This paper shows that the level of 2,4-dichlorophenoxyacetic acid (2,4-D) in the medium determines the level of auxin-binding proteins in the membranes of carrot, Daucus carota, cells grown in suspension. This induction takes slightly more than 2 hours to complete and can be elicited by natural as well as synthetic auxins. The auxin binding sites thus generated, which are pronase-sensitive, bind 2,4-D, indoleacetic acid, and naphthalene-acetic acid (NAA) equally well. However both alpha- and beta-NAA bind, whereas only alpha-NAA is effective in the inductive process. Cells committed to embryogeny (proembryogenic masses) do not respond to auxin, i.e. their level of auxin-binding proteins remains very low, and they do not seem to synthesize the hormone, as indicated by inhibitor studies. Sensitivity to, and production of, auxin, begins when the embryo becomes polarized, i.e. at postglobular stage.

The temperature-sensitive carrot cell variant ts11c, arrested in somatic embryogenesis after the globular stage, was characterized. The sensitivity to a shift from 24° C (permissive temperature) to 32° C (non-permissive temperature) is greatest at the globular stage of embryogenesis, while cells proliferating in unorganized fashion and plantlets are not affected. Embryogenesis in ts11c is also arrested at the permissive temperature by replacement of conditioned culture medium with fresh medium. The timing of sensitivity of ts11c to medium replacement coincides with the sensitivity to temperature shift. Both sensitivities are recessive in somatic hybrids between ts11c and wild-type cells. Extracellular glycoproteins synthesized by ts11c at the non-permissive temperature contain much less fucose than those synthesized by the wild type. The glycoproteins synthesized by the variant under non-permissive conditions do not accumulate at the periphery of the embryo, as their wildtype counterparts do, but instead show a diffuse distribution throughout the embryo. The defect in ts11c can be fully complemented by the addition of extracellular wild-type proteins. A revertant of ts11c was isolated that simultaneously reacquired temperature insensitivity and normal glycosylation ability. Collectively, these observations indicate that ts11c is not able to perform proper glycosylation at the non-permissive temperature and suggest that the activity of certain extracellular proteins, essential for the transition of globular to heart stage somatic embryos, depends on the correct modification of their oligosaccharide side-chains.

Thispapershowsthatthelevel of2,4-dichlorophenoxyacetic acid(2,4-D) inthemediumdetermines thelevel ofauxin-binding proteins inthemembranes ofcarrot, Daucuscarota, cells grown insuspension. Thisinduction takesslightly morethan2hoursto complete andcanbeelicited bynatural aswellassynthetic auxins. Theauxinbinding sites thusgenerated, whicharepro- nase-sensitive, bind2,4-D, indoleacetic acid, andnaphthalene- acetic acid(NAA)equally well. However botha-andf,-NAA bind, whereas onlya-NAAiseffective intheinductive process. Cells committed toembryogeny (proembryogenic masses) donotre- spondtoauxin, i.e. their level ofauxin-binding proteins remains verylow,andtheydonotseemtosynthesize thehormone, as indicated byinhibitor studies. Sensitivity to,andproduction of, auxin, begins whentheembryobecomespolarized, i.e. atpost- globular stage.