Conditioning period, CO2 and GR24 influence ethylene biosynthesis and germination of Striga hermonthica

Germination of witchweed (Striga hermonthica [Del,] Benth), an important ro ot parasite on poaceous crops, requires pretreatment 'conditioning' in a wa rm moist environment and a subsequent exposure to a stimulant. The roles of conditioning period, CO2 and a strigol analogue (GR24) in ethylene biosynt hesis and germination of the parasite were investigated, Conditioning incre ased the seeds' capacity to oxidize exogenous 1-aminocyclopropane-1-carboxy lic acid (ACC), Exogenous CO2 increased the seeds capacity to oxidize ACC b y 3- to 9-fold, A combination of GR24 and ACC increased ethylene production by more than 3-foId in comparison with the rates obtained using these comp ounds separately. Aminoethoxyvinylglycine (AVG) completely inhibited ethyle ne induction by GR24, but not by ACC. A GR24 treatment, made subsequent to conditioning in GR24, did not induce ethylene. However, seeds conditioned i n GR24 and then given 1 mM ACC produced 293 nl l(-1) ethylene, ACC oxidase (ACCO) activity in crude extracts was increased by conditioning and CO2. Th e enzyme displayed an absolute requirement for ascorbate. Absence of exogen ous Fe2+ reduced enzyme activity only by 14%, GR24 applied during condition ing reduced germination in response to a subsequent GR24 treatment, ACC was , invariably, less effective in inducing S, hermonthica germination than GR 24 even at concentrations which induce more ethylene than concurrent GR24 t reatments. The results are consistent with a model in which conditioning re moves a restriction on the ethylene biosynthetic pathway in S. hermonthica seeds. GR24 modulates the key enzymes in ethylene biosynthesis. The stimula nt suppresses ethylene biosynthesis in unconditioned seeds and promotes it in conditioned ones. Germination of S, hermonthica results from the joint a ction of GR24 and the ethylene it induces.