The influence of sublethal levels of a number of herbicides and plant growth regulators on the germinability of the seeds and the growth and development of seedlings of mouseearcress [Arabidopsis thaliana (L.) Heynh. ♯ ARBTH] was determined. Only 7 of the 22 chemicals tested had a persistent effect on progeny. Amitrole (3-amino-s-triazole) was one of the most effective compounds. It caused a characteristic bleaching only in shoot tips and pods in parent plants and appeared to act directly on the progeny by accumulation in the seed. Two auxin transport inhibitors, TIBA (2,3,5-triiodobenzoic acid) and CPII (5-O-carboxyphenyl-3-phenylisoxazole), and four of the six photosynthetic electron transport inhibitors included in the study also affected progeny. They appeared to act indirectly by interfering with seed development.

Many soil fungi colonize the roots of pines to form symbiotic organs known as ectomycorrhizas. Dichotomous branching of short lateral roots and the formation of coralloid organs are diagnostic of ectomycorrhizas in many pine species, although the regulation of these changes in root morphology is not well understood. We used axenic root cultures of six pine species to examine the role of auxin, cytokinin, ethylene and nutrients in the regulation of root architecture. Surprisingly, extensive dichotomous and coralloid branching of lateral roots occurred spontaneously in Pinus taeda, P. halepensis and P. muricata. In P. sylvestris, P. ponderosa and P. nigra, treatment with auxin transport inhibitors (ATIs), the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) or the ethylene-releasing compound 2-chloroethylphosphonic acid (CEPA or ethephon) induced extensive dichotomous branching and coralloid organ formation. Formation of both spontaneous and ATI-induced coralloid structures was blocked by treatment with an ethylene synthesis inhibitor L-α-(2-aminoethoxyvinyl)glycine; this inhibition was reversed by either ACC or CEPA. In addition, the induction of this unique morphogenetic pattern in pine root cultures was regulated by nutrient levels. The morphology and anatomical organization of the chemically induced dichotomous and coralloid structures, as well as the regulation of their formation by nutrient levels, show a striking similarity to those of ectomycorrhizas.