PARENT SESSION

Symposium S2C Carbon and nitrogen interactions
Monday August 30th, 2004 2:40 PM-4:20 PM Room 510A
Chair: Steve Huber
Co-Chair: Graham Noctor

Regulation of senescence by aspartic protease: CND41 in tobacco and CND41 homologues in Arabidopsis. Yusuke Kato¹, Naomi Saito¹, Yumiko Yamamoto¹, Fumihiko Sato^*,1, ¹ Grad. Sch. Biostudies, Kyoto University, Kyoto, Japan

ABSTRACT- Senescence in plants is not only simply a degenerative process, but is also a recycling process in which nutrients are translocated from the senescing cells to reproductive organs or younger leaves. During senescence, leaf cells undergo highly coordinated changes in structure and gene expression. The key factor(s) in senescence, however, is not yet known, whereas an early indication is the breakdown of chloroplasts. Here we report the novel function of aspartic protease(s), i.e., CND41 in tobacco and CND41 homologues in Arabidopsis, in the regulation of senescence. CND41 has DNA binding activity as well as aspartic protease activity (Murakami et al., FEBS Lett. 468: 15-18, 2000). Our attempts to identify the physiological functions of CND41 using antisense CND41 tobacco lines indicated that low CND41 transformants showed retarded senescence of old leaves at flowering time. Further nitrogen depletion experiments indicated that CND41 could degrade Rubisco, a major soluble protein in chloroplasts, in vivo in senescent leaves. To generalize the physiological role of aspartic proteases in senescence, we have characterized the homologues in Arabidopsis. Among 59 aspartic proteases found in Arabidopsis genome, At5g10770 and At5g10760 were most homologous to tobacco CND41. Then, their physiological functions were characterized in transgenic plants, which were transformed with either overexpression or RNAi vectors for these genes. Whereas At5g10760 RNAi transformants did not show desired reduction of gene expression, other transformants were successfully established and characterized. Interestingly, transgenic Arabidopsis with At5g10770 overexpression showed more enhanced senescence in the dark than wild type. The expression of At5g10770 gene was also found to be induced by the combined treatment of nitrogen-depletion and sucrose addition, i.e., effective senescence-induction-treatment. These results strongly suggest aspartic proteases are functionally involved in the senescence both in tobacco and Arabidopsis, while the detailed mechanism should be more carefully examined.

KEY WORDS: nitrogen depletion, senescence, aspartic protease, Rubisco degradation