elongated mesocotyl1, a Phytochrome-Deficient Mutant of Maize

Abstract

To begin the functional dissection of light signal transduction pathways of maize (Zea mays), we have identified and characterized the light-sensing mutant elm1 (elongated mesocotyl1). Seedlings homozygous for elm1 are pale green, show pronounced elongation of the mesocotyl, and fail to de-etiolate under red or far-red light. Etiolated elm1 mutants contain no spectrally active phytochrome and do not deplete levels of phytochrome A after red-light treatment. High-performance liquid chromatography analyses show that elm1 mutants are unable to convert biliverdin IX␣ to 3Z-phytochromobilin, preventing synthesis of the phytochrome chromophore. Despite the impairment of the phytochrome photoreceptors, elm1 mutants can be grown to maturity in the field. Mature plants retain aspects of the seedling phenotype and flower earlier than wild-type plants under long days. Thus, the elm1 mutant of maize provides the first direct evidence for phytochromemediated modulation of flowering time in this agronomically important species.The phytochrome family of photoreceptors mediates many of the responses that plants display to changes in their light environment (Smith, 2000). The basis of phytochrome action is a reversible photoconversion between a red light (R)-absorbing form (Pr) and a far-red light (FR)-absorbing form (Pfr;Quail, 2002). In lower plants, the family is represented by a small number of nuclear genes (Schneider-Poetsch et al., 1998). However, gene duplication and evolutionary divergence have resulted in the formation of functionally diverse multigene families in flowering plants. In Arabidopsis, the phytochrome family consists of five genes: PHYA, PHYB, PHYC, PHYD, and PHYE (Clack et al., 1994), whereas the grasses have three phytochromes: PhyA, PhyB, and PhyC (Mathews and Sharrock, 1996). In maize (Zea mays), an ancestral genomic duplication has increased the total family size to at least six: PhyA1, PhyA2, PhyB1, PhyB2, PhyC1, PhyC2, and possibly PhyC3 (Christensen and Quail, 1989; Childs et al., 1997; Basu et al., 2000). Although loss-of-function phy mutants have been characterized in a broad range of plants, including Arabidopsis (for review, see Whitelam et al., 1998) The photoactive holoprotein (phy) consists of a PHY apoprotein (PHY) covalently attached to a linear tetrapyrrole (bilin) chromophore, 3E-phytochromobilin (P⌽B; Terry, 1997). The first committed step in the synthesis of P⌽B is the conversion of heme to biliverdin (BV) IX␣ by the enzyme heme oxygenase (Weller et al., 1996). BV IX␣ is then reduced to 3Z-P⌽B by P⌽B synthase and subsequently isomerized to 3E-P⌽B (Terry et al., 1995). Of these three activities, genes encoding the first two have now been cloned (Davis et al., 1999;Muramoto et al., 1999; Kohchi et al., 2001). The HO1 (HY1) gene encodes heme oxygenase, which is targeted to the plastid (Muramoto et al., 1999). The HY2 gene encodes P⌽B synthase, a ferredoxin-dependent BV reductase, which is also plastid localized (Kohchi et al., 2001). It is not yet known whether the isomerization of 3Z-...

Photomorphogenic Responses in Maize Seedling Development

“…Because shade avoidance would be detrimental to yield, artificial (human) selection is likely to drive the attenuation of shade avoidance responses. The correlation between R/FR responsiveness and subpopulation and the finding that phytochrome-deficient plants result in early flowering in maize (Sawers et al, 2002) is consistent with the notion that breeding practices have selected for reduced phytochrome response in maize.…”

“…Furthermore, significant differences in light response were observed between semitropical (ST) varieties and North American Cornbelt-derived stiff stalk (SS) and nonstiff stalk (NSS) subpopulations. These results are consistent with the theory that modern breeding practices have selected for lines with reduced light response (Sawers et al, 2002). We also show a limited role for light in C4 photosynthetic development.…”

“…We also have utilized the phytochrome deficient elm1 mutant to directly assess the contribution of phytochrome signaling pathways to the inhibition of mesocotyl elongation and the expression of photosynthetic genes. The elm1 mutant is defective in phytochrome responses due to a block in phytochrome chromophore biosynthesis that severely reduces the active phytochrome pools in maize (Sawers et al, 2002). Together, these resources have enabled us to perform a comprehensive survey of seedling light responses in this agronomically important plant species.…”

“…Phytochrome mutants have only been described recently in sorghum (Sorghum bicolor; Childs et al, 1997), barley (Hordeum vulgare; Hanumappa et al, 1999), rice (Oryza sativa; Izawa et al, 2000;Takano et al, 2001), and maize (Sawers et al, 2002) 1 This work was supported by the National Science Foundation (grant nos. IBN-0110297 to T.P.B.…”

“…The elm1 mutation conditions seedlings that are pale green with an elongated mesocotyl under all light conditions. In the field, elm1 plants tend to lodge and flower slightly earlier than the wild-type (Sawers et al, 2002). The elm1 mutant is impaired in phytochrome responses due to a block in phytochrome chromophore biosynthesis (Sawers et al, 2002) and, therefore, provides a means of examining phytochrome-mediated responses under any light regime.…”