Expression of glyoxysomal ascorbate peroxidase in cotton seedlings during postgerminative growth

In previous research, cottonseed glyoxysomal ascorbate peroxidase (gmAPX) was identified as a membrane-bound APX that scavenged H2O2 generated within the glyoxysomes of cotyledons during the conversion of stored lipids into carbohydrates. To corroborate the importance of gmAPX as a H2O2 scavenger during the establishment of oil-rich seedlings in the soil, the mRNA and protein levels of gmAPX were compared with those for marker enzymes of the glyoxylate cycle and photorespiration, namely isocitrate lyase (ICL) and hydroxypyruvate reductase (HPR), respectively. During the first 72 h of growth in the dark, steady-state levels of mRNAs encoding gmAPX and ICL were coordinately expressed in cotton cotyledons. These increases in transcripts were followed by an accumulation of both proteins that peaked at 48 h indicating that the temporal appearance of gmAPX was regulated by the abundance of gmAPX transcripts. After 72 h, ICL levels declined whereas gmAPX levels increased in the dark-grown seedlings. Exposure of the 48-h, dark-grown seedlings to light accelerated the decrease in the protein levels of ICL and gmAPX but increased the transcript and protein/activity levels of HPR. gmAPX transcripts also were more abundant in the greening cotyledons than in the cotyledons of the dark-grown seedlings. These results indicated that like HPR, the accumulation of transcripts encoding gmAPX was partially regulated by light. The protein levels of gmAPX increased in the cotyledons at 120 h for both the dark- and light-grown seedlings. Results from DNA gel blot analyses suggested that two copies of the gene that encode gmAPX were present in cotton. The concerted accumulation of gmAPX with ICL in the dark and with HPR in the light suggests that gmAPX along with catalase may serve as another important peroxide scavenger in the cotyledons of oilseed seedlings not only during the conversion of stored lipids to carbohydrates but also during photorespiration. (C) 1997 Elsevier Science Ireland Ltd.