Mixture screening and mixture-amount designs to determine plant growth regulator effects on shoot regeneration from grapefruit (Citrus paradisi macf.) epicotyls

Abstract

The objective of this study was to improve shoot regeneration from grapefruit. Because many commercially grown citrus types are apomictic, important in vitro applications such as Agrobacterium-mediated transformation commonly use epicotyl explants from in vitro seedlings; thus, adequate adventitious shoot production is an important prerequisite for efficient use of these applications. Eight plant growth regulators were studied—six cytokinins (6-benzylaminopurine, kinetin, zeatin trans-isomer, 6-[γ,γ-dimethylallylamino] purine, zeatin riboside trans-isomer and meta-topolin) and two auxins (α-naphthalene acetic acid and indole-3-acetic acid). An iterative design strategy was followed that included mixture and mixture-amount experimental designs suitable for resolving proportional and concentration effects; in vitro effects of cytokinins and auxins are affected by both proportion and concentration. One-centimeter-long explants were excised from the epicotyl of etiolated, in vitro-grown seedlings. Explants were placed onto experimental formulations and cultured in growth cabinets at 27°C over 6 wk, which included 2 wk in the dark followed by 4 wk in the light. The results indicated that (1) 6-benzylaminopurine or zeatin riboside were the most effective cytokinins for inducing shoot regeneration in citrus; (2) zeatin riboside singly or in combination with indole-3-acetic acid resulted in the highest quality, the greatest number of explants with buds/shoots, and the greatest shoot number; and (3) 6-benzylaminopurine and indole-3-acetic acid improved shoot regeneration vs. 6-benzylaminopurine at a considerably lesser cost than zeatin riboside and indole-3-acetic acid.