“ESSENTIAL METABOLIC PATHWAYS required for the production and movement of energy within the plant showed significantly lower levels of gene expression variation across development, abiotic stress and natural genetic variation. This suggests that gene expression for these pathways is constrained across a range of external, internal and genetic stimuli. This constraint agrees with the essentiality of these pathways, e.g. aerobic respiration and the TCA cycle, to an individual cells survival.”
http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0001838