How do organelles “see” the environment and how do they “talk” to each other?

The aim of this discovery area is to understand aspects of inter-organelle and intercellular communication. Our goal is to discover key genes, metabolites and proteins that function in signalling pathways that mediate environmental signals leading to altered plant development. Our approach is to dissect critical signalling events in response to environmental stimuli and developmental cues (e.g., stresses such as changes in light intensity, temperature shock and water and/or nutrient supply) at the genetic, biochemical and physiological level. We anticipate that one outcome from an understanding of these processes will be the development of tools and strategies to modulate the function of select signalling pathways, thereby enabling a rational approach to directed plant development.

To understand how organelles talk to each other, to the rest of the cell, and to other tissues we aim to:

• Discover how organelle function is regulated by environmental cues and affected by abiotic stresses.
• Identify components in the signal transduction pathways that detect and respond to these signals
• Identify how the expression of genes encoding organelle proteins is regulated by these signal transduction pathways
• Examine select aspects of the communication between organelles and plant development processes.

Research Highlight

Comparative proteomics of light stress

Dr Britta Forster (Centre affiliate), Dr Mathesius (ARC Centre of Excellence in Integrative Legume Research) and Barry Pogson at ANU produced a two-dimensional reference map of the soluble sub-proteome representing about 1500 proteins in the green alga Chlamydomonas reinhardtii, which is a model system to study photosynthesis and light stress. Comparative proteomics of wild type and two very high light resistant mutants revealed complex alterations in response to excess light. Britta was invited to speak on this research at the 12th International Conference on the Cell & Molecular Biology of Chlamydomonas, Portland, USA.

Forster B, Mathesius U, Pogson BJ (2006) Comparative proteomics of high light stress in the model alga Chlamydomonas reinhardtii.Proteomics 6:4309-20

How is organelle function regulated by environmental cues and affected by abiotic stress?

Employing the diverse and cutting-edge technology platforms available at the Centre, we are able to undertake the measurement of changes in the key plant energy processes of photosynthesis, respiration and photorespiration in response to environmental signals such as changes in light, water availablility and temperature. Arabidopsis plants with defined mutations or having phenotypes indicative of stress phenomena are being generated and analysed at multiple molecular levels, using our technologies to identify:

• alterations in gene transcription
• changes in the organelle proteomes, notably the detection of proteins damaged or modified in response to stresses

• altered metabolite fluxes and induction of novel metabolites

• shifts in targeted cellular processes such as photosynthesis, mitochondrial electron transport and production of protective antioxidants.

These data will be integrated with relevant data from the Centre’s organelle biogenesis and metabolomics discovery activities, and with publicly available datasets, using the Centre’s capacity for a “systems” approach to plant biology.

What components in the signal transduction pathways detect and respond to environmental cues?

To identify key genes, proteins and metabolites that function in the signal transduction pathways that detect and respond to signals we are applying reverse genetics, forward genetics, and alterations to key components of energy metabolism, in combination with proteomics and metabolomics.

The reverse genetics approach has already revealed a number of transcription factors that are up-regulated in leaves of plants transferred from low to high light. Work has begun to isolate and analyse transgenic plants with alterations in expression of these transcription factors either using over-expression or insertional knockouts. Employing forward genetics we have generated a novel set of Arabidopsis mutations that result in altered responses to drought and high light intensity. We are determining the molecular basis for the key responses we have identified. Using a combination of metabolomic and genomic approaches we are investigating the coordination of energy and redox metabolism throughout the plant cell.

Research Hightlight

Uncovering a link between responses to drought and high light

Dr Jan Bart Rossel (ARC APD), Pip Wilson (PhD student), Dr Luke Hendrickson (Research Fellow) and others at the ANU, CSIRO and Essex, UK, have identified links between drought and high light responses using Arabidopsis drought-tolerant mutants. Rossel and Pogson were invited to speak at Keystone Symposia: Plant responses to Abiotic Stress in Colorado, USA and 17th International Botanical Congress, Vienna, Austria on this and related work.

Rossel JB, Walter PB, Hendrickson L, Chow W, Poole A, Mullineaux P, Pogson BJ (2006) A mutation affecting ascorbate peroxidase 2 gene expression reveals a link between responses to high light and drought tolerance. Plant, Cell and Environment, 29:269-281.

Communication between organelles and plant development processes

Changes in organelle function often profoundly affect plant development, indicating that the cellular processes of energy biology are required for correct formation of tissues and growth of the whole plant. Organelle biogenesis and activity must be coordinated with developmental processes, implying organelle signalling pathways that are for the most part unknown. The mechanisms of inter-organelle, organelle-to-nucleus and intercellular signalling are complex and multifaceted. To model this signalling we have chosen to focus on the biosynthesis of carotenoid pigments and their regulatory interactions. This choice has the potential to deliver both economic and social benefit, in that carotenoids are essential not only for human nutrition and photosynthesis, but also play a role in the synthesis of known and novel plant hormones. Carotenoids can act as signalling molecules as indicated by the interaction of the plant growth regulator, auxin, with a novel carotenoid-derived hormone to alter the number of branches that a plant produces. We have identified two nuclear mutations that regulate the expression of two different steps in the carotenoid biosynthetic pathway, and are studying their role in these processes.