Vegetative Primordia Research Articles

Comparative analysis of axillary and floral meristem development

Axillary and floral meristems are shoot meristems that initiate postembryonically. In Arabidopsis, axillary meristems give rise to branches during vegetative development while floral meristems give rise to flowers during reproductive development. This review compares the development of these meristems from their initiation at the shoot apical meristem up to the establishment of their specific developmental fates. Axillary and floral meristems originate from lateral primordia that form at flanks of the shoot apical meristem. Initial development of vegetative and reproductive primordia are similar, resulting in the formation of a morphologically defined primordium partitioned into adaxial and abaxial domains. The adaxial primordial domain is competent to form a meristem, while the abaxial domain correlates with the formation of a leaf. This review proposes that all primordia partition into domains competent to form the meristem and the leaf. According to this model, a vegetative primordium develops as leaf-bias while a reproductive primordium develops as meristem-bias.Key words: SHOOTMERISTEMLESS, LATERAL SUPPRESSOR, AINTEGUMENTA, adaxial primordial domain, abaxial primordial domain, shoot morphogenesis.

Clausa, a tomato mutant with a wide range of phenotypic perturbations, displays a cell type-dependent expression of the homeobox gene LeT6/TKn2.

Class I knox genes play an important role in shoot meristem function and are thus involved in the ordered development of stems, leaves, and reproductive organs. To elucidate the mechanism underlying the expression pattern of these homeobox genes, we studied a spontaneous tomato (Lycopersicon esculentum) mutant that phenotypically resembles, though is more extreme than, transgenic plants misexpressing class I knox genes. This mutant was found to carry a recessive allele, denoted clausa:shootyleaf (clau:shl)-a newly identified allele of clausa. Mutant plants exhibited abnormal leaf and flower morphology, epiphyllus inflorescences, fusion of organs, calyx asymmetry, and navel-like fruits. Analysis by scanning electron microscopy revealed that such fruits carried ectopic ovules, various vegetative primordia, as well as "forests" of stalked glandular trichomes. In situ RNA hybridization showed a peculiar expression pattern of the class I knox gene LeT6/TKn2; expression was restricted to the vascular system and palisade layer of mature leaves and to the inner part of ovules integuments. We conclude that CLAUSA regulates various aspects of tomato plant development, at least partly, by rendering the LeT6/TKn2 gene silent in specific tissues during development. Considering the expression pattern of LeT6/TKn2 in the clausa mutant, we suggest that the control over a given homeobox gene is maintained by several different regulatory mechanisms, in a cell type-dependent manner.

Protease activity, organisation of vegetative primordia and effect of putrescine in tobacco thin layers

Since in tobacco thin layers exogenous putrescine alters the physiological and mor‐phogenic responses induced by IAA (indole‐3‐acetic acid) and/or BA (benzylade‐nine), the effect of this polyamine on protease activity and on the formation of meristemoids and vegetative primordia was studied during morphogenesis. Superficial thin layer explants, excised from the stem of tobacco (Nicotiana tabacum L. cv. Samsun) plants in the vegetative stage, were cultured under various hormonal conditions (IAA, IAA+BA, BA) and in a hormone‐free medium, in the presence or absence of 100 μM putrescine. Histological analysis showed that no meristemoids were formed on the control medium or with putrescine alone and only a few were formed on IAA‐treated explants with or without putrescine. An increasing number of meristemoids was observed in IAA+BA and BA treatments during culture; in both cases this number was enhanced by the presence of exogenous putrescine. Protease activity was evaluated spectrophotometrically using two synthetic substrates, azocasein and N‐benzoyl‐DL‐arginine‐p‐nitroanilide (BAPNA). In the former, maximum protease activity was observed in IAA+BA‐ and BA‐treated explants on days 10 and 15, respectively, while with IAA activity was lowest, the maximum occurring on days 5–10. In this case exogenous putrescine enhanced protease activity in the presence of IAA alone or with BA, while it decreased it in the presence of BA. BAPNA‐mediated proteolytic activity (serine‐proteases) was highest in IAA+BA‐treated explants, intermediate in BA‐ and not different from controls in IAA‐treated explants. Putrescine only affected proteolytic activity in IAA+BA treatments. The use of specific inhibitors of protease activities indicated that these enzymes belong to two main classes of proteases, that is serine‐ and thiol‐proteases. The pattern of proteolytic activities during culture appeared to be related to the differentiation of meristemoids into vegetative primordia. The effect of exogenous putrescine on protease activity was different depending on different synthetic substrates, developmental patterns, pH and ionic strength.

Protease activity, organisation of vegetative primordia and effect of putrescine in tobacco thin layers

Protease activity, organisation of vegetative primordia and effect of putrescine in tobacco thin layers

The tomato 66.3-kD polyphenoloxidase gene: molecular identification and developmental expression.

A gene coding for a polypeptide abundant in tomato floral meristems was isolated and shown to represent a tomato 66.3-kD polyphenoloxidase. Analysis of cDNA clones and a corresponding intronless genomic clone indicated that the plastid-bound 587-residue-long polypeptide, designated P2, contains two conserved copper-binding domains, similar to those found in fungal and mammalian tyrosinases. P2 transcripts and polypeptides are accumulated in the arrested floral primordia of the anantha mutant inflorescences and are equally abundant in primordia of wild-type flowers; the gene continues to be expressed at high levels in developing floral organs. In young expanding leaves, P2 protein is concentrated in palisade cells and in epidermal trichomes. Expression patterns of P2 in plant meristems permit molecular distinction between floral and vegetative primordia, and, in a companion study, comparison with dUTPase suggests that the two genes mark two alternative complementary developmental programs in the floral and vegetative meristems of the tomato plants.

Translocation of 14C-labelled 2,4-Dichlorophenoxyacetic Acid in Barley and Oats

THE ‘hormone’ herbicides 2-methyl-4-chlorophenoxyacetic acid (MCPA) and 2,4-dichlorophenoxyacetic acid (2,4-D) have repeatedly been shown to produce deformities in the straw and spikes of cereal plants when applied before five or six leaves have developed. This connexion between deformation and age of plants has led to the assumption that floral and vegetative primordia are particularly sensitive to compounds with hormone activity but become more resistant with progressing development. Experiments with 2,4-D labelled with carbon-14 in the carboxyl group, however, suggest that this is scarcely likely to be the true explanation.