Light‐quality and irradiance‐level control of light‐harvesting complex of photosystem 2 in maize mesophyll cells. Evidence for a low fluence‐rate threshold in blue‐light reduction of mRNA and protein

Abstract

Levels of pigment‐proteins and mRNA coding for proteins associated with the light‐harvesting complex of photosystem 2 (LHCP2) were reduced in maize (Zea mays L. cv. OP Golden Bantum) plants grown for 14 days in 8.0 nmol m‐2s‐1 of blue light compared to those in plants grown under an equal irradiance of red light. At the same time, there was a small increase in steady state levels of mRNA for the Dl protein of PS2 (psbA) in blue‐grown plants. The reduction of LHCP2 mRNA and the increase in psbA mRNA were observed in both 5‐ and 10‐day‐old blue‐light‐grown leaves, but the degree of reduction or increase was much greater in 10‐day‐old leaves. Maize grown under 6 different mixtures of blue and red light, each with a total irradiance level of 8.0 μmol m‐2 s‐1, showed the same degree of LHCP2 mRNA reduction relative to red light. This is different from the behavior of psbA which increased in a linear manner with increasing amounts of blue light. The amounts of Chi a and Chi b in these mixed‐light samples were not significantly different froi those found in pure red light. This indicates that a low fluence level of blue light, even when combined with red light, is sufficient to reduce equilibrium levels of proteins and mRNA of LHCP2, and this reduction is independent of pigment formation. It also suggests that the mechanisms of blue‐light regulation of mRNA may operate differently at the nuclear and chloroplast levels.