Abstract Detail



Paleobotany

Tomescu, Alexandru [1].

Origin of conducting tissues: revisiting sterilization.

Given the crucial role of conducting tissues in plant biology, their origin has seen surprisingly little scrutiny.  Recent studies indicate that class III HD-ZIP genes (C3HDZ) are expressed in vascular tissue in all tracheophytes and in sporogenous tissue in all embryophytes. These data suggests ancestral functions in sporangium/spore development for C3HDZs across embryophytes, and in vascular tissue development, across tracheophytes.  Despite these distinct ancestral functions, embryophyte C3HDZ phylogenies indicate that the tracheophyte ancestor inherited a single C3HDZ from bryophyte-grade precursors, implying that this ancestral gene had roles in both reproductive and vascular development.  These considerations are relevant to the origin of vascular tissues and the evolution of their developmental regulation.  The early euphyllophyte Psilophyton exhibits conspicuous anatomical continuity between the tissues of sporangia and subtending axes, and direct contact between the vascular strand entering the sporangium and the spore mass.  The intimate association of these specialized tissues suggests continuity of their precursor tissues, which entails shared identity and underlying regulation, thus, evolutionary-developmental relatedness of sporogenous and vascular tissues.  At the turn of the 20th century, F.O. Bower hypothesized that progressive sterilization of an expanding sporophyte was a central driver in the evolution of complex sporophyte organization.  The common regulatory and developmental origin of sporogenous and vascular tissues suggested by molecular data and fossils, fits a scenario of vascular tissue evolution by sterilization of sporogenous tissue: an ancestral C3HDZ specifies pre-sporogenous identity in the central region of the sporophyte, which expands into a central strand as sporophytes evolve larger size through delayed reproductive development and extended vegetative growth; during central strand differentiation, delayed initiation of the regulatory program that completes reproductive development allows for expression of an enhancer for vascular identity.  The ancestral tracheophyte C3HDZ role in the pre-sporogenous-provascular pathway could have originated in bryophytes: in mosses, C3HDZs are expressed in sporangium and seta, as well as gametophyte conducting tissues, and have demonstrated potential for the type of interaction with miR165/166 that regulates vascular development in tracheophytes.  Testing this sterilization hypothesis entails identifying the different putative mechanisms that could have inhibited deployment of the reproductive program and initiated vascular development in the precursor tissue specified by C3HDZs.  Potential candidate mechanisms can be identified based on knowledge of the regulation of vascular development and the transition to reproductive development.  These will require testing, alongside detailed developmental studies, in mosses that possess large sporophytes and well-developed gametophytic and sporophytic conducting tissues.


1 - Humboldt State University, 1 Harpst St., Arcata, CA, 95521, United States

Keywords:
conducting tissues
sporogenous tissue
xylem
Evo-Devo
evolution
sterilization
HD-ZIP
Psilophyton
bryophyte
moss
tracheophyte
C3HDZ.

Presentation Type: Oral Paper
Number: 0007
Abstract ID:961
Candidate for Awards:None


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