Genetic Regulation of Meristem Indeterminacy and the Evolution of Plant Development.

E-mail: emengs@wm.edu

Mailing Address: Department of Biology, P.O. Box 8795, Williamsburg, Virginia  23187-8795

Visiting Research Professor of Biology (Initial appointment: 2005.  Assistant Professor of Biology 2005-2012)

Ph.D. Stanford University (2002)

B.A. Reed College (1993)

The role of HAM genes in regulating meristem indeterminacy--

Indeterminate growth, that is, the capacity for the continuous generation and growth of organs and tissues  throughout the life-cycle of an organism, is a fundamental feature of plant development.  Stems and roots elongate (primary growth) and increase in girth (secondary growth).  Leaves, the primary photosynthetic organs of most plants, are disposable organs, retained for a period of time before being discarded via programmed senescence and abscission, to be replaced by generation of new leaves.  This capacity for continuing organogenesis and growth throughout the life-span permits plants to adaptively regulate their development in response to dynamic environments, which, as sessile organisms, they cannot relocate away from in response to adverse conditions.  Indeterminate growth further permits some species of woody perennials to live for thousands of years. 

In plants, indeterminate growth is endowed by shoot and root meristems.  Leaves, floral organs, and stems are derived from shoot meristems, located at shoot apices.  Root meristems, internal meristems located immediately above the columella of root apices, generate the radially organized tissues of the root.  The primary shoot apical and root apical meristems arise during embryogenesis, while secondary meristems arise de novo during post-embryonic development.  Meristems must balance two competing functions; specification of determinate tissues, which reduces the pool of undifferentiated and pluripotent “stem cells”, and maintenance of indeterminacy, which requires the retention of a stem cell niche, from which cells lost to differentiating tissues may be replaced.  

We are working to understand the role of HAM proteins, members of the plant-specific GRAS family of transcriptional regulators, in regulating shoot and root indeterminacy.  HAM loss-of-function mutants exhibit progressive loss of organ indeterminacy in both the shoot and the root, demonstrating an absolute requirement for HAM proteins in growth and development.  However, the cellular and molecular basis of HAM function is almost entirely unknown.  We are using genetic and molecular genetic approaches to generate new loss and gain-of-function ham alleles and to investigate the relationship of HAM signaling to other, generally better characterized, pathways regulating meristem structure and function.  Many of our current approaches take advantage of the fact that HAM genes are targets of post-transcriptional regulation by microRNAs  

The evolution of ethylene signaling--

Ethylene is a phytohormone required for a broad suite of responses to biotic and abiotic stressors, and for regulation of growth and differentiation.  Ethylene-mediated processes, including germination, flowering, fruit ripening and senescence, are major agricultural concerns.  Ethylene is synthesized from the metabolic precursor S-adenosyl-methionine (SAM) in a two stage process mediated by the enzymes ACC SYNTHASE (ACS) and ACC OXIDASE (ACO).  Synthesis of ACC from SAM by ACS is frequently the rate-limiting step in ethylene biosynthesis.

Synthesis of ethylene by ACS and ACO is generally regarded as a trait restricted to seed plants, based largely upon ACC feeding experiments that have failed to induce elevated ethylene production in more basal plant lineages.  However, many basal plants exhibit well documented responses to ethylene and possess homologs of many or all of the major molecular components of ethylene perception characterized in flowering plants.  We are employing a combination of molecular phylogenetics, and quantitative characterization of ethylene production and ethylene responses in Selaginella moellendorffii in an effort to understand if ethylene is an endogenous hormone in Selaginella and to better understand the evolutionary history of ethylene signaling.

Specificity in hormone signaling may be generated through regulation of hormone biosynthesis, transport, and perception.  Transport is not generally regarded as a point of regulation in ethylene signaling, as ethylene is a highly mobile, highly soluble gas, but regulation of ethylene biosynthesis is a well characterized point of regulation in ethylene signaling.  ACOs have received comparatively little attention relative to ACSs, and the common assumption that ACOs are constitutively expressed and active has been subjected to limited experiment testing.  We are examining the expression and function of ACOs in Arabidopsis thaliana.