Wheat seedling grown with their shoot bottom exposed to red light (400 micromol m(-2) s(-1)) either with constant illumination or light-dark cycles did not accumulate chlorophyll. This near-etiolation response was manifested by a critical threshold intensity of red light and did not need continuous illumination. The inhibition of the greening process resulted from reduced synthesis of glutamate-1-semialdehyde and consequent reduction in tetrapyrrole precursor 5-aminolevulinic acid. Red light perceived by the shoot bottom down regulated the protein and/or gene expression of enzymes involved in the biosynthesis of tetrapyrroles. The contents of endogenous cytokinins, i.e., isopentenyl-adenosine and dihydrozeatinriboside, were reduced in seedlings grown in red light having their shoot bottom exposed. Application of exogenous cytokinin and its analogue to roots of seedlings grown in red light reversed the down regulation of the greening process. The reversal of red-light-induced near-etiolation morphogenesis by far-red (200 micromol m(-2) s(-1)) or blue (25 mumol m(-2) s(-1)) light suggests that it could be a very high red-irradiance response of phytochrome, in the meristematic layers of the shoot bottom, that works in concert with blue light receptor(s).

The present study was conducted to observe the role of the root-shoot transition zone in the development of PS I and PS II in red light. The development of PS II and PS I was severely inhibited when root-shoot transition zones of wheat seedlings were exposed to red light (670 nm) of intensity 500 micromol m(-2) s(-1). Chlorophyll biosynthesis was also inhibited in these seedlings. Most of the PS I and PS II proteins (D1, LHCPII, CP47, OEC33) and their transcript levels were severely inhibited but cyt b6f complex proteins were only partially inhibited. Protein and transcript levels of Rubisco large subunit and protochlorophyllide (Pchlide) biosynthesis were also severely inhibited in these seedlings. When incubated in the dark with or without the precursor of chlorophyll biosynthesis ALA, these plants accumulated most of the Pchlide, as non-phototransformable Pchlide, suggesting low activity of NADPH:protochlorophyllide oxidoreductase (EC 1.6.99.1) in these plants. These effects were not observed when the seedlings were grown in red light with their root-shoot transition zones covered. These results suggest that the root-shoot transition zone plays an important role in the overall greening process involving transcription and translation of photosynthetic genes.

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