Plant science. How plant cells go to sleep for a long, long time.

S eeds are plant spores—desiccated, dormant cells in which metabolism and growth have been drastically slowed, so that the organism can wait out adverse conditions. This is especially important for plants, which cannot run away from environmental threats. Instead of motility, they use dormancy to outlast adversarial conditions caused by pathogens or severe weather. Indeed, some seeds remain viable for hundreds of years. On page 1373 of this issue, Nishimura et al. ( 1) and four other groups ( 2– 5) report the initial biochemical mechanisms that allow the complex cellular machinery of this multicellular eukaryote to live for so long in a dormant state. Since its discovery fi ve decades ago, the plant hormone abscisic acid has been known to control dormancy. For example, after fertilization, when the zygote (fertilized egg cell) has divided and formed the embryo, the concentration of abscisic acid rises, triggering the desiccation and dormancy process. Conversely, after seeds absorb water, they only break dormancy and germinate when the abscisic acid concentration falls. Genetic analyses in the flowering plant Arabidopsis thaliana, maize, and other model plants have revealed some key players in the abscisic acid response, including a transcription factor encoded by the viviparous gene (vp1). Mutant maize embryos lacking this gene ignore the abscisic acid signal to become desiccated and dormant, and instead germinate right on the cob inside the ear ( 6) (see the fi gure). Another key gene, abscisic acid–insensitive 1 (ABI1), encodes a protein phosphatase, whose activation is required to transduce the cellular response to abscisic acid. Over the past decade, mainly through genetic studies of Arabidopsis and rice, the receptor proteins for each of the fi ve traditional plant hormones (auxin, gibberellin, cytokinin, ethylene, and abscisic acid) have gradually become known, but the abscisic acid receptor has been the most diffi cult to identify, despite several disappointing, false claims. The situation improved with the publication of two recent studies ( 7, 8) indicating that receptors for this hormone are a group of small soluble proteins referred to as Pyrabactin Resistance 1 (PYR1), or PYR-like (PYL), encoded by a dozen or so genes. When abscisic acid enters a plant cell, the receptor forms a binary complex with it that can then immediately bind to, and inhibit, the ABI class of protein phosphatases. The PYR1/PYL receptors are members of a family called PathogenesisRelated proteins (PR-10), characterized by steroidogenic acute regulatory lipid transfer (START) sequence motifs. Although recent biochemical and genetic data showed that this family of proteins was most likely a receptor ( 7, 8), the weakest part of the evidence was the abscisic acid binding analysis and the lack How Plant Cells Go to Sleep for a Long, Long Time PLANT SCIENCE