Title: Leaf patterning in maize: establishment and maintenance during development and growth

Speaker: Josh Strable, NSF Postdoctoral Fellow, School of Integrative Plant Science, Plant Biology Section, Cornell University

Abstract: Understanding the genetic control of leaf patterning and growth is a central topic in plant biology with important applications in agriculture.  Maize leaves—composed of the distal blade and supporting midrib, the proximal sheath and the hinge-like auricle and accompanying ligule—are a striking display of early patterning events.  Variations in the morphology of sheath, ligule, auricle and/or blade tissues can dramatically impact leaf architecture, ultimately influencing canopy structure.  Two fundamental questions motivate my research on leaf development: what are the factors that pattern leaves, and how is patterning maintained during organogenesis and growth of the leaf?  I will present vignettes that illustrate how I have approached these questions and will introduce my future plans to leverage development, genetics and genome editing to understand basic plant biology and to apply to crop improvement.

In a collaborative project, I sought to identify early genetic determinants of proximal-distal patterning of maize leaves.  We used laser microdissection followed by RNA sequencing to discover boundary-like expression profiles across four contiguous microdomains at the base of early leaf primordia.  This work uncovered 1,045 differentially expressed genes across the microdomains.  We found proximal expression of knotted1-like homeobox genes, which are known to play key roles in regulating development of the blade-sheath boundary, along with other boundary genes.  I used expression and genetic interaction analyses to reveal boundary functions of previously uncharacterized, proximally-enriched zinc-finger homeodomain genes.  This study underscores the complexity of genes and networks that pattern organ boundaries during leaf development.

A parallel and ongoing project is to understand the function of ethylene signaling genes in maize, and to examine natural variation of ethylene sensitivity in diverse inbred lines.  The hormone ethylene is a key regulator of plant development and growth.  Ethylene is also a pivotal stress response signal, making its production and/or the pathways it regulates potential targets for crop adaptation to climate extremes.  However, ethylene response is not well understood in maize or related cereals.  To this end, I characterized mutations in ZmETHYLENE INSENSITIVE3-LIKE (ZmEIL) genes, maize homologs of the Arabidopsis EIN3/EIL1 master regulators of ethylene response.  Stacking independently-derived mutations in ZmEIL genes conditions a range of leaf and internode phenotypes in higher-order Zmeil mutants.  Zmeil mutants are largely insensitive to the ethylene precursor ACC and mutant phenotypes are sensitive to genetic background and environment.  These findings indicate ZmEIL genes are necessary for normal leaf morphology and plant architecture.  To evaluate natural variation of ethylene response, I screened coleoptile, mesocotyl and root growth in maize diversity lines in the presence of ACC under dark-grown conditions.  Preliminary observations suggest that maize exhibits significant tissue-specific variation in ethylene sensitivity.  These analyses provide novel insights into the understanding of ethylene response in maize.

Host: Steve Whitham