Gibberellin Control of Reproductive Transitions in Brassica oleracea Curd Development

Cauliflower (Brassica oleracea var. botrytis) and broccoli (B. oleracea var. italica) differ in the developmental stage of the reproductive meristem at harvest. A cauliflower head is formed by arrest at the inflorescence meristem stage and broccoli at the flower bud stage, and the horticultural value of the crop depends on synchronous development across the head. In other plant species, gibberellin (GA) can promote floral development and is therefore a candidate for providing the early developmental cues that shape the curd morphology. This research investigated the effect of GAs on the two horticulturally important transitions of the reproductive meristem: initiation of the inflorescence meristem and initiation of floral primordia on the proliferated inflorescence meristems. GA is known to affect the former in many species, but effects on the latter have not been determined. It is also not known whether one or both active forms produced by the two GA biosynthetic pathways is involved in the reproductive transitions in this crop. GAs from the early-13 hydroxylation pathway (GA3) and the non-13 hydroxylation pathway (GA4D7) were applied to the shoot apical meristems of cauliflower and broccoli at three developmental stages: adult-vegetative, curd initiation, and curd enlargement. GAs applied during the adult vegetative stage caused the curd to form faster and after fewer additional nodes in both cauliflower and broccoli. GAs applied to the inflorescence meristem did not cause floral primordia to form nor did the expression of transitionassociated genes change. Integrator genes BoLFY and SOC1 had constant expression over 24 hours, and meristemidentity genes BoAP1-a and BoAP1-c remained undetectable. However, GAs applied early during the reproductive phase increased bract development in cauliflower curds. This study shows that GAs from both pathways can trigger the vegetative-to-reproductive transition in both cauliflower and broccoli, resulting in earlier curd formation. However, GAs did not advance the inflorescence-meristem-to-floral-primordium transition; on the contrary, they increased bract incidence in cauliflower, a sign of reversion toward the vegetative stage, suggesting that another pathway is responsible for this second transition in cauliflower and broccoli. There are three main steps in the process that leads to flowering: a switch from vegetative to inflorescence meristem, a switch from inflorescence meristem to floral primordium, and floral bud development. Distinguishing these processes in most plants has been difficult, because they may occur simultaneously in different regions of a small meristem. The large meristem size, synchronized development, and eventual developmental arrest in cauliflower and broccoli inflorescences allow greater temporal resolution of these transitions. The curd phenotype in cauliflower is comprised of inflorescence meristems that share characteristics of both vegetative and reproductive apices (Sadik, 1962). In broccoli, the head is composed of flower buds, because developmental arrest occurs before anthesis (Fujime and Okuda, 1996). This arrest is accompanied by an extended proliferation of meristems followed by the coordinated induction of floral primordia. Distinguishing the regulation of individual developmental steps is of practical importance for cauliflower and broccoli breeding and production. In breeding, it is difficult to restore the cauliflower phenotype to commercial standards in the progeny of a cross outside cauliflower, resulting in a very narrow genetic base among cauliflower cultivars and limited introgression of disease resistance genes. A better understanding of the genetic or hormonal cues involved in curd development could result in breeding procedures to select early-generation progeny that are parents of high-quality hybrid cauliflower. In field production of broccoli, asynchronous development of the curd can occur in warm temperatures, rendering the broccoli commercially unacceptable (Farnham and Björkman, 2011). A better understanding of flower initiation and bud enlargement can lead to new preventive measures. Vegetative to Inflorescence Transition In arabidopsis (Arabidopsis thaliana), reproductive induction (vegetative to inflorescence meristem) is controlled by a complex network of five flowering pathways that involve response to daylength, vernalization, carbohydrates, gibberellins, and an age-related autonomous pathway. However, the five pathways ultimately converge on the floral integrator genes leafy (LFY), suppressor of overexpression of constans 1 (SOC1), and flowering locus T (FT), upregulating their expression (Blazquez and Weigel, 2000; Simpson and Dean, 2002).