Coordinating the morphogenesis-differentiation balance by tweaking the cytokinin-gibberellin equilibrium

Morphogenesis and differentiation are important stages in organ development and shape determination. However, how they are balanced and tuned during development is not fully understood. In the compound leaved tomato, an extended morphogenesis phase allows for the initiation of leaflets, resulting in the compound form. Maintaining a prolonged morphogenetic phase in early stages of compound-leaf development is dependent on delayed activity of several factors that promote differentiation, including CIN-TCP transcription factor (TF) LA, the MYB TF CLAU and the plant hormone Gibberellin (GA). Here, we investigated the genetic regulation of the morphogenesis-differentiation balance by studying the relationship between LA, CLAU and GA. Our genetic and molecular examination suggest that LA is expressed more broadly than CLAU and determines the spatio-temporal context of CLAU activity. We demonstrate that both LA and CLAU affect the Cytokinin/Gibberellin (CK/GA) balance. LA reduces the sensitivity of the leaf margin to CK, shown before to be also affected by CLAU. CLAU affects leaf active GA content and sensitivity, shown previously to be also influenced by LA. Therefore, LA and CLAU likely function in parallel pathways to promote leaf differentiation by converging on common downstream processes, including the CK/GA balance.

Observations on the development of plants

The influence of light intensity and daylength on four short-day species of <i>Chenopodium</i> was analysed. The following species were tested: <i>Ch. ficifolium, Ch. glaucum, Ch. rubrum</i> and <i>Ch. hybridum</i>. Under short day, generative initiation was accelerated, the abundance of flowering, and growth and leaf differentiation processes were reduced. Under light of low intensity, the rhythm of development of the plants growing under long day is similar to that under short day. The development of axillary buds indicates a decrease in the intensity of vegetative differentiation and elongation growth, with an enhancement of the generative tonus; the buds in the axils of the highest growing leaves form directly inflorescences without preceding leaf differentiation. Inversion of the shape of leaves on lateral shoots was noted.

Multiple shoot formation in Hypericum perforatum L.and hypericin production

Hypericum perforatum is a traditional medicinal plant with wound healing and antidepressive properties. Among the secondary compounds of interest is hypericin, a naphtodianthrone that seems to participate in the medicinal effects of this species. The aim of this work was to obtain an efficient micropropagation system of H. perforatum and to compare the hypericin content between in vitro and field-grown plants. Cultures were initiated from nodal segments of mature plants inoculated onto MS medium supplemented with 4.5 muM BA, kinetin, thidiazuron, individually or in combination with 0.05 muM NAA. Organogenic explants were observed on medium with either BA or kinetin alone or in combination of these with NAA. Subculture of organogenic explants onto the proliferation medium containing 4.5 muM BA promoted the organogenic response. The highest average of shoot production (52.6 shoots) was obtained on those explants induced in the presence of BA and NAA. Rooted plantlets were successfully acclimated. Analysis of hypericin contents showed that levels found in callus represented only 0.11 % of what was detected in adult plants, while shoots and leaves from in vitro plants showed similar hypericin levels to those found in the leaves of the field-grown plants, suggesting that the accumulation of this compound is related to leaf differentiation.

Mechanisms that control knox gene expression in the Arabidopsis shoot

Knotted1-like homeobox (knox) genes are expressed in specific patterns within shoot meristems and play an important role in meristem maintenance. Misexpression of the knox genes, KNAT1 or KNAT2, in Arabidopsis produces a variety of phenotypes, including lobed leaves and ectopic stipules and meristems in the sinus, the region between lobes. We sought to determine the mechanisms that control knox gene expression in the shoot by examining recessive mutants that share phenotypic characteristics with 35S::KNAT1 plants. Double mutants of serrate (se) with either asymmetric1 (as1) or asymmetric2 (as2) showed lobed leaves, ectopic stipules in the sinuses and defects in the timely elongation of sepals, petals and stamens, similar to 35S::KNAT1 plants. Ectopic stipules and in rare cases, ectopic meristems, were detected in the sinuses on plants that were mutant for pickle and either as1 or as2. KNAT1 and KNAT2 were misexpressed in the leaves and flowers of single as1 and as2 mutants and in the sinuses of leaves of the different double mutants, but not in se or pickle single mutants. These results suggest that AS1 and AS2 promote leaf differentiation through repression of knox expression in leaves, and that SE and PKL globally restrict the competence to respond to genes that promote morphogenesis.

Comparative leaf development of juvenile and adult Pseudopanax crassifolius

We describe and compare leaf development in juvenile and adult shoots of Pseudopanax crassifolius, a strongly heteroblastic tree native to New Zealand. The shoot apical meristem is significantly larger in adult plants than in juvenile plants. Leaf primordia of the two forms are morphologically comparable at inception. The allometry of leaf length and width is similar in both forms up to a length of 7 mm. However, a shape index based on the relative position of maximum leaf width indicates that their morphology diverges when leaf primordia are 300 μm long. Laminae are initiated when the leaf primordium is shorter in adult shoots than in juvenile shoots. Maturation processes of the two leaf forms are similar. Cells and tissue types expand and differentiate in an acropetal direction. When leaf length, expressed as a proportion of mature leaf length, is used as a developmental index, the timing of all stages of leaf differentiation is comparable for the two leaf forms. The juvenile form is considered to be derived from the adult form by accelerated growth of the primordial leaf axis. Key words: Pseudopanax crassifolius, heteroblasty, leaf shape, development, allometry, New Zealand.