Changes in mRNA levels and polypeptide subunits of ribulose 1,5‐bisphosphate carboxylase in response to supplementary ultraviolet‐B radiation

Abstract

Abstract. Pea plants (Pisum sativum L., cv. Greenfeast) were grown for 17d (150 μmol photons m−2 s−1; 12h light/12 h dark) and then exposed to moderate levels of supplementary ultraviolet‐B radiation (UV‐B: 280–320 nm) during the light cycle. The total soluble leaf protein, maximum Rubisco activity, polypeptide and mRNA transcript levels for Rubisco subunits were then determined in the mature third leaf pair from the base of the plants. Total soluble protein per unit leaf area showed little change after 1 d but declined by 33% during 3d of UV‐B exposure. However, there was no change on a unit chlorophyll basis. Total RNA per unit area declined by 15% and 37% after 1 or 3d of UV‐B treatment, respectively. Maximum Rubisco activity declined by 38% after 1 d and 71% after 3d of UV‐B exposure. Rubisco polypeptide subunits showed some decrease (∼16%) after 1d exposure, but declined by 56% over 3d. The decrease in Rubisco is probably the major reason for the reduction in soluble protein. In contrast to the relatively slow decline in total soluble protein and Rubisco, the level of the mRNA transcripts for both rbc L and rbc S showed a dramatic decrease within hours of UV‐B exposure. The mRNA transcripts for rbc S were reduced to >20% of control values after 4h of UV‐B exposure, while the rbc L transcripts were reduced by 60% after 8h. Further exposure to UV‐B reduced the mRNA transcripts to either trace or undetectable levels. The decrease in rbc S mRNA levels with the UV‐B exposure can be partially ameliorated by higher photosynthetically active irradiance during the period of UV‐B exposure. Plants that were exposed to supplementary UV‐B radiation for short periods (4h or 8h) and returned to control conditions, showed no recovery after 24h. However, after a further 2d, the rbc L and rbc S mRNA transcripts had recovered to ca. 60% of the control values, showing that the effect upon the mRNA transcripts is a reversible response.