We are studying how the environmental stimuli of light and gravity alter patterns of growth and development in plants. We are using molecular approaches to characterize proteins that are critically involved in the coupling of light and gravity stimuli to growth changes in plants.
In our research on light signaling, we have discovered an enzyme activity that plays a key role in mediating how light activation of the photoreceptor phytochrome leads to a rapid decrease in the growth rate of hypocotyls of etiolated seedlings. The enzyme activity, unexpectedly, is that of two closely related apyrases (NTPDases), enzymes that remove the terminal phosphate from ATP and ADP. The expression of two genes that encode these enzymes in Arabidopsis, APY1 and APY2, is closely correlated with growth: both are strongly expressed in rapidly growing tissue such as etiolated hypocotyls and pollen tubes; the suppression of both of them results in a severely dwarf phenotype. Red-light induced suppression of hypocotyl growth is preceded by red-light induced loss of apyrase protein, which occurs in less than 5 min after irradiation. Light also suppresses the abundance of transcripts encoding these enzymes in less than 15 min.
In light-grown plants a significant fraction of the immunodetectable apyrase protein co-purifies with plasma membranes, with its active site facing to the outside of these membranes. Since ATP is the preferred substrate of this enzyme, its activity in plant walls raises the question of the function of extracellular ATP (eATP). Our data indicate that plant cells release ATP into their walls during their growth, and that the accumulation of this eATP can suppress growth. We are studying how the reduction of [eATP] by the action of ectoapyrases plays a critical role in growth control in plants.
In our gravitational studies, we found that the APY2 gene is strongly expressed in the elongation zone of the root (Wu et al., Plant Physiol., 2007), and the single knockout mutant of APY2 shows significantly reduced gravitropic curvature, as illustrated in a video taken with the Phytomorph system developed in the laboratory of Edgar Spalding (U. Wisconsin). In other studies we have developed a single-cell model system, germinating spores of Ceratopteris richardii, in which gravity orients the direction of a trans-cell calcium current that, in turn, directs the polarity of subsequent nuclear migration, cell division, and rhizoid development and growth. We are using this system to define the genes needed for plants to respond to gravity. As a first step in this investigation we have completed a Shuttle experiment (STS-93, July 1999) that has allowed us to identify genes that are differentially expressed in microgravity, and we are currently examining the role of these genes in mediating the gravity response.