Role of an LRR-RLK in regulating cell expansion in Arabidopsis
The regulation of cell expansion is a primary determinant in the size and shape of plant organs. Understanding how cells regulate this process is crucial in understanding the development of plant form. The Arabidopsis root is an excellent model system to dissect this process as it has a relatively simple, well defined architecture and mutants that alter cell elongation can easily be identified by their effect on root length. In the root, two distinct regions of cell expansion can be distinguished. Both longitudinal and radial cell expansion occurs in the root apical meristem, defining the root diameter and moving cells into the elongation zone of the root. In the elongation zone, just above the meristem, elongation rates are also uniform, but are much higher and occur almost exclusively in the longitudinal direction. This polar cell expansion is known as anisotropy.
Both the extent and orientation of cell expansion is regulated in plants. Cell expansion can be a dynamic process, and the orientation of expansion can change in response to various stimuli, such as wounding and hormonal treatments. Expansion of cells is driven by turgor pressure and the relative alignment and composition of cell wall material, which determines both the extent and orientation of elongation. The plant cell wall is comprised of cellulose microfibrils that are crosslinked with glycans and are associated with a pectin matrix and various extracellular proteins. The primary load bearing elements of the cell wall are the cellulose microfibrils, and thus their orientation and crosslinking are key factors in both the direction and extent of cell expansion. In anisotropically elongating cells, the cellulose microfibrils are wound in a helical spiral transversely around the cell, in a manner that has been likened to hoops around a barrel. This arrangement allows expansion of the cell specifically in the longitudinal direction by stretching of this cellulose “spring”, but restricts expansion in the transverse or radial direction. Cellulose microfibrils are synthesized at the plasma membrane by a hexameric protein complex called the terminal complex or rosette, and the polysaccharides made by this complex are extruded into the extracellular space through some type of a pore in the plasma membrane, where they then associate with other cellulose chains to form microfibrils.
A major question regarding cell expansion is how is the deposition of the cellulose microfibrils is regulated to give, for example, primarily transverse microfibrils in root cells. Early studies revealed that the cytoplasmic microtubules were aligned transversely in some plant cells, correlated to the orientation of the cellulose microfibrils in those cells, and this and other data led to the idea that the microtubules act as a template for the synthesis of the cellulose microfibrils.
Plant hormones influence microtubule organization, and in turn cell elongation. For example, treatment with GA leads to a transverse organization of cortical microtubules and an increase in longitudinal expansion, and treatment with ABA has the opposite effects. Ethylene inhibits hypocotyl and root elongation by inhibiting longitudinal cell expansion and promoting radial expansion, which is correlated with a change in the orientation of the microtubules from transverse to primarily longitudinal, with a concomitant change in the orientation of deposition of cellulose microfibrils. Re-orientation of microtubules in response to ethylene is first evident 30 minutes after treatment with the hormone, which precedes the effect of ethylene on radial cell expansion. This proposal seeks to explore the links between ethylene, cell expansion and a pair of receptor-like kinase gene that we have evidence may link these processes.
Using a yeast two-hybrid screen, we identified a number of clones that encoded proteins that interact with the C-terminal domain of ACS5. One of these was a member of the leucine-rich receptor like Ser/Thr protein kinase (LRR-RLK) family in Arabidopsis that we call ACS5 Interacting Kinase1 (AIK1). This gene has a very close homolog (82% amino acid identity throughout the entire predicted protein) in the Arabidopsis genome, and we have called it AIK2. Disruption of either gene has no effect on Arabidopsis development, but the double mutant displayed radially swollen roots specifically in the presence of high sucrose.
Personnel: Shouling Xu, Graduate student
Joe Kieber, PI
Both the extent and orientation of cell expansion is regulated in plants. Cell expansion can be a dynamic process, and the orientation of expansion can change in response to various stimuli, such as wounding and hormonal treatments. Expansion of cells is driven by turgor pressure and the relative alignment and composition of cell wall material, which determines both the extent and orientation of elongation. The plant cell wall is comprised of cellulose microfibrils that are crosslinked with glycans and are associated with a pectin matrix and various extracellular proteins. The primary load bearing elements of the cell wall are the cellulose microfibrils, and thus their orientation and crosslinking are key factors in both the direction and extent of cell expansion. In anisotropically elongating cells, the cellulose microfibrils are wound in a helical spiral transversely around the cell, in a manner that has been likened to hoops around a barrel. This arrangement allows expansion of the cell specifically in the longitudinal direction by stretching of this cellulose “spring”, but restricts expansion in the transverse or radial direction. Cellulose microfibrils are synthesized at the plasma membrane by a hexameric protein complex called the terminal complex or rosette, and the polysaccharides made by this complex are extruded into the extracellular space through some type of a pore in the plasma membrane, where they then associate with other cellulose chains to form microfibrils.
A major question regarding cell expansion is how is the deposition of the cellulose microfibrils is regulated to give, for example, primarily transverse microfibrils in root cells. Early studies revealed that the cytoplasmic microtubules were aligned transversely in some plant cells, correlated to the orientation of the cellulose microfibrils in those cells, and this and other data led to the idea that the microtubules act as a template for the synthesis of the cellulose microfibrils.
Plant hormones influence microtubule organization, and in turn cell elongation. For example, treatment with GA leads to a transverse organization of cortical microtubules and an increase in longitudinal expansion, and treatment with ABA has the opposite effects. Ethylene inhibits hypocotyl and root elongation by inhibiting longitudinal cell expansion and promoting radial expansion, which is correlated with a change in the orientation of the microtubules from transverse to primarily longitudinal, with a concomitant change in the orientation of deposition of cellulose microfibrils. Re-orientation of microtubules in response to ethylene is first evident 30 minutes after treatment with the hormone, which precedes the effect of ethylene on radial cell expansion. This proposal seeks to explore the links between ethylene, cell expansion and a pair of receptor-like kinase gene that we have evidence may link these processes.
Using a yeast two-hybrid screen, we identified a number of clones that encoded proteins that interact with the C-terminal domain of ACS5. One of these was a member of the leucine-rich receptor like Ser/Thr protein kinase (LRR-RLK) family in Arabidopsis that we call ACS5 Interacting Kinase1 (AIK1). This gene has a very close homolog (82% amino acid identity throughout the entire predicted protein) in the Arabidopsis genome, and we have called it AIK2. Disruption of either gene has no effect on Arabidopsis development, but the double mutant displayed radially swollen roots specifically in the presence of high sucrose.
Personnel: Shouling Xu, Graduate student
Joe Kieber, PI