(Gälweiler et al. 1998). Further PIN proteins and their subcellular localization
were identified. In general, expression and localization of PIN proteins is cell-type
dependent (Michniewicz et al. 2007). Further efflux carriers encompass proteins of
the ABC-B/MULTI-DRUG RESISTANCE/P-GLYCOPROTEIN (ABCB/MDR/PGP)
family. They are localized symmetrically in the plasma membrane and are involved
in auxin homeostasis (Cho and Cho 2013).
The polar auxin transport must work more efficiently on one side of the
gravistimulated organ after reorientation of the plant, so that according to Cholodny
and Went an asymmetric auxin distribution can occur. As a result, stronger growth
on one side will lead to bending of the plant. It was known that the cell polarity of
PIN localization depends on cell type and developmental stage of the tissue and
therefore allows directed auxin flow. PIN1 and PIN7 have different polarized
localizations in pro-embryo and adult plant (Friml 2003). For PIN3 it was shown
that gravistimulation has an influence on the localization of the efflux carrier. PIN3
relocalization in columella cells always appears at the new physiological bottom
(Friml et al. 2002). An accumulation of PIN3 at the new physiological bottom would
favor auxin transport along the new lower side of the root and lead to root bending.
PIN proteins are localized via vesicular transport. An important component of
these vesicular transport systems are ENDOSOMAL SORTING COMPLEXES
REQUIRED FOR TRANSPORT (ESCRT) that are formed by various VESICULAR SORTING PROTEINS (VSPs). Mutations in the regulatory system of ESCRT
lead to different localizations of PIN1, PIN2 and AUX1. A double mutant of
CHARGED MULTIVESICULAR BODY PROTEIN/CHROMATIN MODIFYING PROTEIN1A (CHMP1A) and CHMP1B leads to accumulation of AUX1,
PIN1 and PIN2 in late endosomes and only marginal localization at the plasma
membrane (Spitzer et al. 2009).
6.5 Microgravity Research and Modifying Gravitational
Acceleration Changed Our Perspective
on Gravitropism
Centrifugation has been widely used to alter the amount and the direction of mass
acceleration that acts on the sedimentable masses in gravity sensing systems. In a
pioneering experiment, Sir Thomas Andrew Knight used a water-driven horizontal
wheel to show that plant roots neither grow in the direction of gravity nor in the
direction of the centrifugal acceleration but grow in the direction of the resulting
acceleration angle, thus, providing evidence that plants sense the direction of
accelerations not gravity per se (Knight 1806). Centrifugation intensifies weak
gravity responses (improved graviorientation in starchless Arabidopsis mutants),
renders intracellular processes more clearly visible and was used to characterize
gravisensitive membranes by moving statoliths (cf. Chap. 4). Very high acceleration
86
6 Gravitropism in Higher Plants: Cellular Aspects
were identified. In general, expression and localization of PIN proteins is cell-type
dependent (Michniewicz et al. 2007). Further efflux carriers encompass proteins of
the ABC-B/MULTI-DRUG RESISTANCE/P-GLYCOPROTEIN (ABCB/MDR/PGP)
family. They are localized symmetrically in the plasma membrane and are involved
in auxin homeostasis (Cho and Cho 2013).
The polar auxin transport must work more efficiently on one side of the
gravistimulated organ after reorientation of the plant, so that according to Cholodny
and Went an asymmetric auxin distribution can occur. As a result, stronger growth
on one side will lead to bending of the plant. It was known that the cell polarity of
PIN localization depends on cell type and developmental stage of the tissue and
therefore allows directed auxin flow. PIN1 and PIN7 have different polarized
localizations in pro-embryo and adult plant (Friml 2003). For PIN3 it was shown
that gravistimulation has an influence on the localization of the efflux carrier. PIN3
relocalization in columella cells always appears at the new physiological bottom
(Friml et al. 2002). An accumulation of PIN3 at the new physiological bottom would
favor auxin transport along the new lower side of the root and lead to root bending.
PIN proteins are localized via vesicular transport. An important component of
these vesicular transport systems are ENDOSOMAL SORTING COMPLEXES
REQUIRED FOR TRANSPORT (ESCRT) that are formed by various VESICULAR SORTING PROTEINS (VSPs). Mutations in the regulatory system of ESCRT
lead to different localizations of PIN1, PIN2 and AUX1. A double mutant of
CHARGED MULTIVESICULAR BODY PROTEIN/CHROMATIN MODIFYING PROTEIN1A (CHMP1A) and CHMP1B leads to accumulation of AUX1,
PIN1 and PIN2 in late endosomes and only marginal localization at the plasma
membrane (Spitzer et al. 2009).
6.5 Microgravity Research and Modifying Gravitational
Acceleration Changed Our Perspective
on Gravitropism
Centrifugation has been widely used to alter the amount and the direction of mass
acceleration that acts on the sedimentable masses in gravity sensing systems. In a
pioneering experiment, Sir Thomas Andrew Knight used a water-driven horizontal
wheel to show that plant roots neither grow in the direction of gravity nor in the
direction of the centrifugal acceleration but grow in the direction of the resulting
acceleration angle, thus, providing evidence that plants sense the direction of
accelerations not gravity per se (Knight 1806). Centrifugation intensifies weak
gravity responses (improved graviorientation in starchless Arabidopsis mutants),
renders intracellular processes more clearly visible and was used to characterize
gravisensitive membranes by moving statoliths (cf. Chap. 4). Very high acceleration
86
6 Gravitropism in Higher Plants: Cellular Aspects