Hormonal influence on photocontrol of the protandry in the genus Helianthus

Abstract

Under natural photoperiodic conditions protandry in hermaphrodite disc flowers of sunflower (Helianthus annuus L.) is determined by the different elongation rates of the style and filaments. The elongation of the filament and style starts simultaneously after the daily dark period, but the style growth rate is slower. When plants close to anthesis are exposed to continuous white light (WL) a loss of protandry occurs: the filaments do not grow far enough to extrude the anthers from the corolla. The histological analyses show that the number of filament epidermal cells remains unaltered after organ elongation and that cells respond to photoperiod only by cell expansion. Emasculation does not substantially inhibit filament cell expansion, whereas isolation of the filament or stamen from the corolla suggests that this organ could be the perception site of the filament growth stimulus. In vitro treatments with auxin (indole-3-acetic acid, IAA or alpha-naphthaleneacetic acid, NAA) reverses the inhibition of cell expansion caused by continuous WL, whereas gibberellic acid (GA(3)) at high concentrations reproduces the effect of continuous WL. Experiments carried out on various Helianthus spp. show that all these plants have evolved the same photo- and hormonal-control of the protandry. In experiments in which the light treatments were continued for 24 h, the auxins drastically reduced the inhibiting effect of red light (R) and dichromatic treatments FR (far red)+R, whereas GA(3) repressed filament extension regardless of light quality. As far as auxins are concerned, the response of sunflower filaments does not appear to be connected with the polar transport of the hormone. Moreover, the promoting effect of darkness is not mediated by an increase of endogenous free IAA in disc flowers. However, sunflower filaments manifested a similar temporal pattern of response to the light/dark cycle and to auxin.

The extreme dwarf phenotype of the GA-sensitive mutant of sunflower, dwarf2, is generated by a deletion in the ent-kaurenoic acid oxidase1 (HaKAO1) gene sequence

“…In particular, the anther filaments remain inside the corolla tube hindering the selfing of disc flowers. Notably, GA 3 rescued the normal length of filaments before the anthesis but hormone treatment was ineffective to promote the spectacular cell expansion that in sunflower characterizes the filaments in the day of anthesis (Lobello et al 2000).…”

“…In dw2, the dark-green color was not so much Values shown are average ± SD of three independent experiments on progenies or plants, with 10-20 replicates each (plants or flowers). Asterisks indicate significant differences from wild type (WT): ** P \ 0.01, * P \ 0.05, respectively (Student t test) a The data were collected the day of anthesis (Lobello et al 2000) b The data were collected one day before the anthesis due to higher pigment content, but was correlated with a changed leaf morphology: the wrinkled and thicker dw2 leaves was reversed by GA 3 (Table 3).…”

“…These results could suggest a defect in a GA20ox gene. However, this was unlikely because the occurrence of a The data were collected the day of anthesis (Lobello et al 2000) GA20ox gene family in the sunflower genome (Carzoli et al 2009) unfit with the extreme dwarf phenotype of the mutant. Moreover, mutants with no functional GA20ox genes generally accumulated GA 53 and showed only small variations in the GA 19 level with respect to normal plants (Spielmeyer et al 2002;, whereas in dw2 plants GA 19 was undetectable (Table 5).…”

“…Morphological analysis of floral organs was performed as follows: number of ray flowers and floral organs, presence of ovules, and size of corollas, anthers, and styles were recorded in WT and mutant at the middle stage of the inflorescence anthesis. The filament length was measured either 1 day before the anthesis and in the same day of the anthesis (Lobello et al 2000). Organ lengths were measured with an eyepiece micrometer, using a Wild Makroskop M420 (Wild Heerbrugg Ltd, Heerbrugg, Switzerland) inverted microscope (Lobello et al 2000).…”

“…The filament length was measured either 1 day before the anthesis and in the same day of the anthesis (Lobello et al 2000). Organ lengths were measured with an eyepiece micrometer, using a Wild Makroskop M420 (Wild Heerbrugg Ltd, Heerbrugg, Switzerland) inverted microscope (Lobello et al 2000).…”