Bimodal patterns of floral gene expression over the two seasons that kiwifruit flowers develop

Polymerase chain reaction fragments with homology to the Arabidopsis floral meristem identity genes LEAFY and APETALA1 have been isolated from kiwifruit (Actinidia deliciosa [A. Chev.] C. F. Liang and A. R. Ferguson) and have been named ALF and AAP1, respectively. Northern hybridisation analyses have shown that ALF and AAP1 have bimodal patterns of annual expression in developing first-order axillary buds and their subsequent shoots. This pattern of expression is consistent with the 2-year cycle of axillary bud, flower and fruit development observed in kiwifruit. The first period of expression was early in first-order bud development (late spring of the first growing season), when second-order meristems are initiated, and the second, approximately 10 months later, when those meristems differentiate flowers (late spring of the second growing season). In situ hybridisation analyses on axillary buds collected during late spring of the first growing season have shown ALF expression throughout the developing first-order buds and AAP1 expression was localised in developing second-order axillary meristems. During the spring of the second growing season, transcript accumulation for both ALF and AAP1 is localised in differentiating flowers. Our results show that important developmental events are occurring very early in kiwifruit first-order axillary bud development (spring of the first growing season) and it is likely that this includes floral commitment (evocation).