Research Interests
Proline metabolism and its role in stress resistance, osmoregulation and osmotic adjustment, abscisic acid signaling and metabolism
Research Summary
The ability of plants to adapt to a wide range of adverse environmental conditions including drought, temperature extremes and salinity is a key factor in maintaining crop productivity and in determining the distribution of plant species across natural environments. My laboratory is interested in how plants respond to changes in water availability. To respond to a change in water availability, a plant must first sense the change in water status, most likely through a loss of water and change in cell volume or turgor. This this initial event causes a serious of down stream signaling and responses needed for adaptation (Figure 1). My laboratory's interest is focused on two of these responses: proline metabolism and osmoregulation. As proline metabolism is controlled in part by abscisic acid (ABA), we also have an active interest in ABA perception, metabolism and signaling.
Proline Metabolism
Accumulation of proline and changes in proline metabolism occur in response to many stresses including drought and salinity as well as biotic stresses such as pathogen infection. Why proline accumulation occurs and how it may enhance stress resistance are still matters of active debate. The accumulation of proline under stress likely involves changes in both its synthesis and its degradation; however, the contribution of these processes to proline accumulation and the regulatory mechanisms controlling proline metabolism are not well understood. Proline synthesis occurs in the cytoplasm while proline catabolism occurs in the mitochondria, forming a "proline cycle" (Figure 2). It has been proposed that proline synthesis may occur predominantly in the shoot while proline degradation is active in root tissue under stress. Whether such a proline cycle actually operates under stress, the transporters and regulatory proteins involved, and its significance in stress resistance are not known.
My laboratory is addressing these questions through forward and reverse genetics using both T-DNA knockout mutants of genes related to proline accumulation as well as map based cloning and analysis of new mutants isolated in a screen for altered proline accumulation (Verslues and Bray, 2004). We also seek to identify proteins that interact with and may regulate the key enzymes of proline metabolism.
Osmoregulation
All organisms must control cell volume or, in walled cells, turgor. This is done largely by controlling the amount of solutes present inside the cell and thus controlling the osmotic gradient for water movement into or out of the cell. To do this effectively, cells must also be able to sense and respond to changes in the environment that cause water to flow into or out of the cell. These poorly understood processes are referred to as osmosensing and osmoregulation and are the subject of active research across the range of biology. In plants, there is even more reason to be interested in these topics as accumulation of additional solutes to prevent water loss, referred to as osmotic adjustment, is often studied by agronomists as one factor that may help plants better withstand drought stress.
By measuring the total solute content (osmotic potential) and relative water content after low water potential (reduced water availability) treatment on agar plates, the osmoregulatory properties of Arabiopsis thaliana seedlings can be rapidly quantified. Previous screening for mutants with altered proline accumulation also identified several mutants with altered osmoregulation (Verslues and Bray 2004). We are using map based cloning to identify the mutated genes as well as additional screening and reverse genetics to identify still other genes involved in osmoregulation.
ABA signaling
ABA is required for proline accumulation in stressed plants yet ABA applied to unstressed plants elicits a much lower level of proline accumulation (Verslues and Bray 2006). This could be caused by an ABA-independent effect of stress on proline or a stress-induced change in the response to ABA. Thus, in addition to being of interest in its own right, proline accumulation can also be a marker for altered ABA signaling. The Verslues laboratory is also investigating proline accumulation mutants that may be altered in their ABA response.
Figure 1: Conceptual diagram of the cellular events in a plant cell responding to water loss such as would occur during soil drying. Water loss from the cell is perceived by an unknown mechanism which then activates upstream signaling events. These upstream signaling events then activate the synthesis and accumulation of ABA which is in turn perceived by specific ABA receptors which activate more down stream responses such as changes in gene expression. The response to ABA is also modified by internal signals that reflect the metabolic and developmental state of the plant. The perception of water loss and upstream signaling also activate other, ABA-independent events, such as osmoregulatory accumulation of solutes. Proline accumulation appears to respond both to these osmoregulatory events and to ABA signaling. Thus proline accumulation is of interest both in its own right as an important stress adaptation mechanism but can also serve as an indicator of more upstream signaling events. The Verslues lab is interested in the sensing and signaling events that control osmoregulation and the metabolic changes needed for stress-induced proline accumulation.
Figure 2: Diagram of major metabolic pathways related to proline metabolism. Proline synthesis in the cytoplasm and proline degradation in the mitochondria may form a "proline cycle". The importance of this proline cycle in stress resistance as well as the the importance of intracellular and intercellular transport and the ornithine versus glutamate pathways of proline synthesis remain poorly understood.
Verslues, P.E., and Zhu, J.-K. (2007). New developments in abscisic acid perception and metabolism. Current Opinion in Plant Biology 10, 447-452.
Batelli, G., Verslues, P.E., Agius, F., Qiu, Q., Fujii, H., Pan, S.-q., Schumaker, K.S., Grillo, S., and Zhu, J.-K. (2007). SOS2 promotes salt tolerance in part by interacting with the vacuolar H+-ATPase and upregulating its transport activity. Molecular and Cellular Biology doi:10.1128/MCB.00430-07
Verslues, P.E., Batelli, G., Grillo, S., Agius, F., Kim, Y.S., Zhu, J., Agarwal, M., Katiyar-Agarwal, S., and Zhu, J.-K. (2007). Interaction of SOS2 with NDPK2 and catalases reveals a point of connection between salt stress and H2O2 signaling in Arabidopsis. Molecular and Cellular Biology doi:10.1128/MCB.00429-07
Fujii, H., Verslues, P.E., and Zhu, J.K. (2007). Identification of two protein kinases required for abscisic acid regulation of seed germination, root growth, and gene expression in Arabidopsis. Plant Cell 19, 485-494.
Verslues, P.E., Kim, Y.S., and Zhu, J.K. (2007). Altered ABA, proline and hydrogen peroxide in an Arabidopsis glutamate: glyoxylate aminotransferase mutant. Plant Molecular Biology 64, 205-217.
Verslues, P.E., Agarwal, M., Katiyar-Agarwal, S., Zhu, J.H., and Zhu, J.K. (2006). Methods and concepts in quantifying resistance to drought, salt and freezing, abiotic stresses that affect plant water status. Plant Journal 45, 523-539.
Verslues, P.E., Guo, Y., Dong, C.H., Ma, W., and Zhu, J.K. (2006). Mutation of SAD2, an importin beta-domain protein in Arabidopsis, alters abscisic acid sensitivity. Plant Journal 47, 776-787.
Verslues, P.E., and Bray, E.A. (2006). Role of abscisic acid (ABA) and Arabidopsis thaliana ABA-insensitive loci in low water potential-induced ABA and proline accumulation. Journal of Experimental Botany 57, 201-212.
Borsani, O., Zhu, J., Verslues, P.E., Sunkar, R., and Zhu, J.-K. (2005). Endogenous siRNAs derived from a pair of natural cis-antisense transcripts regulate salt tolerance in Arabidopsis. Cell 123, 1279-1291.
Zhu, J.H., Verslues, P.E., Zheng, X.W., Lee, B., Zhan, X.Q., Manabe, Y., Sokolchik, I., Zhu, Y.M., Dong, C.H., Zhu, J.K., Hasegawa, P.M., and Bressan, R.A. (2005). HOS10 encodes an R2R3-type MYB transcription factor essential for cold acclimation in plants. Proceedings of the National Academy of Sciences of The United States of America 102, 9966-9971.
Verslues, P.E., and Zhu, J.K. (2005). Before and beyond ABA: upstream sensing and internal signals that determine ABA accumulation and response under abiotic stress. Biochemical Society Transactions 33, 375-379.
Verslues, P.E., and Bray, E.A. (2004). LWR1 and LWR2 are required for osmoregulation and osmotic adjustment in Arabidopsis. Plant Physiology 136, 2831-2842.
Verslues, P.E., and Sharp, R.E. (1999). Proline accumulation in maize (Zea mays L.) primary roots at low water potentials. II. Metabolic source of increased proline deposition in the elongation zone. Plant Physiology 119, 1349-1360.
Verslues, P.E., Ober, E.S., and Sharp, R.E. (1998). Root growth and oxygen relations at low water potentials. Impact of oxygen availability in polyethylene glycol solutions. Plant Physiology 116, 1403-1412.
Our laboratory is often looking for new members. Interested people are welcome to email Paul Verslues with a description of your qualifications, experience and how you could contribute our laboratory’s objectives.
Institute of Plant and Microbial Biology, Academia Sinica, Taipei, Taiwan, Republic of China
Tel: +886-2-27899590 Fax: +886-2-27827954
