The last step of the ethylene biosynthesis pathway in turnip tops (Brassica rapa) seeds: Alterations related to development and germination and its inhibition during desiccation

The involvement of ethylene in zygotic embryogenesis is a little known aspe ct of the growth and development in higher plants, In the present work, we study the alterations of the last step of the ethylene biosynthesis pathway during the formation period of turnip tops (Brassica rapa cv, Rapa) seeds and its repercussions in the germination process and post-germinative growt h. For this, we chose 11 different phases of silique development, the first being the recently fertilized pistil and the last being the silique just p rior to its dehiscence (ca, 2 months post-anthesis), In the 11 phases, ethy lene production was detected in both whole silique (with or without seeds) and in the seeds enclosed by the silique wall. The levels of ACC, ACO and e thylene production proved high in seeds belonging to: (1) the pod in the ve ry early phases, when the seeds mere growing but without photosynthetic com petence; (2) the silique at maximum growth, in which the seeds will initiat e desiccation and loss of photosynthetic activity. During the phases prior to dehiscence, there was a marked inhibition in the last step of the ethyle ne biosynthesis pathway, In viable dry seeds, no ACO activity was detected and the ACC levels were 4-fold lower than at the onset of the silique senes cence. Germination brings about a net synthesis of ACC with respect of the stores dry seed. This fact, together with other results presented in this w ork, point towards, as in other seeds, a dependence of ethylene synthesis f or radicle emergence. The possible role played by the silique wall in the c ontrol of ethylene biosynthesis during zygotic embryogenesis, as well as th e participation of ethylene as a hormonal signal in the triggering of seed desiccation in Brassica rapa cv, Rapa, are discussed in depth.