Editorial
The plant vascular system II: From essential functions in resource allocation, interorgan communication and defense, to evolution of the monocot cambium
William J. Lucas, Professor
Special Issue Editor
Department of Plant Biology, University of California, Davis, CA 95616, USA
Chun-Ming Liu, Director General
Special Issue Editor
Institute of Crop Science, CAAS, Beijing, China
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In this Special Issue, a focus is also placed on the role of the xylem as an essential conduit for the
long-distance delivery of water and mineral nutrients from the soil to the vegetative (aboveground) regions of the plant. Xylem cells destined to form tracheids or vessel members, which
will make up the conduit for this water and mineral transport from the roots to the shoots,
undergo apoptosis, a process of programmed cell death. In their review, Venturas et al. (2017)
provide an in-depth analysis of the processes underlying the passage of water through these
mature, and thus, dead, xylem conducting cells. They elegantly describe the physics associated
with this transport of water from the soil to the above-ground tissues and organs of the plant. A
tensional gradient, within the water column, pulls the transpiration stream through the plant and
Venturas et al. (2017) address the challenges that this presents to the plant, including the
unthinkable, in that the water column can rupture, a process termed cavitation. The authors
provide an up-to-date analysis of the debate as to how plants might refill such cavitated xylem
cells, an important topic with respect to tissue hydraulics. This review closes with an insightful
section on the impact of climate change on xylem function.
Four research articles are also included in this Special Issue. Leach et al. (2017) addressed the
role of the vacuole in sucrose storage in their paper, entitled “Sucrose transporter2 contributes to
maize growth, development, and crop yield.” Subcellular localization studies established that the
Zea maize SUCROSE TRANSPORTER2 (ZmSUT2) is localized to the tonoplast, and
physiological experiments conducted on the mzsut2 mutant revealed that leaf sugar levels were
elevated relative to wild-type maize. Importantly, mutant plants were significantly negatively
impacted in terms of overall growth and yield. Thus, this study elegantly shows that
remobilization of vacuolar stored sucrose makes an important contribution to the availability of
fixed carbon for plant growth, ear development and corn grain yield.
The relationship between nitrogen (amino acid) transport through the phloem and fixed carbon
transport, allocation and growth was explored by Santiago and Tegeder (2017) in their paper,
entitled “Implications of nitrogen phloem loading for carbon metabolism and transport during
Arabidopsis development.” In Arabidopsis, the AMINO ACID PERMEASE 8 (AAP8) transport
system function in amino acid loading into the phloem, and these authors used an aap8 mutant to
study the effects of reduced amino acid levels in the phloem on growth and carbon metabolism
during the plant life cycle. The results from these studies were intriguing, as although growth of
the aap8 mutant plants was reduced, with a concomitant decline in biomass, transport of sucrose
to vegetative sink tissues remained unaffected. This situation changed in the aap8 plants during
reproductive development, when in contrast to control plants, photosynthesis decreased, as did
sucrose transport into the siliques. This study indicates that phloem loading of amino acids can
impact carbon allocation to specific sinks, in a development-dependent manner.
Competition for photosynthate can occur not only between various sink tissues and organs, but
also between plant cells and invading pathogens. For example, corn smut disease in maize is
caused by the basidiomycete, Ustilago maydis, and this biotrophic fungus utilizes the plant host
as a nutrient source. This aspect of ‘carbon competition’ is explored by Wittek et al. (2017) in
their paper, entitled “The fungal UmSRT1 and maize ZmSUT1 sucrose transporters in their
battle for plant sugar resources.” A biophysical characterization of the sucrose-H+ cotransport
properties of UmSRT1 and ZmSUT1 was first conducted in Xenopus oocytes. This information
was then used in numerical analysis assays that simulated the interface between the fungus and
the maize cells neighboring the phloem. This study showed that the poor maize SUT1
transporter cannot compete against the UmSRT1 system for apoplasmic sucrose. Indeed, the high
affinity of UmSRT1 for sucrose establishes a thermodynamic gradient such that ZmSUT1 is
reversed in direction, causing it to export sucrose into the plant-fungal interface.
During angiosperm evolution, the vascular cambium was lost from the monocot lineage, and as a
consequence, most monocots are not capable of secondary growth. However, in some monocots,
a novel lateral meristem is formed, termed the monocot cambium (MC), and this gives rise to
stem thickening in plants such as Yucca and Cordyline. Zinkgraf et al. (2017) in their paper,
entitled Transcript profiling of a novel plant meristem, the monocot cambium”, delve into the
likely molecular mechanism underlying the formation and function of this unique MC that
generates an outer secondary cortex and inner secondary vascular bundles (SVB). Interestingly,
these SVB have an atypical cellular arrangement in that, unlike in the dicots, the phloem cells are
located in the center and are surrounded by xylem cells. Based on transcriptomics analyses of
MC and derivative tissues from Yucca and Cordyline and the vascular cambium derived from
poplar and eucalyptus, the authors advance the hypothesis that the evolution of this unique MC
involved cooption of genes and regulatory mechanisms that operate within the vascular
cambium.
William J. Lucas, Professor
Special Issue Editor
Department of Plant Biology
University of California
Davis, CA 95616, USA
Chun-Ming Liu, Director General
Special Issue Editor
Institute of Crop Science, CAAS
Beijing, China
REFERENCES
Leach KA, Tran TM, Slewinski TL, Meeley RB, Braun DM (2017) Sucrose transporter2
contributes to maize growth, development, and crop yield. J Integr Plant Biol 59:
Santiago JP, Tegeder M (2017) Implications of nitrogen phloem loading for carbon metabolism
and transport during Arabidopsis development. J Integr Plant Biol 59:
Ventura MD, Sperry JS, Hacke UG (2017) Plant xylem hydraulics: What we understand, current
research, and future challenges. J Integr Plant Biol 59:
Wittek A, Dreyer I, Al-Rasheid KAS, Sauer N, Hedrich R, Geiger D (2017) The fungal UmSrt1
and maize ZmSUT1 sucrose transporters battle for plant sugar resources. J Integr Plant Biol
59:
Zinkgraf M, Gerttula S, Groover A (2017) Transcript profiling of a novel plant meristem, the
monocot cambium. J Integr Plant Biol 59: