A high-resolution map of auxin distribution in the Arabidopsis root apex.

The gradient and dynamic flow of the plant growth regulator auxin (indole-3-acetic acid [IAA]) in shoot and root apices is thought to be a critical factor in determining meristematic activity (i.e., the maintenance of stem cell niches and the regulated development of organ primordia) (reviewed in Leyser, 2006; Tanaka et al., 2006). Therefore, determining the precise role of auxin and its regulation will require determination of its concentration, biosynthesis, and transport parameters on a cell-specific basis in and around meristematic regions. However, thus far mainly indirect methods have been used to estimate auxin concentrations and distribution patterns within plant tissues, such as the use of IAAresponsive promoter elements to drive marker gene expression. Now, Petersson et al. (pages 1659–1668) present a direct method for quantification of IAA in specific cell types using a combination of fluorescence-activated cell sorting (FACS) and precise mass spectrometry–based IAA quantification and biosynthesis measurements and produce a high-resolution auxin distribution map of the Arabidopsis root apex. Their results provide direct support for the existence of an auxin gradient having a distinct maximum in the organizing quiescent center of the root apex. They also show that all of the cells in the root apex display a high capacity for auxin synthesis, indicating that local biosynthesis and polar transport of auxin combine to produce auxin gradients and maxima in the root tip. The authors made use of 14 transgenic Arabidopsis lines expressing green fluorescent protein (GFP) driven by promoters having distinct expression patterns in the root apex. Protoplasts were isolated from root tissue of these lines and were sorted by FACS into GFP-expressing cells and nonexpressing cells, and IAA was then quantified using gas chromatography–selected reaction modemass spectrometry. The data from the different GFP-expressing Arabidopsis lines were combined to construct an IAA distribution map for the root apex (see figure). In addition, rates of IAA synthesis were measured in four of the lines representing all of the cell types in the root apex, which revealed a high capacity for IAA synthesis in all regions of the root apex. This approach allowed for the cell-specific quantification of IAA accumulation and biosynthesis rates from lines with GFP expression in as few as four to seven cells per root. The IAA distribution map confirmed the existence of a gradient of IAA concentration within the root apex, with the highest IAA concentration occurring in the quiescent center cells. Relatively high IAA concentrations also were observed in the cortex, endodermis, and apical parts of the stele, whereas the columella and epidermis showed lower levels than the background average. Taken together with the IAA biosynthesis data, the results suggest that the auxin gradient in the root apex is in a highly dynamic state, which is likely maintained by a combination of biosynthesis, transport, and catabolism/conjugation. In addition to providing a precise map of auxin distribution in the root apex, this work also provides important information on the relationship between steady state auxin and auxin-responsive reporter signals, which are often used as an indicator of auxin activity.