Annals of Botany

Rubisco renewed as ravages of age are reversed

Rubisco is by far the most abundant protein in green leaves. It can comprise 50 % or more of total leaf protein and contain up to 35 % of leaf N. Its synthesis is affected by both light and N supply, and its expression is regulated at both transcriptional and translational levels. In senescing leaves it is an important source of N and C for recycling as synthesis declines and existing protein is broken down. However, it is apparent from the work of Imai et al., Sendai, Japan (pp. 135–144) that the senescence-related loss can be reversed. From their very thorough study we focus on the dynamics of Rubisco synthesis and turnover in leaves of Oryza sativa. Senescence was studied in the 24 days following full leaf expansion. Plants were supplied with 1.0 or 4.0 mM N [supplied as (NH₄)₂SO₄] at intervals. As expected, Rubisco synthesis declined during this time. Transcriptional regulation was evident in the decline of mRNAs encoding both the large and small subunits; there was also regulation at the translational level in that the translational efficiencies of both mRNAs were decreased. This involved both the cytoplasmic and chloroplastic translation systems. However, all these features could be reversed, at least temporarily, by increased N-flux, especially at 4 mM. Thus, as shown by other authors for other aspects of leaf physiology, the senescence programme can be turned around. In the present work, this reversal was especially marked in the most extreme treatment when all tillers and all leaves except for the eighth were removed from plants fed with N at 4 mM. Even at a late stage of senescence, 16 days after full expansion, this treatment led to an increased Rubisco mRNA population (especially of the nuclear-encoded small subunit mRNA) and increased translational efficiency of both mRNAs. This resulted in an increase in Rubisco protein in leaves that had previously been well advanced in senescence.