Research Interests

The primary research interest in my laboratory is to study how cells communicate during plant development. Cells have to constantly communicate with each other and interact with the environment to carry out their functions. Cells of all organisms produce products that are secreted into the environment to form an extracellular matrix (ECM) which is a complex arrangement of polysaccharides, proteoglycans and proteins. In multicellular organisms the interactions between extracellular and intracellular compartments play an important role in controlling many developmental decisions. In animal cells the ECM molecules are constituents of selective cell-cell recognition and cell-ECM adhesion events and important in the regulation of developmental processes such as cellular polarity, differentiation, cell division, cell death and cell migration. In plants, the ECM that encompasses each cell is referred to as the cell wall, and this forms a single continuous matrix through the body of the plants. The cell wall is vital for plant development and interactions with the changing environments, yet little is known about the molecular mechanisms of how cell wall interacts with and regulates intracellular compartments.

My laboratory has been investigating how a family of five Arabidopsis cell wall associated protein kinases (WAK), WAK1-5, plays roles in plant functions. WAKs are plasma membrane proteins whose extracellular domains contain several repeats homologous to vertebrate Epidermal Growth Factor (EGF), a motif found in many proteins involved in extracellular interactions. The carboxyl region encodes a cytoplasmic serine/threonine protein kinase which can interact with other components through protein phosphorylation/dephosphorylation (Figure 1). We hypothesize that signals from adjacent cells and/or environment can be sensed by WAKs' extracellular domains and be further transduced to downstream components possibly through protein phosphorylation.

Figure 1. The topology of the cell wall associated receptor protein kinase (WAK).

In the last a few years, research progresses made in several labs including mine have revealed that WAK1 is a pathogenesis-related (PR) protein and WAK members are expressed in specific organs and differentially regulated by various biotic and abiotic factors. To gain further insight of how WAKs function during development, we have used a glucocorticoid inducible system to ectopically express the WAK4 antisense gene. The induced expression of the antisense WAK4 gene results in a significant decrease of WAK proteins. Ninety-six hours after the induction of antisense WAK4 expression, WAK proteins become undetectable. Cell elongation is impaired and lateral root development is blocked. The level of WAK protein can be controlled by the concentration of the applied inducer, dexamethasone, and is correlated with the severity of the cell elongation inhibition phenotype (Figure 2). These results suggest that the WAKs serve a vital role in cell elongation and are required for plant development (Lally et al., 2001 Plant Cell ).

	Figure 2. WAK4 Antisense Expression Affects Cell Elongation. (A) and (B) Scanning electron micrographs of DEX-treated leaves from either a wild-type plant (A) or a W4A plant (B). Bars = 100 µm. (C) and (D) Light microscope images of the epidermis of inflorescence stems from a DEX-treated wild-type plant (C) or a DEX-treated W4A plant (D). Hand peels of epidermis were stained with Lugol's solution and viewed with a compound microscope. Bars = 30 µm.

We have recently identified a large family of genes (Figure 3) with sequence similarity to the cell wall-associated kinase (WAK) genes and these new discovered genes are named WAKLs for WAK-like kinases (Verica and He, 2002, Plant Physiology, in press). Like the WAKs, these genes exist in multiple gene clusters; and our analyses suggest that they encode functional protein kinases that are associated with the cell wall. The large number of genes in this family may provide Arabidopsis with the potential to recognize and respond to a diverse array of ligands. The observations that WAKs play roles in both the pathogen response and cell elongation suggests that they function in some manner that is common to be processes.

Figure 3. The WAKL and WAK genes are distributed among all five chromosomes.

Chromosomes (I-V) are indicated by the vertical bars. Centromeres are indicated by the darkened circles. Horizontal bars indicate the location of each of the WAKL/WAK genes, and their physical position is given in centiMorgans.

We are currently using a combination of approaches including techniques in molecular genetics, bioinformatics, biochemistry, as well as cell biology, to gain a better understanding of the specific functions of each WAK/WAKL members.