Changes in protein ubiquitination and the expression of ubiquitin‐encoding transcripts in daylily petals during floral development and senescence

Abstract

The occurrence of ubiquitin‐protein conjugates and the expression of ubiquitin genes during floral development and senescence of Hemerocallis fulva (daylily) is reported. Daylily petal proteins were extracted at various stages of floral development and the abundance of ubiquitinated proteins determined by western blotting with affinity purified antibody to ubiquitin. Up to 100 ubiquitin conjugates in the molecular mass range 22–220 kDa were detected within the petals during bud development through to flower opening. The intensity of several of the ubiquitinated protein bands changed markedly during flower opening and/or senescence. Ubiquitinated proteins of apparent molecular masses 160, 105 and 29 kDa were intense in the daylily bud, but rapidly disappeared after flower opening, whereas ubiquitinated proteins of apparent molecular masses 26 and 15.5 kDa increased in intensity as the flowers senesced. Treatment of daylily buds with a 1‐h pulse of cycloheximide as the flowers commenced opening prevented these changes; this treatment also delayed the onset of visual signs of senescence by 5–6 days. Analysis by northern blotting demonstrated differential expression of ubiquitin transcripts by the multi‐ubiquitin gene family during floral development and senescence. Inhibition of petal senescence by treatment with cycloheximide resulted in the accumulation of several ubiquitin‐encoding transcripts and the down‐regulation of a 3.4 kb transcript. Overall, these studies suggest that ubiquitin is involved in the degradation of many petal proteins during floral development and senescence and that, during senescence, this selective degradation is occurring against a background of net proteolysis.