Research Accomplishment Reports 2007

Environmental and Genetic Determinants of Seed Quality and Performance

A.B. Downie, D.B. Egli, C.C. Baskin, J.M. Baskin, S.E. Perry, R.L. Geneve
Department of Horticulture

 

Project Description

Knowledge of seed germination can be used to advance the efficiency with which agricultural products are grown. My goal is to understand the physiological/molecular events controlling seed germination and how these events are influenced by the external environment.

Using mutant screens of arabidopsis I discovered a key component in the phytochrome signal transduction pathway. In many plant species, seed germination, hypocotyl elongation, chlorophyll biogenesis, etc. are regulated by the phy system, exclusively or in concert with other photoreceptors. Our identification of the ctg10 f-box focused attention on the scf ub ligase group (1 of 4 ub ligase groups in plants) as causal for pif degradation (another component of phy signal transduction). Characterization of this aspect of phy signal transduction will revolutionize identification of ctg10 homologues and elucidation of this step in photomorphogenesis, in plant (including crop) species.I am exploring the functional significance of protein repair mechanisms with Prof. Steven Clarke, UCLA. The proteome is subject to deleterious alteration over time. One repair process involves protein-l-isoaspartate methyltransferase (ec 2.1.1.77; pimt) converting isoaspartyl (isoasp) residues, formed spontaneously from l-aspartate or l-asparagine, back into l-aspartate, usually recovering some or all of the substrate protein's function/catalytic capacity.

Plants producing seeds capable of surviving dehydration require additional repair capacity for their proteome which, in the dehydrated state, is subjected to extended periods of stress and ionic insult. Exposure to stress increases isoaspartyl formation in many species. By ascertaining the protein targets most requiring pimt-based repair, and/or by increasing endogenous pimt activity, we hope to extend the longevity of seeds in storage.

Mr. Jianchang Gao is collaborating in studies of brownseed1 mutation in tomato. The bs1 mutation is one of five mutations in tomato causing darker testa color, tougher testa, poor percentage and speed of seed germination, enhanced activity of at least one enzyme capable of scavenging reactive oxygen species, and for some, increased fruit pH and delayed flowering time and turning stage. Despite the obvious phenotype produced in the testa, the mutation is not strictly dependent on the maternal genotype! The inheritance of mutations led us to discover a communication between the endosperm/embryo (paternal genetic composition) and the testa (strictly maternal genotype) whereby the endosperm/embryo influence testa color and physical toughness. This inheritance pattern is distinct from that of all known transparent testa (arabidopsis) and anthocyaninless (tomato) mutations that are totally dependent on the maternal plant genotype. None of the brownseed genes or the blackseed gene has been cloned to date. We are attempting to clone bs1. Dr. Pradeep Kachroo (Plant Pathology) and I achieved a significant breakthrough identifying a class of small, endogenous molecules that evoked plant protection from a fungus.

Impact

In the project on phy signal transduction, leaders in photobiology throughout the world have expressed intense interest in this finding and this has led to three collaborative agreements between my lab and that of, Prof. Richard Vierstra, Univ. Wisconsin, Madison; Prof. Peter Quail, Univ. California, Berkeley; and Dr. Enamul Huq, University of Texas, Austin, TX. This project has been the focus of a collaborative research proposal between Dr. Huq and I that, while ranked excellent, was not funded. It has been resubmitted on the urging of the NSF panel manager.

With Steven Clarke (UCLA) and Randy Dinkins, (USDA) we have discovered that knock down of both arabidopsis protein isoaspartyl methyltransferase genes results in poor completion of germination, as does germination of any of the pimt single knockdown mutants on micromolar amounts of cycloheximide (paper in preparation). We determined that the second pimt in arabidopsis is under a complex transcriptional control which allows the proteins derived from these various MMAs to be targeted throughout the cell (paper now accepted in plant journal). In addition, using phage display, we have identified 5 potential pimt target proteins in seeds (paper in preparation). This NSF funded project will continue into 2009, is the subject of a grade 11 student's research project (Ms. Alyssa Eliopolous), and possibly that of a visiting scientist (Dr. Mohamed Karduoush) from Aleppo University, Syria, if we can arrange his visa.

For the brownseed1 cloning project, we have sequenced 190,000 contiguous bp on chromosome 1 and discovered 30 molecular markers between lycopersicon esculentum (currently styled solanum esculentum) and S. pimpinellifolium. Mr. Gao discovered two markers (sslp8 and caps8) that bracket the gene of interest with less than 4 centimorgans between them (~3 million base pairs). Mr. Gao is using additional markers, of his own design, to bracket bs1 within a shorter interval.