Light differentially regulates cell division and the mRNA abundance of peanucleolin during de-etiolation

The abundance of plant nucleolin mRNA is regulated during de-etiolation by phytochrome. A close correlation between the mRNA abundance of nucleolin an d mitosis has also been previously reported. These results raised the quest ion of whether the effects of light on nucleolin mRNA expression were a con sequence of light effects on mitosis. To test this we compared the kinetics of light-mediated increases in cell proliferation with that of light-media ted changes in the abundance of nucleolin mRNA using plumules of dark-grown pea (Pisum sativum) seedlings. These experiments show that S-phase increas es 9 h after a red light pulse, followed by M-phase increases in the plumul e leaves at 12 h post-irradiation, a time course consistent with separately measured kinetics of red light-induced increases in the expression of cell cycle-regulated genes. These increases in cell cycle-regulated genes are p hotoreversible, implying that the light-induced increases in cell prolifera tion are, like nucleolin mRNA expression, regulated via phytochrome. Red li ght stimulates increases in the mRNA for nucleolin at 6 h post-irradiation, prior to any cell proliferation changes and concurrent with the reported t iming of phytochrome-mediated increases of rRNA abundance. After a green li ght pulse, nucleolin mRNA levels increase without increasing S-phase or M-p hase. Studies in animals and yeast indicate that nucleolin plays a signific ant role in ribosome biosynthesis. Consistent with this function, pea nucle olin can rescue nucleolin deletion mutants of yeast that are defective in r RNA synthesis. Our data show that during de-etiolation, the increased expre ssion of nucleolin mRNA is more directly regulated by light than by mitosis .