Genetic analysis of the roles of phytochromes A and B1 in the reversed gravitropic response of the lz‐2 tomato mutant

The lz‐2 mutation in tomato ( Lycopersicon esculentum ) causes conditional reversal of shoot gravitropism by light. This response is mediated by phytochrome. To further elicit the mechanism by which phytochrome regulates the lz‐2 phenotype, phytochrome‐deficient lz‐2 plants were generated. Introduction of au alleles, which severely block chromophore biosynthesis, eliminated the reversal of hypocotyl gravitropism in continuous red and far‐red light. The fri 1 and tri 1 alleles were introduced to specifically deplete phytochromes A and B1, respectively. In dark‐grown seedlings, phytochrome A was necessary for response to high‐irradiance far‐red light, a complete response to low fluence red light, and also mediated the effects of blue light in a far‐red reversible manner. Loss of phytochrome B1 alone did not significantly affect the behaviour of lz‐2 plants under any light treatment tested. However, dark‐grown lz‐2 plants lacking both phytochrome A and B1 exhibited reduced responses to continuous red and were less responsive to low fluence red light and high fluence blue light than plants that were deficient for phytochrome A alone. In high light, full spectrum greenhouse conditions, lz‐2 plants grew downward regardless of the phytochrome deficiency. These results indicate that phytochromes A and B1 play significant roles in mediating the lz‐2 phenotype and that at least one additional phytochrome is involved in reversing shoot gravitropism in this mutant.