The heterogeneity of turnip-tops (Brassica rapa) seeds inside the silique affects germination, the activity of the final step of the ethylene pathway, and abscisic acid and polyamine content

Abstract

The mature silique of turnip-tops (Brassica rapa L. cv. Rapa) contains seeds that are heterogeneous in colour. From these seeds, we have selected three homogeneous lots: black (B), dark brown (DB) and light brown (LB). The dry seeds of these lots contained different levels of free and conjugated 1-aminocyclopropane-1-carboxylic acid (ACC), polyamines (PA) and ABA, the levels of the latter being inversely related to the germinative capacity. The water uptake (WU) rate was much faster in LB seeds than in B. This fact was probably related to the breaking of the seed coat, the speed of which was B >> DB > LB. The ABA, spermidine (Spd) and spermine (Spm) contents decreased in the seeds during germination, whereas the putrescine (Put) levels rose sharply (B > DB > LB). For the first time in seeds, heterogeneity is reported with respect to ethylene sensitivity and synthesis. Whereas exogenous ethylene did not alter the percentage of germination in lot B, germination was higher in DB and LB (LB >> DB) in the presence of ethylene. The final step of the ethylene pathway was altered concomitantly with this change in germinating capacity, affecting the levels of 1-(malonylamino)cyclopropane-1-carboxylic acid (MACC), ACC, ACC–oxidase (ACO) and ethylene production. The gene BrACO1, recently characterised by us, is expressed differently in the three seed lots, particularly in the LB, where little transcription occurs. Finally, ethylene inhibits Put, Spd and Spm levels at different intensities in the three lots. The results point towards variation in the channelling of ACC towards synthesis of ethylene and / or PA, caused by the heterogeneity.