No abstract

GPI-anchored proteins (GPI-APs) are essential for plant growth and development; knockout mutations in enzymes responsible for anchor biosynthesis or attachment are gametophyte or embryo lethal. In a genetic screen targeted to identify genes regulating stomata formation, we discovered a missense mutation in the Arabidopsis (Arabidopsis thaliana) homolog of GPI8/PIG-K, a Cys protease that transfers an assembled GPI anchor to proteins. The Arabidopsis genome has a single copy of AtGPI8, and the atgpi8-1 mutation reduces the efficiency of this enzyme, leading to reduced accumulation of GPI-anchored proteins. While the atgpi8-1 mutation strongly disrupts plant growth, it is not lethal. Phenotypic analysis of atgpi8-1 mutants suggests that GPI-APs are important for root and shoot growth, stomata formation, apical dominance, transition to flowering, and male gametophyte viability. In addition, atgpi8-1 mutants accumulate higher levels of callose and have reduced plasmodesmata permeability. Genetic interactions of atgpi8-1 with mutations in ERECTA family (ERf) genes suggest the existence of a GPI-AP in a branch of the ERf signaling pathway that regulates stomata formation. Activation of the ERf signal transduction cascade by constitutively active YODA rescues stomata clustering in atgpi8-1, indicating that a GPI-AP functions upstream of the MAP kinase cascade. TOO MANY MOUTHS (TMM) is a receptor-like protein that is able to form heterodimers with ERfs. Our analysis demonstrates that tmm-1 is epistatic to atgpi8-1, indicating that either TMM is a GPI-AP or there is another GPI-AP regulating stomata development whose function is dependent upon TMM.

Plant organ shape and size are established during growth by a predictable, controlled sequence of cell proliferation, differentiation, and elongation. To understand the regulation and coordination of these processes, we studied the temporal behavior of epidermal and cortex cells in Arabidopsis pedicels and used computational modeling to analyze cell behavior in tissues. Pedicels offer multiple advantages for such a study, as their growth is determinate, mostly one dimensional, and epidermis differentiation is uniform along the proximodistal axis. Three developmental stages were distinguished during pedicel growth: a proliferative stage, a stomata differentiation stage, and a cell elongation stage. Throughout the first two stages pedicel growth is exponential, while during the final stage growth becomes linear and depends on flower fertilization. During the first stage, the average cell cycle duration in the cortex and during symmetric divisions of epidermal cells was constant and cells divided at a fairly specific size. We also examined the mutant of ERECTA, a gene with strong influence on pedicel growth. We demonstrate that during the first two stages of pedicel development ERECTA is important for the rate of cell growth along the proximodistal axis and for cell cycle duration in epidermis and cortex. The second function of ERECTA is to prolong the proliferative phase and inhibit premature cell differentiation in the epidermis. Comparison of epidermis development in the wild type and erecta suggests that differentiation is a synchronized event in which the stomata differentiation and the transition of pavement cells from proliferation to expansion are intimately connected.