Crowd control in the shade

research news literature Crowd control in the shade Robson, P.R.H., McCormac, A.C., Irvine, A.S. and Smith, H. (1996) Genetic engineering of harvest index in tobacco through overexpression of a phytochrome gene, Nat. Biotechnol. 14, 995-998 Plants have evolved a fascinating array of strategies with which to survive under highly competitive conditions. For example, individual plants can sense when neighbours are nearby, competing for resources, by employing phytochromes to detect far-red light. In dense canopies most of the visible radiation is absorbed by the chlorophylls, while far-red light is reflected and causes the conversion of phytochrome into a biologically active form. This results in a shade avoidance response: there is an increase in stem elongation growth which projects leaves back into the light. For individuals the advantages of the shade avoidance response appear obvious, but this may not be the case in artificial crop monocnltures as competition can reduce overall productivity. Robson et al. have examined the impact of altering the shade avoidance response for field-grown plants by manipulating the plant's phytochrome complement. In lightgrown adult plants, phytochrome B (and others) modulates stem growth in response to the red : far-red light ratio. In contrast, in etiolated seedlings, phytochrome A predominates and acts to inhibit stem growth. Typically phytochrome A is not active in light conditions as it is down-regulated by light. However, when expressed constitutively in transgenic plants, phytochrome A is relatively stable and results in a dwarfed phenotype. In exact agreement with these observations the present study revealed that transgenic overproduction of oat phytochrome A in tobacco prevented shade avoidance. Furthermore, transgenic plants grown at high densities had a higher proportion of assimilate allocated to leaves rather than to stems. There are several exciting aspects to the work by Robson et al. The potential increase in harvest index, and relatively simple means by which assimilates can be reallocated, is of direct interest for agricultural exploitation. The results also make an ecological point, indicating that of all the potential signals that might be associated with crowding, tlie relative ratio of red and far-red light appears to be of paramount importance to the plant. Finally, it is enlightening to discover that what appears to be a highly complex phenomenon - shade avoidance can nevertheless be radically and predictably altered by the manipulation of a single gene. © 1996 Elsevier Science Ltd

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Methylation and development Ronemus, M.J., Galbiati, M., Ticknor, C., Chen, J. and Dellaporta, S.L. (1996) Demethylation-induced developmental pleiotropy in Arabidopsis, Science 273, 654--657 It has been known for some time that the addition of methyl groups to DNA could be an important means of modifying gene expression and plant development. Indeed, work with Arabidopsis mutants ~ddm 'decrease in DNA methylation') has shown that altered levels of methylation may be associated with developmental abnormalities. What has been lacking was the means to perturb the methytation process so that the demethylated condition could be assessed. Ronemus et at. set out to examine this by transforming Arabidopsis plants with an antisense construct to a cytosine methyltransferase gene - thereby reducing the amount of methylation in transformed plants. Methylation patterns in the transgenic lines were examined using Southern (DNA) hybridization experiments. It was possible to identify 'strong' antisense lines - in which a large proportion of cytosines were demethytated relative to the wild-type - by restricting genomic DNA with enzymes whose ability to cleave is affected by methylation. Probes for examining repetirive DNA were a centromeric repeat or a

5S ribosomal DNA sequence ,both sequences are methylated in the wild-type~. Single-copy gene probes were also used in these hybridization experiments: substantial demethylation of these genes also appeared to occur in the strong antisense lines. This demonstrates that antisense expression had the effect of reducing the level of methylation in both repetitive and single copy D N A - previous analysis o f d d m l had indicated that demethylation was confined to non-coding repetitive DNA. High-performance liquid chromatography revealed that in strong antisense lines total genomic levels of 5-methylcytosine were reduced by up to 71% relative to the wild-type. The phenotypes of the strong antisense lines were severely altered and took far longer to flower, had more leaves and more secondary branches on the primary inflorescence shoot. Some of the flowers also had abnormalities, such as a threefold increase in stamen number. In summary, these results provide a strong link between cytosine methylation and the regulation of gene expression during plant development.

Flavonols and fertility Bubulis, I.E., lacobucci, M. and Shirley, B.W. (1996) A null mutation in the first enzyme of flavonoid biosynthesis does not affect male fertility in Arabidopsis, Plant Cel/ 8, 1013-1025 Flavonoids are no longer merely regarded as pigments that serve to attract insects to flowers: their importance as signalling molecules in the establishment of plantRhizobium symbioses and in pathogen resistance is now widely appreciated. One intriguing discovery that has received much recent attention concerns the requirement for fiavonols in male fertility. In both maize and petunia, mutants have been isolated in which chalcone synthase (CHS) activity is lost, with an accompanying loss of fertility. Flavonols appear to be involved in this process - application of flavonols restored fertility in mutant plants and stimulated pollen to germinate in vitro. It was therefore assumed that pollen of other species would have a similar requirement for fiavonols. The present study now reveals this assumption to be incorrect and demonstrates that mutant Arabidopsis plants lacking CHS activity are fully fertile. The mutant tt4 ('transparent testa 4') was isolated by screening ethyl methanesul-

fonate treated plants for yellow seed coloration. Further analysis indicated that the mutation mapped to the CHS locus and that tt4 plants lack anthocyanins and ultraviolet light-absorbing flavonoids. Sequence analysis established that the mutation corresponded to a G-to-A transition in the AG consensus sequence at the 3' splice acceptor site of the single intron of the CHS gene. Shirley's lab has investigated this phenomenon to determine whether any alternative splicing sites could act to restore CHS mRNA processing, but none appear to be capable of providing a full length transcript. Further confirmation that no CHS protein expression occurred in the mutants was obtained by immunoblotting seedling and flower extracts. In addition to highlighting the importance of appropriate 3' acceptor sites for mRNA processing, the work also demonstrates the need for care in extrapolating data from a limited number of plant species - an important reminder when so much research is conducted on just a few model species. October 1996, Vol. 1, No, !0

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