Inter-organellar crosstalk in higher plants: impaired chloroplast development affects mitochondrial gene and transcript levels

Go-ordination of gene expression between the three genomes present in plast ids, mitochondria and nucleus is of crucial importance for plant cells. Pre vious studies revealed that in white leaves of the albostrians (Hordeum vul gare cv. Haisa) mutant, photosynthesis-related plastid and nuclear genes ar e expressed only at an extremely low level. The plastids of this mutant lac k ribosomes, photosynthetic activity and have only rudimentary membrane sys tems. Here we report on the expression of mitochondrial genes in albostrian s barley. Steady-state RNA levels of the mitochondrial genes encoding cytoc hrome oxidase or ATPase subunits, coxII, coxIII, atpA, atp6, atp9 and cob, were observed to be consistently elevated in the white leaves but not in ro ots. Investigation of mitochondrial DNA revealed an about three-fold enhanc ed mitochondrial gene copy number in white compared to green leaf cells, bu t no differential amplification of mitochondrial genes. Analysis of plants in which the white albostrians plastids were combined with a new nuclear ba ckground showed that the enhanced transcript levels were a consequence of t he impaired plastids and not of the nuclear albostrians allele. Furthermore , plants bleached by the carotenoid biosynthesis inhibitor norflurazon also showed an enhanced mitochondrial transcript level. These findings allow th e conclusion that lack of chloroplast activity in an otherwise fully differ entiated leaf leads to an increase in mitochondrial gene copy number and an elevated level of mitochondrial transcripts. Our results indicate an influ ence of plastids on the genetic apparatus of mitochondria in leaves but not in roots.