Beyond the darkness: recent lessons from etiolation and de-etiolation studies

The state of etiolation is generally defined by the presence of non-green plastids (etioplasts) in plant tissues that would normally contain chloroplasts. In the commonly used dark-grown seedling system, etiolation is coupled with a type of growth called skotomorphogenesis. Upon illumination, de-etiolation occurs, marked by the transition from etioplast to chloroplast, and, at the seedling level, a switch to photomorphogenic growth. Etiolation and de-etiolation systems are therefore important for understanding both the acquisition of photosynthetic capacity during chloroplast biogenesis and plant responses to light—the most relevant signal in the life and growth of the organism. In this review, we discuss recent discoveries (within the past 2–3 years) in the field of etiolation and de-etiolation, with a particular focus on post-transcriptional processes and ultrastructural changes. We further discuss ambiguities in definitions of the term ‘etiolation’, and benefits and biases of common etiolation/de-etiolation systems. Finally, we raise several open questions and future research possibilities.

Control by Phytochrome of the Level of Nicotinamide Nucleotides in the Cotyledons of the Mustard Seedling

Long-term and short-term effects of phytochrome on the levels ("tissue contents") of NAD, NADH2 , NADP and NADPH2 were measured in the cotyledons of the mustard (Sinapis alba L.) seedling. It was found that long-term far-red light (which is considered to operate exclusively via active phytochrome, Pfr) strongly increases the levels of NADP and NADPH2 , whereas this light treatment suppresses the levels of NAD and NADH2 below the levels present in the cotyledons of the dark grown seedling. The high levels of NADP and NADPH2 as well as the low levels of NADP2 (and probably also NAD) are actively being maintaned by the far-red light. In experiments with light pulses it was found that a red light pulse causes a rapid but transient rise in the level of NADPH2 under all circumstances (dark-grown seedlings and seedlings pretreated with long-term far-red lig h t). The operational criteria for the involvement of phytochrome (Pfr) are fulfilled. In the case of NADP a significant rise of the level following a red light pulse could only be observed if the seedlings were pretreated with long-term far-red light. Dark-grown seedlings do not significantly respond. In the case of NAD and NADP2 no significant changes could be induced by light pulses either in dark-grown seedlings or in seedlings pretreated with long-term far-red light. It is concluded that NADPH2 does neither originate from NADP2 nor from NADP. It is further concluded that it is unlikely that nicotinamide nucleotides are links in any causal chain originating from Pfr and leading to phenomena of photomorphogenesis. We favour the con­cept that the phytochrome-mediated changes caused by light pulses occur in the plastids.

On the Regulation of Enzyme Levels (phenylalanine ammonia-lyase) in Different Organs of a Plant (Sinapis alba L.)

The time courses of formation of the enzyme phenylalanine ammonia-lyase (PAL; EC 4.3.1.5) mediated by far-red light (i. e. by phytochrome, Pfr) in cotyledons and hypocotyl of the mustard seedling (Sinapis alba L.) are described. While PAL activity in the cotyledons can scarcely be detected in the dark-grown seedling, the enzyme can be synthesized in the hypocotyl even in the dark. However, the degree of induction by far-red light is much greater in the cotyledons than in the hypocotyl. In the cotyledons the enzyme is not stable. The enzyme level eventually returns to nearly zero even under continuous far-red light. The time course of the level of PAL in the cotyledons (Fig. 1) can be explained by the following 3 factors (Fig. 2): 1. Induction of PAL synthesis by Pfr, whereby Pfr is continuously required; 2. Inactivation (degradation) of PAL by an inactivating principle; 3. Repression of PAL synthesis. The time course of the level of PAL in the hypocotyl is completely different (Fig. 1). An explanation of the hypocotyl data is presented which is based on the assumption that PAL synthesis in the dark and PAL synthesis mediated by phytochrome are independent phenomena which occur in different tissues of the hypocotyl. It appears that the occurrence of dark synthesis of a stable enzyme in the hypocotyl explains the seemingly dramatic difference between the cotyledons and the hypocotyl with respect to PAL.