An Arabidopsis mosaic death1 ( mod1 ) mutant, which has premature cell death in multiple organs, was isolated. mod1 plants display multiple morphological phenotypes, including chlorotic and curly leaves, distorted siliques, premature senescence of primary inflorescences, reduced fertility, and semidwarfism. The phenotype of the mod1 mutant results from a single nuclear recessive mutation, and the MOD1 gene was isolated by using a map-based cloning approach. The MOD1 gene encodes an enoyl-acyl carrier protein (ACP) reductase, which is a subunit of the fatty acid synthase complex that catalyzes de novo synthesis of fatty acids. An amino acid substitution in the enoyl-ACP reductase of the mod1 mutant causes a marked decrease in its enzymatic activity, impairing fatty acid biosynthesis and decreasing the amount of total lipids in mod1 plants. These results demonstrate that a deficiency in fatty acid biosynthesis has pleiotropic effects on plant growth and development and causes premature cell death.
INTRODUCTIONCell death has been classified into physiological cell death and nonphysiological cell death (Vaux and Korsmeyer, 1999). The former refers to a process programmed by the organism for the purpose of killing its own cells, including apoptosis, aging, terminal differentiation, and defense against pathogen infection. The latter includes cell death from injury caused by external events, such as poisons or lack of nutrients, or from intrinsic defects, such as a mutation in an essential enzyme or expression of an altered gene product that is toxic to the cells.In plants, two types of physiological cell death have received considerable attention in recent years. One is developmentally programmed cell death (PCD), which includes the degeneration of suspensor and aleurone cells (Yeung and Meinke, 1993), the development of xylem tracheary elements (Fukuda, 1997), and the execution of senescence (Bleecker and Patterson, 1997). The other is the hypersensitive response (HR), which results in cell death at the site of infection by an avirulent pathogen. The HR produces dry lesions that are clearly delimited from the surrounding healthy tissues (Dangl et al., 1996). A large number of mutants characterized by the spontaneous lesion phenotype, which mimics the HR defense responses, have been identified in maize (Gray et al., 1997), barley (Wolter et al., 1993), rice (Marchetti et al., 1983), and Arabidopsis (Dietrich et al., 1994; Greenberg et al., 1994). Because the lesions form in the absence of pathogen infection, these mutants have been collectively called the lesion mimic mutants. In Arabidopsis, the lesionstimulating disease mutant lsd1 has been well characterized. LSD1, a zinc finger protein, has been proposed to function as a negative regulator of plant PCD in response to signals emanating from the cells undergoing pathogeninduced HR-mediated cell death (Dietrich et al., 1997).Blockage of metabolic processes also induces cell death in plants. For example, the maize lesion mimic mutant Les22 produces minute necrotic sp...
