Systems Biology of Gibberellin Induced Plant Cell Growth

the growth-repressing DELLA proteins. It was also possible to predict a gradient in DELLA proteins which provides a reasonable explanation for the observation of a reduction in growth exhibited in cells close to the end of the elongation zone (Band et al., 2012). The authors also simulated systems in which GA levels were reduced by genetic or pharmacological approaches. Accordingly by modeling these scenarios it was possible to predict how dilutioninduced spatial variations in DELLA concentration can explain the growth dynamics of Arabidopsis root cells (Band et al., 2012). In a similar vein a recent purely experimentally based comparison of molecular changes in transcript and metabolite levels demonstrated that a low GA level mainly affects growth by uncoupling growth from carbon availability (Ribeiro et al., 2012). It is now more than 50 years ago that GA was first proposed to act as an inhibitor of an inhibitor of growth (Stowe and Yamaki, 1959), a concept reinforced by the recently identified molecular mechanisms of GA action (Davière et al., 2008; Yamaguchi, 2008; Achard and Genschik, 2009). The GA-responsive inhibitors of plant growth proposed by Brian (1957) are now known as DELLA proteins (usually referred to as DELLAs). DELLAs are a subfamily of the GRAS family of putative transcriptional regulators and are named for a conserved set of N-terminal amino acids (Asp-Glu-Leu-Leu-Ala; Bolle, 2004). It is well known that GA binds its receptor, GID1, to form a complex that mediates the degradation of DELLAs (Hedden and Thomas, 2012). By this means, GAs relieves DELLA-dependent growth repression and therefore control plant growth by regulating the degradation of growth-repressing DELLA proteins (Sun, 2011; Hedden and Thomas, 2012). It is noteworthy that GA also regulates the expression of GID1, GA20ox, By analogy to their animal counterparts, plant hormones were originally recognized as regulators of growth and development (van Overbeek, 1966; Galston and Davies, 1969; Santner and Estelle, 2009). They have subsequently been demonstrated to achieve this by modulating both processes in response to both intrinsic and environmental cues (Wolters and Jurgens, 2009). Accordingly it is currently accepted that hormones not only exert intrinsic growth control but also mediate adaptation of plant development to transiently changing conditions (Wolters and Jurgens, 2009). Equally accepted is the fact that they play key roles in the regulation of immune responses to microbial pathogens, insect herbivores, and beneficial microbes (Pieterse et al., 2012). In brief the classical developmental growth regulators are considered as auxin, brassinosteroid, cytokinin, and gibberellin (GA), whilst abscisic acid, ethylene, jasmonic, and salicylic acid are often implicated in stress responses (Browse, 2009; Vlot et al., 2009; Wolters and Jurgens, 2009). However, the very recent identification of strigolactones as a further developmental growth regulators (Gomez-Roldan et al., 2008; Umehara et al., 2008; Alder et al., 2012) suggests that our current list of phytohormones may not yet be complete. Growth hormone studies over the past 20 years have been dominated by the analysis of mutants and transgenic plants. These studies provided us with extensive “parts lists” of genes involved in hormone synthesis and catabolism or encoding receptors and signaling components for most classical hormones (Hedden and Phillips, 2000; Hedden, 2003; Yamaguchi, 2008; Wolters and Jurgens, 2009). However, although many components of the plant hormone signaling network have been identified in this way, less consideration has been given to understanding the precise mechanisms by which hormones can regulate plant growth and development per se. Two recent elegant studies have integrated experimental data and multi-scale mathematical modeling approaches in order to explore the interplay between GAs signaling and the root elongation zone (Band et al., 2012) and to investigate the role of the various feedback loops in GA signaling (Middleton et al., 2012). In these ground-breaking studies the authors utilize experimental data to validate their model by comparing predicted mRNA levels with that obtained from transcriptomics. As such they not only provide important fundamental insight into hormonally regulated plant growth but also serve as powerful examples of the marriage of experimental and theoretical approaches in plant systems biology. Understanding the development of root systems is vital in efforts toward maintaining/improving food security. Roots both provide anchorage and facilitate acquisition of water and nutrients from the soil with growing roots exploring their local environments in order to exploit these resources (Lynch, 1995). The understanding of the regulation of developmental and physiological process underlying plant growth is of particular importance since greater growth will lead to a larger leaf area which may ultimately compound final gains in productivity. Thus in order to explore the relationship between GAs and root growth Band et al. (2012) used a multi-scale mathematical model associated with measurements of root growth dynamics and investigated the distribution of GA within the root elongation zone. A significant gradient in GA levels was simulated in the model based on the fact that rapid cell expansion is associated with an expected dilution of GA. The incorporation of the GA signaling network allowed an in silico simulation of how GA would affect downstream components including Systems biology of gibberellin induced plant cell growth