THE CONTROL OF FLOWERING IN WHEAT AND BARLEY - WHAT RECENT ADVANCES IN MOLECULAR-GENETICS CAN REVEAL

The combined forces of developmental biologists, studying primordium i nitiation at the stem apex, and mathematical modellers, developing sim ulations of crop growth and development, have brought about considerab le advances in the understanding of the control of flowering in wheat and barley. Nevertheless, there are still major gaps in this understan ding including: what determines the basic rate of development (magnitu de of the phyllochron or plastochron); how temperature and photoperiod interact to bring about the transition from vegetative to reproductiv e development; and how flowering occurs eventually in the absence of i nductive conditions. Although geneticists have tended to measure cerea l flowering in terms of 'days from sowing or emergence to heading', re sults of studies using aneuploids and molecular markers are compatible with the roles for photoperiod and low-temperature vernalization esta blished in purely-physiological or developmental investigations. They have also revealed the existence of 'earliness per se' loci, whose det ailed roles have yet to be established. Progress towards isolating and characterizing wheat and barley loci is hampered by the poor resoluti on of mapping (location to a precision of tens of thousands of base pa irs). Neither of these broad approaches promises a rapid resolution of the factors controlling the induction of flowering. Two expanding are as of molecular generics now provide potential for greater understandi ng of cereal flowering. First, the extensive homoeology among members of the Gramineae can be employed to establish the existence and locati on of genes or quantitative trait loci in rice which correspond to con trolling loci in wheat or barley. Since the rice genome is 1/30th of t he size of the wheat genome, the accuracy of mapping loci can be much higher, and there is greater potential for precise location of loci us ing techniques such as chromosome walking. With the ultimate cloning o f individual genes, and the isolation of gene products, the relative r oles of the 20 loci apparently involved in the induction of flowering of wheal could be explored. However, progress in the molecular genetic s of Arabidopsis (the second area) may provide a more rapid route to u nderstanding the control of flowering in cereals for several reasons: its small genome (1/4 that of rice); the likelihood of extensive homoe ology with cereals, in spite of differences in codon usage between mon ocots and dicots; the existence of a wide range of flowering-time muta nts; and the control of floral induction by a similar range of environ mental factors including photoperiod and low temperature. It is likely that the MCDK (Martinez-Zapater, Coupland, Dean and Koornneef, 1994. In: Meyerowitz EM, Somerville CR. Arabidopsis. New York: Cold Spring H arbor Laboratory, 403-433) model, formulated to explain the genetic an d environmental control of flowering in Arabidopsis, could be employed usefully in the formulation of experimental work on flowering in whea t and barley. This paper reviews these issues, paying particular atten tion to the significance of 'earliness per se' loci and the 'constitut ive floral pathway' for wheat and barley. (C) 1998 Annals of Botany Co mpany.