It's not easy being green
As seeds sprout, they consume a finite energy pack contained within them. Once those resources are depleted, the plant cell nucleus must be ready to switch on a photosynthetic program. A team of researchers led by postdoctoral fellow Jesse Woodson, in the lab of Joanne Chory, recently showed a new way that those signals are relayed. In a study published in Current Biology, Chory and her group identified a signaling factor that turns on photosynthesis-related genes—a finding that may help achieve greater crop yields and better plant health.
Although in plants and animals most genes reside in the nucleus, small DNA rings of genes are found in other cellular venues such as energy-producing mitochondria. Plant chloroplasts, whose primary function is to turn light and carbon dioxide into energy and carbohydrates required for growth, also contain genes that regulate photosynthesis-related factors encoded in the plant cell nucleus.
The Chory lab previously identified mutations in five genes in Arabidopsis thaliana plants that were unable to synthesize molecules such as chlorophyll or respond to signals generated by intermediates of the chlorophyll biosynthetic pathway. Those studies suggested that when plants undergo stress, an intermediate accumulates that tells the nucleus to stop "turning green."
These mutants—called GUN (for genomes uncoupled) 1 to 5—lack proteins necessary to generate signals or to relay information from these signals from stressed chloroplasts to the nucleus. Chloroplasts in normal plants might deploy those signals when plants encounter stress, such as too much heat or too little water. Inhibitory signals could also be sent when germinating sprouts are not yet mature enough to make the leap from relying on the seed energy pack to generating their own energy using sunlight.
Suspecting that healthy chloroplasts also generate signals, the team screened Arabidopsis for factors that switched photosynthetic proteins on, rather than off. What they found was a gene designated gun 6, the first gun mutant indicating that signals are sent from functioning chloroplasts, as well as chloroplasts that are not functioning well.
"Overall, this work answers basic questions regarding how a plant grows, builds chloroplasts and harvests light energy in order to turn into a photosynthetic organism," says Chory. "Understanding how plants coordinate gene expression between the chloroplast and nucleus will ultimately increase crop yields in the field, where plants often encounter multiple stresses during the growing season."