April 2018, Volume 60 Issue 4, Pages 265-357.


Cover Caption: Maintenance of suspensor identity
Suspensor plays an important role in embryogenesis, especially in establishing the apical©\basal polarity. Loss of function of RPL18aB resulted in suspensor cell proliferation rather than undergoing programmed cell death in the wildtype. In this issue, Xie et al. (pp. 266¨C269) show that RPL18aB is required for the maintenance of suspensor identity. This work provides a clue to understanding how the suspensor cells maintain its identity during early embryogenesis.

 

          Letters to the Editor
RPL18aB helps maintain suspensor identity during early embryogenesis  
Author: Fei Xie, Hailong Yan, Yang Sun, Yameng Wang, Hong Chen, Wanying Mao, Liyao Zhang, Mengxiang Sun and Xiongbo Peng
Journal of Integrative Plant Biology 2018 60(4): 266-269
Published Online: November 29, 2017
DOI: 10.1111/jipb.12616
      
    

During embryogenesis, plants are thought to use a mechanism that allows the suspensor to maintain its identity. Here, we reported that RPL18aB is involved in this mechanism in Arabidopsis thaliana. The suspensor cells proliferated in rpl18aB and formed a multicellular structure rather than undergo programmed cell death, as in wild type. Suspensors of rpl18aB expressed the embryo proper marker, DRN::GFP, but not the suspensor marker, WOX8::GFP. In addition, auxin accumulated throughout the suspensors of rpl18aB proembryos. Suspensor©\specific expression of RPL18aB could rescue the cell proliferation defects in rpl18aB suspensors. These findings supported a role for RPL18aB in maintaining suspensor identity.

Abstract (Browse 406)  |  References  |  Full Text HTML  |  Full Text PDF  |  Cited By       
Suspensors of rpl18aB mutant lose cell identity with the mis©\expression of the embryo proper marker DRN::GFP and by not expressing the suspensor marker WOX8::GFP. Moreover, suspensor©\specific expression of RPL18aB could rescue the cell proliferation defects in rpl18aB suspensors. These findings supported a role of RPL18aB in maintaining suspensor cell identity.
Differential requirement of BAK1 C©\terminal tail in development and immunity  
Author: Di Wu, Yanan Liu, Fan Xu and Yuelin Zhang
Journal of Integrative Plant Biology 2018 60(4): 270-275
Published Online: December 11, 2017
DOI: 10.1111/jipb.12623
      
    

BRI1©\ASSOCIATED RECEPTOR KINASE 1 (BAK1) plays critical roles in plant developmental and immune signaling pathways. BAK1 and a large number of leucine©\rich repeat receptor©\like kinases (LRR©\RLKs) harbor a mysterious carboxyl©\terminal tail (CT) beyond their kinase domain. In this study we analyzed the biological significance of this CT region using a unique bak1 mutant allele which causes deletion of the CT region. We showed that BAK1 CT promotes its kinase activity and is required for pathogen©\associated molecular pattern (PAMP)©\triggered immunity, but it is dispensable for brassinosteroid responses and BAK1/BKK1©\inhibited cell death signaling. Therefore the BAK1 C©\terminal tail is differentially required for its functions in development and immunity.

Abstract (Browse 305)  |  References  |  Full Text HTML  |  Full Text PDF  |  Cited By       
In this study, we showed that the C©\terminal tail of BAK1 promotes its kinase activity and is required for PAMP©\triggered immunity, but is dispensable in brassinosteroid responses and cell death control, suggesting that the C©\terminal tail is differentially required for the functions of BAK1 in development and immunity.
          Cell and Developmental Biology
The dioxygenase GIM2 functions in seed germination by altering gibberellin production in Arabidopsis  
Author: Wei Xiong, Tiantian Ye, Xuan Yao, Xiong Liu, Sheng Ma, Xi Chen, Ming-Luan Chen, Yu-Qi Feng and Yan Wu
Journal of Integrative Plant Biology 2018 60(4): 276-291
Published Online: December 4, 2017
DOI: 10.1111/jipb.12619
      
    

The phytohormones gibberellic acid (GA) and abscisic acid (ABA) antagonistically control seed germination. High levels of GA favor seed germination, whereas high levels of ABA hinder this process. The direct relationship between GA biosynthesis and seed germination ability need further investigation. Here, we identified the ABA©\insensitive gain©\of©\function mutant germination insensitive to ABA mutant 2 (gim2) by screening a population of XVE T©\DNA©\tagged mutant lines. Based on two loss©\of©\function gim2©\ko mutant lines, the disruption of GIM2 function caused a delay in seed germination. By contrast, upregulation of GIM2 accelerated seed germination, as observed in transgenic lines overexpressing GIM2 (OE). We detected a reduction in endogenous bioactive GA levels and an increase in endogenous ABA levels in the gim2©\ko mutants compared to wild type. Conversely, the OE lines had increased endogenous bioactive GA levels and decreased endogenous ABA levels. The expression levels of a set of GA©\ and/or ABA©\related genes were altered in both the gim2©\ko mutants and the OE lines. We confirmed that GIM2 has dioxygenase activity using an in vitro enzyme assay, observing that GIM2 can oxidize GA12. Hence, our characterization of GIM2 demonstrates that it plays a role in seed germination by affecting the GA metabolic pathway in Arabidopsis.

Abstract (Browse 288)  |  References  |  Full Text HTML  |  Full Text PDF  |  Cited By       
The phytohormone GA plays a critical role in regulating seed germination. We demonstrate how GIM2, a member of 2OGD family, catalyzes the oxidation of GA12 in GA biosynthetic pathway, thus modulating seed germination in Arabidopsis.
Natural variation of hormone levels in Arabidopsis roots and correlations with complex root architecture  
Author: Sangseok Lee, Lidiya I. Sergeeva and Dick Vreugdenhil
Journal of Integrative Plant Biology 2018 60(4): 292-309
Published Online: December 2, 2017
DOI: 10.1111/jipb.12617
      
    

Studies on natural variation are an important tool to unravel the genetic basis of quantitative traits in plants. Despite the significant roles of phytohormones in plant development, including root architecture, hardly any studies have been done to investigate natural variation in endogenous hormone levels in plants. Therefore, in the present study a range of hormones were quantified in root extracts of thirteen Arabidopsis thaliana accessions using a ultra performance liquid chromatography triple quadrupole mass spectrometer. Root system architecture of the set of accessions was quantified, using a new parameter (mature root unit) for complex root systems, and correlated with the phytohormone data. Significant variations in phytohormone levels among the accessions were detected, but were remarkably small, namely less than three©\fold difference between extremes. For cytokinins, relatively larger variations were found for ribosides and glucosides, as compared to the free bases. For root phenotyping, length©\related traits—lateral root length and total root length—showed larger variations than lateral root number©\related ones. For root architecture, antagonistic interactions between hormones, for example, indole©\3©\acetic acid to trans©\zeatin were detected in correlation analysis. These findings provide conclusive evidence for the presence of natural variation in phytohormone levels in Arabidopsis roots, suggesting that quantitative genetic analyses are feasible.

Abstract (Browse 358)  |  References  |  Full Text HTML  |  Full Text PDF  |  Cited By       
A range of hormone levels were quantified in root extracts of Arabidopsis accessions. Except for biologically active hormones, most of their intermediates showed narrow ranges of variation. Correlations between hormone levels and root traits were observed, which showed antagonistic interactions of shaping root system architecture.
          Molecular Physiology
OTS1©\dependent deSUMOylation increases tolerance to high copper levels in Arabidopsis
Author: Erbao Zhan, Huapeng Zhou, Sha Li, Lei Liu, Tinghong Tan and Honghui Lin
Journal of Integrative Plant Biology 2018 60(4): 310-322
Published Online: December 4, 2017
DOI: 10.1111/jipb.12618
      
    

The conjugation of SUMO (small ubiquitin©\like modifier) to protein substrates is a reversible process (SUMOylation/deSUMOylation) that regulates plant development and stress responses. The essential metal copper (Cu) is required for normal plant growth, but excess amounts are toxic. The SUMO E3 ligase, SIZ1, and SIZ1©\mediated SUMOylation function in plant tolerance to excess Cu. It is unknown whether deSUMOylation also contributes to Cu tolerance in plants. Here, we report that OTS1, a protease that cleaves SUMO from its substrate proteins, participates in Cu tolerance in Arabidopsis thaliana (Arabidopsis). OTS1 loss©\of©\function mutants (ots1©\2 and ots1©\3) displayed increased sensitivity to excess Cu. Redox homeostasis and the balance between SUMOylation and deSUMOylation were disrupted in the ots1©\3 mutant under excess Cu conditions. The ots1©\3 mutant accumulated higher levels of Cu in both shoots and roots compared to wild type. Specific Cu©\related metal transporter genes were upregulated due to the loss©\of©\function of OTS1, which might explain the high Cu levels in ots1©\3. These results suggest that the SUMOylation/deSUMOylation machinery is activated in response to excess Cu, and modulates Cu homeostasis and tolerance by regulating both Cu uptake and detoxification. Together, our findings provide insight into the biological function and regulatory role of SUMOylation/deSUMOylation in plant tolerance to Cu.

Abstract (Browse 256)  |  References  |  Full Text HTML  |  Full Text PDF  |  Cited By       
OTS1, a protease that cleaves SUMO from its substrate proteins, participates in Cu tolerance in Arabidopsis thaliana.
          Photosynthesis and Crop Physiology
Overexpression of microRNA408 enhances photosynthesis, growth, and seed yield in diverse plants
Author: Jiawei Pan, Dahui Huang, Zhonglong Guo, Zheng Kuang, He Zhang, Xinyu Xie, Zengfeng Ma, Shaopei Gao, Manuel T. Lerdau, Chengcai Chu and Lei Li
Journal of Integrative Plant Biology 2018 60(4): 323-340
Published Online: January 13, 2018
DOI: 10.1111/jipb.12634
      
    

The ability of a plant to produce grain, fruit, or forage depends ultimately on photosynthesis. There have been few attempts, however, to study microRNAs, which are a class of endogenous small RNAs post©\transcriptionally programming gene expression, in relation to photosynthetic traits. We focused on miR408, one of the most conserved plant miRNAs, and overexpressed it in parallel in Arabidopsis, tobacco, and rice. The transgenic plants all exhibited increased copper content in the chloroplast, elevated abundance of plastocyanin, and an induction of photosynthetic genes. By means of gas exchange and optical spectroscopy analyses, we showed that higher expression of miR408 leads to enhanced photosynthesis through improving efficiency of irradiation utilization and the capacity for carbon dioxide fixation. Consequently, miR408 hyper©\accumulating plants exhibited higher rate of vegetative growth. An enlargement of seed size was also observed in all three species overproducing miR408. Moreover, we conducted a 2©\year©\two©\location field trial and observed miR408 overexpression in rice significantly increased yield, which was primarily attributed to an elevation in grain weight. Taken together, these results demonstrate that miR408 is a positive regulator of photosynthesis and that its genetic engineering is a promising route for enhancing photosynthetic performance and yield in diverse plants.

Abstract (Browse 302)  |  References  |  Full Text HTML  |  Full Text PDF  |  Cited By       
This work shows that a conserved small RNA, miR408, promotes photosynthesis and vegetative growth in diverse plant species. In rice, over expression of miR408 leads to enlarged seed size and increased grain weight without compromising other aspects of plant growth, demonstrating the biotechnological potential of miR408 in crop improvement.
          Plant-biotic Interactions
Xanthomonas axonopodis pv. punicae uses XopL effector to suppress pomegranate immunity
Author: Madhvi Soni and Kalyan K. Mondal
Journal of Integrative Plant Biology 2018 60(4): 341-357
Published Online: November 29, 2017
DOI: 10.1111/jipb.12615
      
    

Xanthomonas axonopodis pv. punicae (Xap) causing bacterial blight is an important pathogen that incurs significant losses to the exportability of pomegranate. Xap uses the Xop TTSS©\effector, via the type three secretion system, to suppress pomegranate immunity. Here, we investigate the role of XopL during blight pathogenesis. We observed that XopL is essential for its in planta growth and full virulence. Leaves inoculated with Xap ΔxopL produced restricted water©\soaked lesions compared to those inoculated with wild©\type Xap. XopL supports Xap for its sustained multiplication in pomegranate by suppressing the plant cell death (PCD) event. We further demonstrated that XopL suppresses immune responses, such as callose deposition and production of reactive oxygen species (ROS). RT©\qPCR analysis revealed that immune responsive genes were upregulated when challenged with Xap ΔxopL, whereas upregulation of such genes was compromised in the complemented strain containing the xopL gene. The transiently expressed XopL::EYFP fusion protein was localized to the plasma membrane, indicating the possible site of its action. Altogether, this study highlights that XopL is an important TTSS©\effector of Xap that suppresses plant immune responses, including PCD, presumably to support the multiplication of Xap for a sufficient time©\period during blight disease development.

Abstract (Browse 315)  |  References  |  Full Text HTML  |  Full Text PDF  |  Cited By       
Bacterial blight causes havoc loss to pomegranate. The pathogen, Xanthomonas axonopodis pv. Punicae employs XopL effector to subvert pomegranate immunity. We demonstrated that XopL acts as a suppressor of immune responses like PCD, callose deposition, ROS production and transcript abundance of defense genes to facilitate bacterial multiplication in the invaded tissues.
 
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