~OMMENT Mutants shed light on plant development EDUARDO R. BEJARANO AND CONRAD LICHTENSTEIN DEPARTMENT OF BIOCHEMISTRY, IMPERIAL COLLEGE,LONDON S'~r7 2AZ, UK. That light provides the source of energy, via photosynthesis, for ma~aufacturing organic molecules is obvious; but light is also an import- ant environmental signal for some of the more dramatic decisions in plant development. These are col- lectively called photomorphogen- esis. Such responses include seed germination, leaf formation, stem growth, flowering and fruit ripen- ing, some of which also involve photoperiodic responses (i.e. to day length) and phototropism (growth towards light). Chloroplast formation, synthesis of components of the photosynthetic apparatus and anthocyanin accumulation are also affected by light (reviewed in Ref. 1). The web of responses is com- plex, bt, t begins with detection of light by photoreceptors. In higher plants, physiological experiments suggest that at least three photo- receptors exist: phytochrome (a far- red/red light receptor), a blue/ UV-A light receptor and a UV-B light receptor. Only phytochrome has heen identified at the molecu- lar and biochemical level 23. It con- sists of a protein attached to a tetra- pyrmle chromophore that can exist in two photochemicaily different forms: the Pr form can absorb red light: the Pf~ form can absorb far- red light. Red light converts P~ to Pf~; far-red light converts Pf~ to Pr" Pf~ is the physiologically active form. It is now known that a gene family encodes a set of slightly dif- ferent phytochrome apoproteins. Subtle differences in the properties of these phytochromes, and differ- ent patterns of expression, may account for the diversity of phyto- chrome responses. The mechanisms by which light signals the cascade of events lead- ing to regulation of gene expression are unknown. Recently, genetic approaches have been taken to- wards unmasking this signal trans- duction pathway. Curiously, the classical genetic approaches taken, though enhanced by gene cloning e 1092 Elsc~ier ."idt.ncc Publinht'r', lid (1"KP technology, would have been poss- ible long ago. The reason for their belated application to this problem is cultural; it is the recent world- wide adoption of Arabidopsis tbaliana as the plant model sys- tem ~. This small weed is a perfectly respectable flowering plant show- ing the typical photomorphogenic responses of dicotyledenous angio- sperms, as summarized in Table 1. In the dark, the hypocotyl of seed- lings elongates. In the light, hypo- cotyl elongation is inhibited, the cotyledons ()pen and enlarge, and develop chloroplasts and differenti- ated cell types. Leaves develop and anthocyanin synthesis commences. Plants with defects in these responses can be generated by mutagenesis and selfed to recover homozygous recessive progeny. One approach taken was to screen for mutant seedlings that failed to show white light-induced suppression of hypocotyl elong- ation ~.~'. These so-called h.r mutants fall into six complementation groups, indicating that at least six genetic loci exist, t0'1-6. In adult plants the i~.rl, hy2 and 10'6 mu- tants display pleiotropic effects to varying extents, such as increased apical dominance, paler green colour, and reduced leaf size. These mutants lack fully functional (photoreversible) phytochrome but do express the apoprotein ~,.'. The other mutants have wild-type phytochrome, suggesting that a later step in control of hypocotyl growth is defective; interestingly, their pleiotropic effects are also less severe. Expression of anthocyanins in hi'l, hy2 and hy6 is normal, in- dicating that their expression is either not regulated by phyto- chrome or that other photo- receptors can compensate. Recently, Liscum and Hangarter s refined this approach by isolating mutants, designated bht (for blue light uninhibited), that have long hypocotyls when grown under intense blue light. But, as in wild- type plants, hypocotyl elcmgation is still inhibited by far-red light. Liscum and Hangarter defined three genetic loci responsible for this effect and showed that progeny of bhtxhl' crosses are wild type. In blu mutants, cotyledon expansion is also reduced btnt chlorophyll content is nortnal: mc)reover, there is little effect on mature pl:mts. Their data corroborate physiologi- cal evidence for an independent blue light signal transduction p:tth- way: however, it is not known whether the genetic lesions affect TAmE1. Phenotypes for photomorphogenic responses of wild-type plants and constitutive Ught-induced mutants Wild tTpe cop l det l det2 Dark Light Dark Dark Dark Chloroplast biogenesis - + + + - Light-specific gene expression - + + + + Seedling development Hypocotyl inhibition - + + + + Cotyledon expansion - + + + + Leaf formation - + + + + Anthocyanin synthesis - + + + + Germination - + - + Dark adaptation a + - + - aDark adaptation is defined at the molecular level as the changes in the mRNA levels of some genes when a plant, grown in light, is transferred to the dark. The other characteristics were analysed in seedlings. T1G JANI'ARY 1992 VOL 8 NO. 1 U