CK and IAA are key hormones that regulate root development, its vascular differentiation and root gravitropism; these two hormones, together with ethylene, regulate lateral root initiation.

No abstract

To elucidate the role of auxin in flower morphogenesis, its distribution patterns were studied during flower development in Arabidopsis thaliana (L.) Heynh. Expression of DR5::GUS was regarded to reflect sites of free auxin, while immunolocalization with auxin polyclonal antibodies visualized conjugated auxin distribution. The youngest flower bud was loaded with conjugated auxin. During development, the apparent concentration of free auxin increased in gradual patterns starting at the floral-organ tip. Anthers are major sites of high concentrations of free auxin that retard the development of neighboring floral organs in both the acropetal and basipetal directions. The IAA-producing anthers synchronize flower development by retarding petal development and nectary gland activity almost up to anthesis. Tapetum cells of young anthers contain free IAA which accumulates in pollen grains, suggesting that auxin promotes pollen-tube growth towards the ovules. High amounts of free auxin in the stigma induce a wide xylem fan immediately beneath it. After fertilization, the developing embryos and seeds show elevated concentrations of auxin, which establish their axial polarity. This developmental pattern of auxin production during floral-bud development suggests that young organs which produce high concentrations of free IAA inhibit or retard organ-primordium initiation and development at the shoot tip.

No abstract

A comparison is made between foliar and axial vascular differentiation. Current thoughts and new evidence are presented on the role of hormones in controlling the differentiation of vascular tissues in organized and tumorous tissues, focusing on the role of auxin and cytokinin in controlling phloem and xylem relationships, vessel size and density, cambium sensitivity, vascular adaptation and xylem evolution in deciduous hardwood trees. The possible role of wounding is also considered. A new hypothesis, namely, the leaf-venation hypothesis, is proposed to explain the hormonal control of vascular differentiation in leaves of dicotyledonous plants. Experimental evidence in support of the hypothesis is presented showing that hydathodes, the watersecreting glands, are the primary sites of auxin synthesis during leaf morphogenesis. Vessel element patterns similar to those found in hydathodes were experimentally induced by exogenous auxin application.