Plants are sessile organisms that are continuously impacted by changes in the environment. As a response a multitude of signal transduction cascades controls the development and metabolism of the plant such that it is continuously adapted to match the environmental challenges. In many instances it is the extent and direction of growth that is changed. Plant growth results from the formation of new cells during division and from the subsequent massive increase in volume during expansion of these newly formed cells. Ongoing research on both processes mainly exploits the model plant Arabidopsis thaliana
, which represents one of the best experimental systems to study developmental processes in higher plants. Its whole genome is sequenced [1
], the development occurs in a highly predictive and well-defined pattern [2
] and cellular growth in the stem or the root can be easily monitored by means of microscopy [3
]. Along the root axis, cells pass through different developmental stages. In the apical meristem, near the root tip, new cells are continuously formed by repeated cycles of division. Upon leaving the meristem, these cells pass the transition zone where they are physiologically and mechanically prepared to undergo rapid elongation. In the zone of rapid elongation these cells increase their length and volume by 300% in three hours. In the adjacent differentiation zone, the cells acquire their final shape and functions. At the root surface this can be seen as the emergence of root hairs on specific epidermal cells, the trichoblasts [4
The massive increase in cell volume contributes substantially to the growth of plants. During the expansion, be it along several (e.g. leaf cells) or only one axis (e.g. root and hypocotyl cells), the cell wall is a centre of activity. Cellulose is the main constituent of the primary cell wall of vascular plants and forms the load-bearing network together with tethering xyloglucans (the main hemicellulose in plants like Arabidopsis). This network is laid down in a highly hydrophilic matrix, which contains pectins and structural proteins like arabinogalactan proteins (AGPs) and hydroxyproline-rich glycoproteins (HRGPs) [5
]. Cellular growth results from the spatial separation of cellulose microfibrils, which requires modifications of the interconnecting xyloglucans and a force that pushes the microfibrils apart. The former is done by several classes of cell wall remodeling proteins, such as expansins [6
] and xyloglucan endotransglucosylase/hydrolases (XTHs; [7
]), both of which are proven to loosen walls, and by different enzyme activities as described in Frankova et al. [9
]. The latter is provided by turgor pressure generated inside the cell. The process of cell expansion/elongation is highly complex and needs tight control as cell lysis by excessive turgor pressure or too loose walls needs to be prevented at all time. As mentioned, several players are known to contribute to this process, but the mechanism by which hormones and stressors exert control remains partly elusive.
In previous work it was shown that the gaseous plant hormone ethylene, administered as its precursor 1-aminocyclopropane-1-carboxylic acid (ACC), can reduce cell elongation in a concentration-dependent manner [3
]. On the cellular level this inhibition is irreversible, root cells that ceased elongation cannot regain this. At the root level on the other hand, removal of ACC leads to normal elongation of these cells that are newly formed in the ACC-free condition. Some answers to the question how ethylene/ACC controls the maximal cell size in roots are found in the published literature and will be briefly discussed here. It is broadly accepted that for normal expansion to occur, expansins need a slightly acidic environment [10
]. It is documented that ethylene/ACC exerts its effect on cell size by altering the auxin content in specific cells in the treated roots by modifying auxin transport and/or bio-synthesis [11
]. As a result plasma membrane H+
-ATPases are locked in their low-activity state, leading to an alkalinisation of cell walls instead of acidification and interfering with expansin-driven weakening of the walls [12
]. At the same time peroxidase-mediated cross-linking activity in the cell wall further prevents cell expansion [13
], resulting in the observed cell elongation phenotype. Since interference with the alkalinisation [12
] or cross-linking activity [13
] never restores growth to 100%, this clearly indicates that other yet to be discovered actors are at play. This study represents an attempt to reveal new actors in the control of cell elongation.
We will reveal differential gene expression levels between control and 3h ACC-treated Arabidopsis roots using CATMA microarray analysis, validate these by quantitative PCR analysis of some of the most altered genes and discuss the ethylene-mediated control of cell expansion. It is striking that genes coding for known cell wall loosening actors are down regulated, and genes coding for specific cell wall components together with their cross-linking enzymes are upregulated. The analysis of the 240 differentially expressed genes reveals that many genes identified in this ACC-evoked response, are also involved in other stress responses. This suggests that many responses may originate from individual elicitors, but that they may converge to a ’common pathway’ further downstream. Moreover, this microarray analysis identified several potential keyplayers, such as transcription factors and auxin-responsive genes, that await further analysis to reveal their exact role in the control of cell elongation.