The plant hormone auxin regulates virtually every aspect of plant growth and development. Much of this control occurs by auxin regulating the fundamental processes of cell division, cell expansion, and cell differentiation. Given the prominent role of auxin in these basic cellular events, it is hardly surprising that plant biologists have long been intrigued by this hormone and have compiled an enormous amount of physiological data concerning the responses of plants to applied auxin. Over the past decade, impressive molecular insight into the molecular mechanisms underlying auxin signaling has been obtained, with the SCFTIR1 ubiquitin-ligase complex emerging as a central regulator. In response to auxin, SCFTIR1 catalyzes the ubiquitin-mediated degradation of members of the Aux/IAA protein family. The degradation of these negative regulators of auxin response derepresses the pathway, resulting in auxin-mediated changes in gene expression and plant growth and development.
The large family of Small Auxin Up RNA (SAUR) genes are among the most rapidly and strongly auxin-induced genes. A primary focus of our current research is the elucidation of SAUR protein function. Utilizing multiple reverse genetic strategies, we have found that many SAUR proteins play important roles in auxin-mediated cell expansion. Mechanistically, this involves SAUR inhibition of members of the D-subfamily of type 2C protein phosphatases (PP2C.D). These PP2C.D phosphatases control the phosphorylation status and hence activity of plasma membrane H+-ATPases, which for over 40 years have been hypothesized to play a central role in the ‘acid-growth’ theory of auxin-mediated cell expansion. However, molecular/biochemical evidence linking auxin to changes in PM H+-ATPase activity has been scant. Our findings that auxin-induced SAURs inhibit PP2C.D activity to trap PM H+-ATPases in the phosphorylated, active state has provided a molecular framework for the acid-growth theory of cell expansion. Ongoing projects in the lab include the elucidation of SAUR-PP2C.D physical interactions; the identification and characterization of distinct SAUR-PP2C.D regulatory modules and their roles in cell expansion, stomatal regulation, and other processes; phosphoproteomic studies aimed at identifying additional phosphoproteins subject to regulation by SAUR-PP2C.D regulatory modules; and the engineering of plants to modulate SAUR/PP2C.D expression in a tissue-specific manner to alter organ size.
We are grateful to the NIH and NSF for supporting our work.
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