Research in the Brusslan Lab

My lab is interested in the phenotype and physiological changes that occur when plants are grown at different light intensities, a process called photoacclimation.  The figure below shows Arabidopsis thaliana plants transferred to three different light intensities for four days of growth.  In low light, leaves are round and green and petioles are long.  As light intensity increases, leaves become smaller, petioles shorten, the purple pigment anthocyanin is produced and total chlorophyll levels decrease.

Our lab originally focused on changes that occur in chlorophyll levels during photoacclimation.  We cloned the chlorophyll a oxygenase gene from Arabidopsis thaliana and demonstrated that there was only one gene that was responsible for chlorophyll b synthesis (Espineda et al., 1999).  We later showed that CAO mRNA levels increase when plants are transferred to lower light intensity (Harper et al., 2004).  In this paper we also showed that a leaky mutation in the H subunit of Mg-chelatase, an enzyme that chelates Mg to the porphyrin ring for chlorophyll synthesis, has reduced chlorophyll b levels and reduced CAO mRNA after transfer to moderate light.  In WT, CAO mRNA levels decrease transiently after 1-2 days in moderate light, while in the Mg-chelatase mutant, cch1, mRNA levels decreased, and remain low even after 7 days.  The Elip1 gene was used as a control to indicate that the plants were undergoing light stress after tranfer to moderate light intensity.

The cch1 mutation was found to be allelic to gun5, a mutation selected on the basis of reduced interorganelle communication between the plastid and the nucleus (Mochizuki et al., 2001).  These mutants have decreased levels of Mg-protoporphyrin, and also have decreased repression of the Lhcb1 mRNA levels during treatment with the carotenoid biosynthesis inhibitor, Norflurozon.  Subsequently, our lab published a review on plastid-nuclear signaling (Brusslan and Peterson, 2002). 

In the last two years, our lab has focused on two genes that encode chloroplast-localized peptidases.  This work is currently funded by an NSF RUI grant.   One gene encodes a glutamyl endopeptidase (AtcGEP, MEROPS family S9D) while the other encodes and acyl aminopeptidase (AtcAAP, MEROPS family S9C).   Interest in these genes stems from a microarray experiment that compared WT and cch1 after transfer to moderate light intensity.  cGEP mRNA levels are increased during senescence, but cgep T-DNA insertion mutants senesce normally.  cAAP mRNA levels are constitutive, but caap T-DNA insertion mutants green more slowly.  We have also made mutations in the cAAP ortholog in Synechocystis 6803, and these mutations demonstrate a mild stress phenotype.

Our lab is also intereted in the regulation of the early-light inducible protein 1 gene (Elip1).  The RNA gel blot, above, demonstrates that Elip1 mRNA levels are highly induced after exposure to higher light intensity.  We have produced promoter::GUS fusions to determine which regions of the promoter are responsible for high light inducibility.  We have performed site-directed mutagenesis on three promising light regulatory elements (LRE) located within 216 bp of the start of transcription, but have not yet defined an element necessary for high light induction.  We are continuing our efforts in site-directed mutagenesis, currently testing four additional motifs in the 216 bp region.

In addition, ongoing studies of the virescent, vir2, mutant continue.  This mutant, shown below, greens more slowly than WT. The location of the gene has been narrowed down to 23 genes on chromosome 1, but the gene identity is still not known.

 

Our lab also is involved in a collaboration among many CSULB researchers who are studying the round stingray population (Urobatis halleri) at Seal Beach, CA.  We take samples every month, and are in the process of genotyping the population using microsatellite loci.  Initial results suggest a stable population of round rays at Seal Beach.