Photomorphogenesis of the root system in developing sunflower seedlings: a role for sucrose

The domestic sunflower (Helianthus annuus L. cv. 'Giganteus') has been used since the 19th century as a model plant for the study of seedling development in darkness and white light (WL) (scoto- versus photomorphogenesis). However, most pertinent studies have focused on the developmental patterns of the hypocotyl and cotyledons, whereas the root system has been largely ignored. In this study, we analysed entire sunflower seedlings (root and shoot) and quantified organ development in the above- and belowground parts of the organism under natural (non-sterile) conditions. We document that seedlings, raised in moist vermiculite, are covered with methylobacteria, microbes that are known to promote root development in Arabidopsis. Quantitative data revealed that during photomorphogenesis in WL, the root system expands by 90%, whereas stem elongation is inhibited, and hook opening/cotyledon expansion occurs. Root morphogenesis may be mediated via imported sucrose provided by the green, photosynthetically active cotyledons. This hypothesis is supported by the documented effect of sucrose on the induction of lateral root initials in sunflower cuttings. Under these experimental conditions, phytohormones (auxin, cytokinin, brassinolide) exerted little effect on root and cotyledon expansion, and no hormone-induced initiation of lateral roots was observed. It is concluded that sucrose not only acts as an energy source to fuel cell metabolism but is also a shoot-derived signalling molecule that triggers root morphogenesis.