The control of flowering in wheat and barley: What recent advances in molecular genetics can reveal

The combined forces of developmental biologists, studying primordium initiation at the stem apex, and mathematical modellers, developing simulations of crop growth and development, have brought about considerable advances in the understanding of the control of flowering in wheat and barley. Nevertheless, there are still major gaps in this understanding including: what determines the basic rate of development (magnitude of the phyllochron or plastochron); how temperature and photoperiod interact to bring about the transition from vegetative to reproductive development; and how flowering occurs eventually in the absence of inductive conditions. Although geneticists have tended to measure cereal flowering in terms of 'days from sowing or emergence to heading', results of studies using aneuploids and molecular markers are compatible with the roles for photoperiod and low-temperature vernalization established in purely-physiological or developmental investigations. They have also revealed the existence of 'earliness per se' loci, whose detailed roles have yet to be established. Progress towards isolating and characterizing wheat and barley loci is hampered by the poor resolution of mapping (location to a precision of tens of thousands of base pairs). Neither of these broad approaches promises a rapid resolution of the factors controlling the induction of flowering. Two expanding areas of molecular generics now provide potential for greater understanding of cereal flowering. First, the extensive homoeology among members of the Gramineae can be employed to establish the existence and location of genes or quantitative trait loci in rice which correspond to controlling loci in wheat or barley. Since the rice genome is 1/30th of the size of the wheat genome, the accuracy of mapping loci can be much higher, and there is greater potential for precise location of loci using techniques such as chromosome walking. With the ultimate cloning of individual genes, and the isolation of gene products, the relative roles of the 20 loci apparently involved in the induction of flowering of wheal could be explored. However, progress in the molecular genetics of Arabidopsis (the second area) may provide a more rapid route to understanding the control of flowering in cereals for several reasons: its small genome (1/4 that of rice); the likelihood of extensive homoeology with cereals, in spite of differences in codon usage between monocots and dicots; the existence of a wide range of flowering-time mutants; and the control of floral induction by a similar range of environmental 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 Harbor Laboratory, 403-433) model, formulated to explain the genetic and environmental control of flowering in Arabidopsis, could be employed usefully in the formulation of experimental work on flowering in wheat and barley. This paper reviews these issues, paying particular attention to the significance of 'earliness per se' loci and the 'constitutive floral pathway' for wheat and barley. (C) 1998 Annals of Botany Company.