Interactions between hy1 and gun mutants of Arabidopsis, and their implications for plastid/nuclear signalling

Lhcb and other nuclear genes for chloroplastic proteins are regulated by se veral signals. Among them, light and retrograde signals from the plastid it self appear to act through closely related mechanisms. To investigate this interaction, we analysed an Arabidopsis mutant, hy1, deficient in plastidic heme oxygenase, hy1 is defective in phytochrome chromophore biosynthesis, which has other indirect effects on tetrapyrrole metabolism. We generated d ouble mutants between hy1-6.2, genetically a null mutation, and three known gun (genome uncoupled) mutants, defective in retrograde plastid signalling . Recent molecular evidence shows GUN5 to be involved in tetrapyrrole metab olism (N. Mochizuki and J. Chory, manuscript in preparation). We observed h y1gon4 to be semi-albino plants, and hy1gun5 albino lethal, in a high-light -sensitive manner. Both double mutants showed defective greening and chloro plast development, and expressed Lhcb at reduced levels specifically in hig h light. Their degree of 'genome uncoupling' (Lhcb expression in the absenc e of functional chloroplasts) was similar to that observed in single mutant s. These results can be interpreted as a metabolic (rather than genetic) in teraction between HY1 and GUN4 or GUN5, and this in turn supports the invol vement of tetrapyrroles as plastid signals. The tetrapyrrole precursor 5-am inolevulinic acid (ALA) inhibited Lhcb expression in hy1. Surprisingly, ALA also rescued photomorphogenesis of hy1. We speculate that either one tetra pyrrole intermediate, which can accumulate anomalously in hy1, or an altere d ratio between two intermediates, plays a role as a repressor of Lhcb expr ession. gun1 did not exacerbate the plastid or Lhcb expression phenotype of hy1. This can be interpreted as a role for gun1 strictly on the same pathw ay as hy1 or, more likely, as evidence for the existence of at least one se parate, non-tetrapyrrole related plastid signal.