Description:

Tip growth and cell motility are highly directed forms of cellular motion. Tip growth is a form of polarized growth where exocytosis, leading to cell expansion, occurs at a single localized site, giving rise to long cylindrical or conical structures. Invasion by many pathogenic fungi and reproduction of higher plants is dependent on tip growth. Cell motility on the other hand involves migration of the entire cell. Many types of cells are able to migrate, including supporting glia of the nervous system, macrophages of the immune system and cancer cells. Both tip growth and cell motility can be directed by external cues to guide a population of cells that would normally grow/move in random directions. Our research has focused on the ionic control and signal transduction pathways that direct normal tip growth and cell motility during the imposition of external guidance cues.

Progress

Tip growth in root hairs involves the release of cell wall monomers through exocytosis, which must polymerize in order to form the strong mature cell wall. Cell walls loosen under extracellular acidification and harden during extracellular alkalinization. Extracellular reactive oxygen species (ROS) have also been linked to cell wall strength although it is still controversial whether they strengthen or weaken cell wall crosslinking.
In this last grant cycle 2 manuscripts were accepted for publication. Computer vision tracking software, developed at the BRC, was used to determine that tip growth in root hairs of Arabidopsis occurs in an oscillating manner. Tip growth measurements were made in combination with imaging of intracellular and extracellular fluorescent indicators. It was found that oscillations in growth were followed closely in time by increases in [Ca2+]i, [H+]i, and extracellular reactive oxygen species (ROS) and a decrease in [H+]ext. The oscillations in [Ca2+]i occurred significantly earlier than the decrease in [H+]ext. These findings are consistent with the hypothesis that a surge in growth leads to a rise in [Ca2+]i that acts to raise extracellular pH and ROS. Extracellular pH and ROS act to harden the newly released cell wall to prevent rupture.
Future work with the computer vision tracker involves identifying the origin of the rise in Ca2+i and determining the nature of its activation following the surge in growth.