Oscillations in extracellular pH and reactive oxygen species modulate tip growth of Arabidopsis root hairs

Abstract

Root hairs show highly localized cell expansion focused to their growing tips. This growth pattern is accomplished through restriction of secretion to the elongating apex and modulation of cell wall properties, with the wall just behind the tip becoming rigidified to resist the lateral expansive forces of turgor. In this report we show that root hairs exhibit oscillating growth that is associated with oscillating increases in extracellular pH and reactive oxygen species (ROS), which lag growth by Ϸ7 s. Consistent with a role for these changes in growth control, artificially increasing extracellular pH arrested root hair elongation, whereas decreasing pH elicited bursting at the tip. Similarly, application of exogenous ROS arrested elongation, whereas scavenging of ROS led to root hair bursting. Roots hairs of the root hair-defective rhd2-1 mutant, which lack a functional version of the NADPH oxidase ATRBOH C, burst at the transition to tip growth. This phenotype could be rescued by elevating the pH of the growth medium to >6.0. Such rescued root hairs showed reduced cytoplasmic ROS levels and a lack of the oscillatory production of ROS at the tip. However, they exhibited apparently normal tip growth, including generation of the tip-focused Ca 2؉ gradient thought to drive apical growth, indicating that ATRBOH C is not absolutely required to sustain tip growth. These observations indicate that root hair elongation is coupled to spatially distinct regulation of extracellular pH and ROS production that likely affect wall properties associated with the polarized expansion of the cell.

Imaging of the Yellow Cameleon 3.6 Indicator Reveals That Elevations in Cytosolic Ca2+ Follow Oscillating Increases in Growth in Root Hairs of Arabidopsis

“…While this has been confirmed for different species of pollen tubes by independent groups (Pierson et al, 1996;Messerli and Robinson, 1997;Watahiki et al, 2004;Hwang et al, 2005), it has not been repeated for tip growth in fungal hyphae (LopezFranco et al, 1994;Sampson et al, 2003). Oscillating growth in root hairs has only recently been reported (Monshausen et al, 2007) and is under further investigation in this report.…”

“…Until recently, whether such oscillatory patterns represent an element in the process of root hair tip growth has been unclear. Figure 1 shows that when made with sufficient resolution, oscillations in growth rate could be monitored in root hairs of Arabidopsis with a frequency of two to four peaks per minute (see also Monshausen et al, 2007). During these oscillations, elongation decelerated and accelerated, with growth rates rising to up to three times basal levels.…”

“…Applying high-resolution tip-tracking software previously used to measure growth of Arabidopsis root hairs (Messerli et al, 1999;Monshausen et al, 2007), we were able to confirm that growth rates of YC3.6-expressing root hairs oscillated at the same frequency of two to four peaks per minute as apical Ca 21 levels ( Fig. 1D) and as in untransformed wild-type root hairs (Monshausen et al, 2007). The magnitude of both the oscillations in Ca 21 and growth rate were variable even within a single root hair ( Fig.…”

“…It has even been suggested that a burst of secretion may precede elongation in Agapanthus pollen (Coelho and Malhó, 2006). Lily pollen tubes in culture (for review, see Messerli et al, 2000) grow five to seven times faster than Arabidopsis (Arabidopsis thaliana) root hairs in culture (Monshausen et al, 2007), and it is unclear whether an equivalent relationship between oscillatory growth and Ca 21 changes exists for the root hair. Although a stretch-activated Ca 21 influx channel has been identified in pollen tubes (Dutta and Robinson, 2004), the Ca 21 channel sustaining the tip-focused Ca 21 gradient in root hairs is thought to be gated by membrane voltage and reactive oxygen species (ROS; Foreman et al, 2003).…”

Windows to cell function and dysfunction: Signatures written in the boundary layers

“…These can include the use of freely diffusible fluorescent reporters with pH sensitivity, such as fluorescein. (71) More powerfully, from our perspective, is the transfection and expression of pH sensitive molecules, such as pHluorin (72) on the surface of cells. Expressing cells can then be aggregated into pseudo-tissues or 'tumor' masses, the former exploiting advances in the field of tissue engineering where dielectrophoretic methods are used to control cell position generating controlled aggregations.…”