The brown alga Ectocarpus siliculosus has a haploid–diploid life cycle that involves an alternation between two distinct generations, the sporophyte and the gametophyte. We describe a mutant, ouroboros ( oro ), in which the sporophyte generation is converted into a functional, gamete-producing gametophyte. The life history of the mutant thus consists of a continuous reiteration of the gametophyte generation. The oro mutant exhibited morphological features typical of the gametophyte generation and accumulated transcripts of gametophyte generation marker genes. Genetic analysis showed that oro behaved as a single, recessive, Mendelian locus that was unlinked to the IMMEDIATE UPRIGHT locus, which has been shown to be necessary for full expression of the sporophyte developmental program. The data presented here indicate that ORO is a master regulator of the gametophyte-to-sporophyte life cycle transition and, moreover, that oro represents a unique class of homeotic mutation that results in switching between two developmental programs that operate at the level of the whole organism.

Rhizobia can infect roots of host legume plants and induce new organs called nodules, in which they fix atmospheric nitrogen. Infection generally starts with root hair curling, then proceeds inside newly formed, intracellular tubular structures called infection threads. A successful symbiotic interaction relies on infection threads advancing rapidly at their tips by polar growth through successive cell layers of the root toward developing nodule primordia. To identify a plant component that controls this tip growth process, we characterized a symbiotic mutant of Medicago truncatula, called rpg for rhizobium-directed polar growth. In this mutant, nitrogen-fixing nodules were rarely formed due to abnormally thick and slowly progressing infection threads. Root hair curling was also abnormal, indicating that the RPG gene fulfils an essential function in the process whereby rhizobia manage to dominate the process of induced tip growth for root hair infection. Map-based cloning of RPG revealed a member of a previously unknown plant-specific gene family encoding putative long coiled-coil proteins we have called RRPs (RPG-related proteins) and characterized by an ''RRP domain'' specific to this family. RPG expression was strongly associated with rhizobial infection, and the RPG protein showed a nuclear localization, indicating that this symbiotic gene constitutes an important component of symbiotic signaling.genetics ͉ symbiosis ͉ coiled-coil I n the symbiotic interaction between legumes and soil bacteria called rhizobia, nitrogen-fixing nodules are formed that allow plant growth to be independent of added combined nitrogen, and the plant provides rhizobia with a carbon source derived from photosynthesis. During initial signal exchange in the rhizosphere, rhizobia respond to plant flavonoids by producing lipochito-oligosaccharidic molecules called Nod factors (NFs). Host-specific recognition of NFs triggers a controlled infection leading to rhizobial internalization, and the induction of a new plant organ, the nodule, in which nitrogen fixation occurs (1).Rhizobial infection of host legumes is generally via root hairs (RHs) that undergo marked curling. Compared with normal RH tip growth in which vesicles, containing cell wall and membrane material, travel in an actin-and microtubule-dependent fashion to the RH tip, where they fuse with the cell membrane (2), this rhizobium-induced growth reorientation involves alterations to the plant cytoskeleton and the redirection of vesicle traffic away from the RH tip to a new site (3, 4). Inside a closed chamber formed by root hair curling (RHC), the plant cell wall is locally degraded and the plasma membrane becomes invaginated. Rhizobia enter a newly formed, plant-derived structure, the infection thread (IT), that undergoes inward tip growth within the RH. Underlying outer cortical cells change into highly polarized pre-IT cells, which guide IT passage to the nodule primordium formed, in M. truncatula, in the inner root cortex (3). Here, bacteria are released into plant cells an...