Roberto A. Gaxiola Ariza, Ph.D.

Title
Associate Professor
Department
Department of Cellular & Molecular Biosciences
Institution
Arizona State University
Address
427 E. Tyler Mall
City, State, Zip
Tempe, AZ 85287-4501
Country
United States
Phone
(480) 965-3337
E-mail
roberto.gaxiola[at]asu.edu
Website
https://sols.asu.edu/people/roberto-gaxiola
Research Field
Plant Genetics
Award Year
1995
Country Of Origin
Mexico
Mentor Name
Gerald R. Fink, Ph.D.

Research

Up-regulation of the vacuolar H+-PPase had a very unexpected effect in plant growth and development. Arabidopsis AVP1OX plants AVP1-1 and AVP1-2 had more rosette leaves (3 and 16, respectively) and significantly greater leaf area (60 and 40% respectively, p<0.05) than wild type plants. Furthermore, AVP1-1 and AVP1-2 exhibited enhanced root growth and dry weight (2.6 and 9.4X, p < 0.01) compared to wild type. Thus, AVP1 appears to function in both shoot and root development. Our initial hypothesis, that was still vacuole-centric, predicted that the cells of the AVP1OX plants would be larger than controls due to enhanced vacuolar functions. This hypothesis was soon challenged when we learned that the enhanced biomass of these plants was due to increased cell numbers. So how could we explain that the overexpression of a vacuolar H+-PPase enhances cell division and biomass? In order to solve this puzzle we had to question the established notion that the H+-PPase functions exclusively at the tonoplast and basically as a backup device to the ubiquitous V-ATPase (Sze et al., 1999; Sze et al., 2002). By integrating several disciplines in plant biology, including hormone metabolism, development, transport, and cell biology, and through the establishment of collaborations with US and international experts in the different fields, we demonstrated that increases and decreases in the expression of AVP1 affect the distribution and abundance of the plasma membrane H+-ATPase with implications in apoplastic (cell wall space) and rhizosphere acidification (acidification of the environment surrounding the root). Furthermore, our data showed that AVP1 plays a key role in development through facilitating the fluxes of auxin (plant growth hormone) that regulate organogenesis in Arabidopsis (Li et al., 2005). Is there a common denominator? However, auxin fluxes do not explain the increase in shoot and root biomass triggered by the H+-PPase up-regulation. How can we explain these phenotypes? Is there a common denominator? Our current working hypothesis is that the up-regulation of the H+-PPase AVP1 enhances sucrose fluxes from source to sink tissues via improving phloem sucrose loading capacity. Sucrose produced by photosynthesis, is the cornerstone of higher plant metabolism. It is the main substrate for respiration and biosynthesis. Thus, an enhanced availability of sucrose could explain both larger and more energized root systems with an enhanced apoplast and rhizosphere acidification capacity. Apoplast acidification is known to enhance auxin transport and rhizosphere acidification is a central mechanism for plant mineral nutrition. In a sense a domino effect for integral plant growth and development. We are using a combined genetic, molecular biology, biochemistry, cytology and ultra-structural approach to address our main hypothesis.