Special Issue: Plant vascular biology (1)   

May 2017, Volume 59 Issue 5, Pages 289每344.

Cover Caption: Plant vascular biology (1)
The cover shows secondary vascular bundles, produced by a unique monocot cambium, in which the phloem is centrally-located, surrounded by a xylem ring. In this issue, first insights into the genetic regulation and evolution of this fascinating meristem are presented in support of the hypothesis that genes regulating this vascular cambia were coopted in the evolution of this novel meristem that is found within some species of the Asparagales.


The plant vascular system I: From resource allocation, inter-organ communication and defense, to evolution of the monocot cambium  
Author: William J. Lucas and Chun-Ming Liu
Journal of Integrative Plant Biology 2017 59(5): 290每291
Published Online: March 22, 2017
DOI: 10.1111/jipb.12541
Abstract (Browse 406)  |  References  |  Full Text HTML  |  Full Text PDF  |  Cited By       
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What actually is the M邦nch hypothesis? A short history of assimilate transport by mass flow  
Author: Michael Knoblauch and Winfried S. Peters
Journal of Integrative Plant Biology 2017 59(5): 292每310
Published Online: March 9, 2017
DOI: 10.1111/jipb.12532

In the 1920s, the German forestry scientist Ernst Münch postulated that photo-assimilate transport is a mass flow driven by osmotically induced pressure gradients between source organs (high turgor) and sink organs (lower turgor). Two crucial components of Münch's hypothesis, the translocation by mass flow from sources to sinks and the osmotic mechanism of pressure flow, were established notions at the time, but had been developed by two institutionally separated groups of scholars. A conceptual separation of whole-plant biology from cellular physiology had followed the institutional separation of forestry science from botany in German-speaking central Europe during the so-called Humboldtian reforms, and was reinforced by the delayed institutionalization of plant physiology as an academic discipline. Münch did not invent a novel concept, but accomplished an integration of the organism-focused and the cell-focused research traditions, reducing the polarization that had evolved when research universities emerged in central Europe. Post-Münch debates about the validity of his hypothesis focused increasingly on the suitability of available research methodologies, especially the electron microscope and the proper interpretation of the results it produced. The present work reconstructs the influence of the dynamic scientific and non-scientific context on the history of the Münch hypothesis.

Abstract (Browse 408)  |  References  |  Full Text HTML  |  Full Text PDF  |  Cited By       
The phloem pressure-flow model for long-distance transport in plants is well supported by experimental data. This review reconstructs the history of the concepts underlying this mass flow hypothesis. The authors examine the impact on the field of a forester, Ernst M邦nch, who in the 1920ies successfully integrated organismal and cellular biology approaches to study the physiology of the phloem.
Sucrose transporters and plasmodesmal regulation in passive phloem loading  
Author: Johannes Liesche
Journal of Integrative Plant Biology 2017 59(5): 311每321
Published Online: April 21, 2017
DOI: 10.1111/jipb.12548

An essential step for the distribution of carbon throughout the whole plant is the loading of sugars into the phloem in source organs. In many plants, accumulation of sugars in the sieve element-companion cell (SE-CC) complex is mediated and regulated by active processes. However, for poplar and many other tree species, a passive symplasmic mechanism of phloem loading has been proposed, characterized by symplasmic continuity along the pre-phloem pathway and the absence of active sugar accumulation in the SE-CC complex. A high overall leaf sugar concentration is thought to enable diffusion of sucrose into the phloem. In this review, we critically evaluate current evidence regarding the mechanism of passive symplasmic phloem loading, with a focus on the potential influence of active sugar transport and plasmodesmal regulation. The limited experimental data, combined with theoretical considerations, suggest that a concomitant operation of passive symplasmic and active phloem loading in the same minor vein is unlikely. However, active sugar transport could well play an important role in how passively loading plants might modulate the rate of sugar export from leaves. Insights into the operation of this mechanism has direct implications for our understanding of how these plants utilize assimilated carbon.

Abstract (Browse 420)  |  References  |  Full Text HTML  |  Full Text PDF  |  Cited By       
In plants, phloem loading can occur by a number of mechanisms, including active sugar loading. Passive phloem loading, through the symplasm, has been proposed as the mechanism operating in many trees. Here, current evidence is evaluated regarding this loading mechanism, and the potential influence of protein-mediated sugar transport and regulation of cell coupling is discussed.
Molecular regulation of sucrose catabolism and sugar transport for development, defence and phloem function  
Author: Jun Li, Limin Wu, Ryan Foster and Yong-Ling Ruan
Journal of Integrative Plant Biology 2017 59(5): 322每335
Published Online: March 17, 2017
DOI: 10.1111/jipb.12539

Sucrose (Suc) is the major end product of photosynthesis in mesophyll cells of most vascular plants. It is loaded into phloem of mature leaves for long-distance translocation to non-photosynthetic organs where it is unloaded for diverse uses. Clearly, Suc transport and metabolism is central to plant growth and development and the functionality of the entire vascular system. Despite vast information in the literature about the physiological roles of individual sugar metabolic enzymes and transporters, there is a lack of systematic evaluation about their molecular regulation from transcriptional to post-translational levels. Knowledge on this topic is essential for understanding and improving plant development, optimizing resource distribution and increasing crop productivity. We therefore focused our analyses on molecular control of key players in Suc metabolism and transport, including: (i) the identification of promoter elements responsive to sugars and hormones or targeted by transcription factors and microRNAs degrading transcripts of target genes; and (ii) modulation of enzyme and transporter activities through protein-protein interactions and other post-translational modifications. We have highlighted major remaining questions and discussed opportunities to exploit current understanding to gain new insights into molecular control of carbon partitioning for improving plant performance.

Abstract (Browse 485)  |  References  |  Full Text HTML  |  Full Text PDF  |  Cited By       
Powered by solar energy, mature leaves convert CO2 and water into sucrose for translocation to the rest of the plant through the phloem. Here, we evaluate the molecular regulation of key enzymes and carriers responsible for sucrose transport and metabolism in relation to development, defence and yield potential.
Transport of chemical signals in systemic acquired resistance  
Author: Archana Singh, Gah-Hyun Lim and Pradeep Kachroo
Journal of Integrative Plant Biology 2017 59(5): 336每344
Published Online: March 17, 2017
DOI: 10.1111/jipb.12537

Systemic acquired resistance (SAR) is a form of broad-spectrum resistance induced in response to local infections that protects uninfected parts against subsequent secondary infections by related or unrelated pathogens. SAR signaling requires two parallel branches, one regulated by salicylic acid (SA), and the other by azelaic acid (AzA) and glycerol-3-phosphate (G3P). AzA and G3P function downstream of the free radicals nitric oxide (NO) and reactive oxygen species (ROS). During SAR, SA, AzA and G3P accumulate in the infected leaves, but only a small portion of these is transported to distal uninfected leaves. SA is preferentially transported via the apoplast, whereas phloem loading of AzA and G3P occurs via the symplast. The symplastic transport of AzA and G3P is regulated by gating of the plasmodesmata (PD). The PD localizing proteins, PDLP1 and PDLP5, regulate SAR by regulating PD gating as well as the subcellular partitioning of a SAR-associated protein.

Abstract (Browse 375)  |  References  |  Full Text HTML  |  Full Text PDF  |  Cited By       
Systemic acquired resistance (SAR) is a form of broad-spectrum resistance against plant pathogens that is regulated by signals including salicylic acid, azelaic acid and glycerol-3-phosphate. This review summarizes the role of these chemical signals in SAR and their differential transport through apoplasmic and symplasmic pathways.
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