, and is also critical for plant root development. Threshold levels of ROS act as signals for elongation and differentiation of root cells. The protein phosphatase LIKE SEX FOUR2 (LSF2) has been reported to regulate starch metabolism in Arabidopsis, but little is known about the mechanism how LSF2 affect ROS homeostasis. Here, we identified that LSF2 function as a component modulating ROS homeostasis in response to oxidative stress and, thus regulate root development. Compared with wild type Arabidopsis, lsf2-1 mutant exhibited reduced rates of superoxide generation and higher levels of hydrogen peroxide upon oxidative stress treatments. The activities of several antioxidant enzymes, including superoxide dismutase, catalase, and ascorbate peroxidase, were also affected in lsf2-1 mutant under these oxidative stress conditions. Consequently, lsf2-1 mutant exhibited the reduced root growth but less inhibition of root hair formation compared to wild type Arabidopsis plants. Importantly, protein phosphatase LSF2 interacted with mitogen-activated protein kinase 8 (MPK8), a known component of ROS homeostasis pathways in the cytoplasm. These findings indicated the novel function of LSF2 that controls ROS homeostasis to regulate root development.

Geminivirus Activates ASYMMETRIC LEAVES 2 to Accelerate Cytoplasmic DCP2-Mediated mRNA Turnover and Weakens RNA Silencing in Arabidopsis.

Aberrant viral RNAs produced in infected plant cells serve as templates for the synthesis of dsRNAs. The derived virus-related small interfering RNAs (siRNA) mediate cleavage of viral RNAs by post-transcriptional gene silencing (PTGS), thus blocking virus multiplication. Here, we identified ASYMMETRIC LEAVES2 (AS2) as a new component of plant P body complex which mediates mRNA decapping and degradation. We found that AS2 promotes DCP2 decapping activity, accelerates mRNA turnover rate, inhibits siRNA accumulation and functions as an endogenous suppressor of PTGS. Consistent with these findings, as2 mutant plants are resistant to virus infection whereas AS2 over-expression plants are hypersensitive. The geminivirus nuclear shuttle protein BV1 protein, which shuttles between nuclei and cytoplasm, induces AS2 expression, causes nuclear exit of AS2 to activate DCP2 decapping activity and renders infected plants more sensitive to viruses. These principles of gene induction and shuttling of induced proteins to promote mRNA decapping in the cytosol may be used by viral pathogens to weaken antiviral defenses in host plants.

PLANT U-BOX PROTEIN10 Regulates MYC2 Stability in Arabidopsis.

MYC2 is an important regulator for jasmonic acid (JA) signaling, but little is known about its posttranslational regulation. Here, we show that the MYC2 C-terminal region interacted with the PLANT U-BOX PROTEIN10 (PUB10) armadillo repeats in vitro. MYC2 was efficiently polyubiquitinated by PUB10 with UBC8 as an E2 enzyme and the conserved C249 in PUB10 was required for activity. The inactive PUB10(C249A) mutant protein retained its ability to heterodimerize with PUB10, thus blocking PUB10 E3 activity as a dominant-negative mutant. Both MYC2 and PUB10 were nucleus localized and coimmunoprecipitation experiments confirmed their interaction in vivo. Although unstable in the wild type, MYC2 stability was enhanced in pub10, suggesting destabilization by PUB10. Moreover, MYC2 half-life was shortened or prolonged by induced expression of PUB10 or the dominant-negative PUB10(C249A) mutant, respectively. Root growth of pub10 seedlings phenocopied 35S:MYC2 seedlings and was hypersensitive to methyl jasmonate, whereas 35S:PUB10 and jin1-9 (myc2) seedlings were hyposensitive. In addition, the root phenotype conferred by MYC2 overexpression in double transgenic plants was reversed or enhanced by induced expression of PUB10 or PUB10(C249A), respectively. Similar results were obtained with three other JA-regulated genes, TAT, JR2, and PDF1.2. Collectively, our results show that MYC2 is targeted by PUB10 for degradation during JA responses.

Enhancing protein stability with retained biological function in transgenic plants.

The final expression level of a transgene-derived protein in transgenic plants depends on transcriptional and post-transcriptional processes. Here, we focus on methods to improve protein stability without comprising biological function. We found that the four isoforms of the Arabidopsis RAD23 protein family are relatively stable. The UBA2 domain derived from RAD23a can be used as a portable stabilizing signal to prolong the half-life of two unstable transcription factors (TFs), HFR1 and PIF3. The increased stability of the TF-UBA2 fusion proteins results in an enhanced phenotype in transgenic plants compared to expression of the TF alone. Similar results were obtained for the RAD23a UBA1 domain. In addition to UBA1/2 of RAD23a, the UBA domain from the Arabidopsis DDI1 protein also increased the half-life of the unstable protein JAZ10.1, which is involved in jasmonate signaling. Taken together, our results suggest that UBA fusions can be used to increase the stability of unstable proteins for basic plant biology research as well as crop improvement.