Jason W. Reed (PI), Punita Nagpal (co-PI). Proposal number IOS-0920418: Functions of Arabidopsis SAUR proteins.
Intellectual merit of the proposed activity The plant hormone auxin regulates plant growth and development by largely unknown mechanisms that include induction of gene expression and regulating cell polarity. The largest class of auxin-inducible genes are a family of SAUR (Small Auxin Up-Regulated) genes of unknown function. SAUR genes are present in all flowering plants and also in moss, and therefore may carry out a function conserved in all plants. This proposal is to determine functions of a subfamily of SAUR genes and the corresponding proteins in the model plant Arabidopsis thaliana. Preliminary results indicate that these SAUR genes regulate elongation of rapidly growing tissues, possibly by affecting auxin transport or some other cell biological function. Genetic and physiological approaches will be used to determine the developmental functions of the selected SAUR genes, their expression patterns, and whether they indeed affect auxin transport. Microscopy experiments will reveal the intracellular localization of SAUR proteins, and whether this is regulated by endogenous or environmental signals. These experiments will establish when and where these SAUR proteins are present, their effects on auxin tranport, and how they may control growth of plant tissues. Broader impact of the proposed activity Understanding how growth is controlled is a necessary prelude to developing a systems-level understanding of plant development incorporating inputs from multiple different hormonal and environmental signals. Knowledge from these experiments may ultimately enable practical approaches to manipulate growth patterns in crop plants. Since SAUR genes are present even in bryophytes, the study will be relevant to development of all plants. In the course of this research one post-doc and one graduate student as well as multiple undergraduates will be trained in research approaches and specific techniques.
Intellectual merit Plants are very sensitive to environmental conditions such as light, temperature, and humidity. Plants sense these environmental variables, and integrate the information to adjust their growth to ambient conditions. Much of this signal integration occurs through changes in levels of or responses to endogenous plant hormones. These in turn signal changes in gene expression or other cellular functions. For example, the growth-promoting hormones auxin and brassinolide can each activate a set of SAUR (Small Auxin Up RNA) genes, whereas the stress hormone absciscic acid can cause the same genes to be turned off. Multiple SAUR genes are present in all land plants, but the biochemical functions of SAUR proteins are not known. Members of a family of nine Arabidopsis SAUR genes were expressed in growing parts of the shoot. Arabidopsis plant lines with decreased expression of these genes had decreased hypocotyl (seedling stem) and flower organ elongation. Conversely, plants expressing fusion proteins that increased the activity of these proteins had increased hypocotyl and flower organ growth. Together these results indicate that these SAUR proteins promote growth in multiple tissues. A SAUR63:HA fusion protein (carrying a small tag to allow monitoring) was partly localized to cell membranes, suggesting that SAUR63 might act through interactions with membrane proteins such as transporters. The circadian rhythm affected SAUR63:HA gene transcription and hence protein level. Bright light increased SAUR63:HA protein half life. These and other studies reveal that several hormonal and environmental signals regulate SAUR protein levels. Fusions of SAUR62 or SAUR63 proteins to the fluorescent tag GFP had much greater growth-promoting activity than did the SAUR63:HA protein, and also caused stems to grow in erratic directions (see picture). The SAUR63:GFP fusion protein was much more stable than SAUR63:HA and it localized exclusively to the plasma membrane. Thus, increased stability and/or membrane localization may explain why SAUR63:GFP protein had higher activity. The plants with increased SAUR63 activity were useful tools to assess how the protein might work. For example, the increased growth conferred by SAUR63 required transport of the hormone auxin, suggesting that SAUR63 might affect auxin transport. Broader impacts One post-doc and several undergraduates were trained in research methods.