ASU SoLS Faculty: David M. Rhoads

David Rhoads received a Ph.D. degree in Biochemistry from Michigan State University in 1992, was as an NSF Postdoctoral Fellow in Plant Biology at Duke University and North Carolina State University, and was a Postdoctoral Fellow at the University of Nebraska, Lincoln.

Our primary research focus is to understand how cellular compartments (organelles) communicate with the nucleus to alter gene expression. We are especially interested in understanding how this communication may function under stress conditions that affect organelle function. More specifically, we have focused on studying how plant mitochondria can affect nuclear gene expression (called mitochondrial retrograde regulation, or MRR) when 1) respiration or carbon metabolism is impaired, which can occur during plant stress; and 2) when plants are exposed to heat stress. Very little progress has been made in understanding MRR in higher plants. We have developed one new system to study MRR associated with plant stress and are studying the classical MRR response to chemical inhibition of respiration using new approaches.

To better understand MRR in plants, we are applying basic molecular genetic techniques, plant molecular biology and biochemistry, forward and reverse plant genetics and genomics approaches. Classical genetics has long been used to approach difficult biological problems. We are using classical genetic techniques with a twist. The twist is to use molecular genetic and plant molecular biology techniques to create an easily observable phenotype. So, for our forward genetic approaches, we produce transgenic lines of Arabidopsis thaliana (Arabidopsis) that contain the firefly luciferase gene coding region under the control of a promoter (gene expression control regions) taken from a gene of interest that responds to specific mitochondrial stress/dysfunction, such as disruption of respiration, and then produce mutant plants for further study. Altered gene expression is relatively easily assayed in whole plants using our photon-counting, super-cooled, CCD camera system and computer analyses. Plants glow when the transgene (promoter region plus luciferase) is turned-on by specific stimuli, such as disruption of respiration. This is our easily observable phenotype. Mutant plants with altered MRR patterns (transgenic plants that do not glow) following disruption of respiration have been isolated through our genetic screens. Using genetic techniques, mutated genes will be identified as important components of the communication pathways between plant mitochondria and nuclear gene expression.

For reverse genetic approaches, we developed transgenic Arabidopsis plants with under- or over-expression of mitochondrial small heat shock protein (mt sHSP) genes. Over-expression of a specific mt sHSP provides increased thermotolerance and, most significantly to us, causes dramatically altered nuclear gene expression patterns (especially of other HSP genes) in response to heat stress. This suggests that plant mitochondria can act as stress sensors that respond to stress by contributing to alterations in nuclear gene expression. This also provides a genetically approachable system to study the contribution of MRR to stress responses in plants. Also, Arabidopsis plants with low or no expression of mt sHSP genes are being utilized to determine the effects of lack of these proteins on thermotolerance and on MRR in response to heat stress.

Functional genomics is a powerful new way to comprehensively examine changes in gene expression of an organism following a specific stimulus. We are using functional genomics to analyze: 1) changes in whole genome expression patterns in response to specific alterations in mitochondrial function, such as impaired respiration or carbon metabolism; 2) the same responses in mutant plants; and 3) changes in whole genome expression patterns in response to heat stress in plants with altered expression of HSPs. These studies are funded by the NSF.

Selected Publications

Rhoads DM, Umbach AL, Subbaiah CC & Siedow, JN (2006) Mitochondrial ROS: contribution to oxidative stress and inter-organellar signaling. Plant Physiology, in press.

Subbaiah CC, Palaniappan A, Duncan K, Rhoads DM, Huber SC, Sachs MM (2006) Mitochondrial localization and putative signaling function of sucrose synthase in maize. J Biol Chem, in press.

White SJ, George RM & Rhoads DM. The response of plants to heat stress includes mitochondria-to-nucleus signaling. In preparation.

Zarkovic J, Yu J, Ruckle ME, Umbach AL, McIntosh L, Hock JJ, Bingham S, White SJ, George RM & Rhoads DM. Mitochondrial dysfunction in leaves causes extensive nuclear gene expression changes directed at defense against pathogens. Plant Molecular Biology. Submitted.

Dojcinovic D, Krosting J, Harris AJ, Wagner DJ & Rhoads DM (2005) Identification of regions of the Arabidopsis AtAOX1a promoter important for developmental and mitochondrial retrograde regulation of expression. Plant Mol. Biol. 158: 159-175.

Zarkovic J, Anderson SL & Rhoads DM (2005) A reporter gene system used to study developmental expression of alternative oxidase and isolate mitochondrial retrograde regulation mutants in Arabidopsis. Plant Mol. Biol. 57: 871-888.

Rhoads DM, White SJ, Zhou Y, Muralidharan M & Elthon TE (2005) Altered gene expression in plants with constitutive expression of a mitochondrial small heat shock protein suggests the involvement of retrograde regulation in the heat stress response. Physiol. Plant. 123: 435-444.

Rhoads DM & Vanlerberghe GC (2004) Mitochondria-Nucleus Interactions: Evidence for mitochondrial retrograde communication in plant cells. In Advances in Photosynthesis and Respiration. DA Day, AH Millar, J Whelan, eds. Kluwer Academic Publishers, Dordrecht, The Netherlands. Vol 17, pp. 83-106.

Lund AA, Rhoads DM, Lund AL, Cerny RL, & Elthon TE (2001) In vivo modifications of the maize mitochondrial low molecular weight heat stress protein, HSP22. J. Biol. Chem. 276: 29924-29929.

Rhoads DM, Umbach AL, Sweet C, Lennon AM, Rauch GS & Siedow JN (1998) Regulation of the cyanide-resistant alternative oxidase of plant mitochondria: Identification of the cysteine residue involved in a-keto acid stimulation and intersubunit disulfide bond formation. J. Biol. Chem. 273: 30750-30756.

Rhoads DM, Kaspi CI, Levings CS, III & Siedow JN (1994) N, N'-Dicyclohexylcarbodiimide cross-linking suggests a central core of helices II in oligomers of URF13, the pore-forming T-toxin receptor of cms-T maize mitochondria. Proc. Natl. Acad. Sci. U.S.A. 91: 8253-8257.

Rhoads DM & McIntosh L (1992) Salicylic acid regulation of respiration in higher plants: alternative oxidase expression. Plant Cell 4: 1131-1139.

Rhoads DM & McIntosh L (1991) Isolation and characterization of a cDNA clone encoding an alternative oxidase protein of Sauromatum guttatum (Schott). Proc. Natl. Acad. Sci. U.S.A. 88: 2122-2126.