Stem Cell Basis for Fractal Patterns: Axillary Meristem Initiation

Whereas stem cell lineages are of enormous importance in animal development, their roles in plant development have only been appreciated in recent years. Several specialized lineages of stem cells have been identified in plants, such as meristemoid mother cells and vascular cambium, as well as those located in the apical meristems. The initiation of axillary meristems (AMs) has recently gained intensive attention. AMs derive from existing stem cell lineages that exit from SAMs and define new growth axes. AMs are in fact additional rounds of SAMs, and display the same expression patterns and functions as the embryonic SAM, creating a fractal branching pattern. Their formation takes place in leaf-meristem boundaries and mainly comprises two key stages. The first stage is the maintenance of the meristematic cell lineage in an undifferentiated state. The second stage is the activation, proliferation, and re-specification to form new stem cell niches in AMs, which become the new postembryonic “fountain of youth” for organogenesis. Both stages are tightly regulated by spatially and temporally interwound signaling networks. In this mini-review, I will summarize the most up-to-date understanding of AM establishment and mainly focus on how the leaf axil meristematic cell lineage is actively maintained and further activated to become CLV3-expressed stem cells, which involves phytohormonal cascades, transcriptional regulations, epigenetic modifications, as well as mechanical signals.

Development of a new in vivo protocol through soil inoculation to investigate sugar beet resistance towards Ditylenchus dipsaci penetration

Summary The stem nematode, Ditylenchus dipsaci, causes severe damage in sugar beet. To date, nematode inoculation through the leaf axil has been used as the standard method to investigate D. dipsaci interaction with sugar beet under in vivo conditions. To get as close as possible to field conditions, we established a new screening mechanism to perform soil inoculation. The most suitable inoculation time point, inoculum level and positioning on sugar beet, as well as rearing process on carrots, were determined. At a 15:8°C day:night temperature regime, penetration rates of D. dipsaci were at maximum following soil inoculation at plant emergence. Up to 115 nematodes penetrated sugar beet seedlings 22 days post-planting with an inoculum level of 1000 nematodes into the soil at plant emergence. Ditylenchus dipsaci penetration rate was higher in plants with soil inoculation than with inoculation on to the leaf axil. High soil moisture increased nematode migration into seedlings when D. dipsaci inoculation was carried out in four holes 1 cm from the plant base. Rearing the nematodes for 35 days at 20°C on carrot discs resulted in an infective inoculum containing up to 50% eggs. We recommend a soil inoculation of 1000 freshly extracted nematodes per pot at plant emergence. The nematode suspension has to be previously reared for 35 days on carrot discs to obtain active D. dipsaci inoculum. This system will allow for the selection of suitable sugar beet genotypes that suppress nematode penetration, in support of breeding for resistance against D. dipsaci.

Novelties on Lippia sect. Goniostachyum (Verbenaceae): a new variety from the Brazilian Cerrado and additional nomenclatural and taxonomic notes

Lippia stachyoides var. guajajarana, a new variety of Verbenaceae endemic from the Emas National Park, Goiás State, is described and illustrated. The new variety belongs to L. sect. Goniostachyum, which is characterized by numerous florescences per leaf axil and tetrastichous floral bracts. A detailed diagnosis, comparison with morphologically similar taxa, a key to the species and varieties from the section, plus habitat information, illustration, and a conservation status assessment are provided. Furthermore, L. morii is included in L. sect. Goniostachyum based on morphological traits and L. rivalis is considered a new synonym of L. origanoides. Lectotypes are designated for L. morii and L. rivalis, contributing to the taxonomy and nomenclature of the section.

Nepenthes fractiflexa (Nepenthaceae), a new Bornean pitcher plant exhibiting concaulescent metatopy and a high degree of axillary bud activation

Nepenthes fractiflexa is described as a new species from Sarawak and Kalimantan. The species is characterised by a number of unusual growth habits: plants readily produce aerial offshoots from their leaf axils, giving rise to a highly branched architecture of a complexity rarely seen in the genus; even those axillary buds not destined to develop further are activated and often elaborated into bract-like prophylls up to 5 cm long; and the inflorescence emerges close to the middle of the internode rather than from the leaf axil, as is typical of all other known species, leading us to make the first proposal for concaulescence in Nepenthes. These traits—coupled with the plant’s distinctive, long-decurrent petiolar wings and fractiflex climbing stems—distinguish it from its putative closest relative, N. mollis. The known distributions of N. fractiflexa and N. mollis are reviewed in light of recent discoveries. The apparent geographical disjunction between the two species is discussed in the context of edaphic and other ecological factors. Owing to its large range and remote, predominantly ridgetop habitat—but small number of recorded localities and individuals—the conservation status of N. fractiflexa is assessed as Near Threatened under the IUCN 3.1 criteria.

Spectrum and Frequency of Mutations Induced by Gamma Radiations in Three Varieties of Nigerian Sesame (Sesamum indicum L.)

Insufficient genetic variability is one of the major problems of plant breeding programmes, especially in sesame. Gamma radiation has been reported to be very effective in creating genetic variability in plants. Three varieties of Nigerian sesame were assessed for spectrum and frequency of mutation induced by Gamma radiations in M1 and M2 generations. The varieties (NCRIBEN-04E, NCRIBEN-01M and NCRIBEN-03L) were treated with four different doses of gamma rays (250, 350, 450 and 550 Gy). The treated and untreated seeds (control) were sown in planting bags (under field condition) to raise M1 plants. Four treatments: V1D5, V2D3, V3D2 and V3D4 (from M1 plants) were selected and bulked to obtain M2 populations. The results of M1 revealed four mutant fruit traits: multicarpellate capsule, multiple capsule per leaf axil, indehiscent capsule and terminal capsules. The highest frequencies of the traits in M1 generation were 2.50×10-2, 9.17×10-2, 1.67×10-2and3.33×10-2 respectively. The highest branching (7) was from NCRIBEN-01M, while the least (2) was from NCRIBEN-04E. The M2 plants were grouped into eight M2 lines. The dose range (250-550 Gy) was proved to be effective in inducing viable mutations in sesame.

NaCl markedly improved the reproductive capacity of the euhalophyte Suaeda salsa

The effect of NaCl on reproductive development was investigated in euhalophyte Suaeda salsa L. under controlled conditions. Results showed that NaCl promoted the reproductive growth of S. salsa and 200 mM NaCl was optimal. This was reflected in the increases of seed yield, seed number, flower number per plant and leaf axil, 1000 seed weight, as well as a decrease in flower abortion percentage with supply of NaCl. NaCl reduced the flower abortion percentage by increasing stigma receptivity instead of pollen viability. The Na+ and Cl– concentration in petals, stems and leaves were increased significantly but slightly in stamen and pistil. In contrast, the K+ concentration decreased markedly in leaves, stems and petals but a little in stamen and pistil. The Na+ and Cl– concentrations also increased significantly in seed from mother plants exposed to NaCl, whereas K+ decreased. However, seed quality was not influenced. Our results showed that high concentration of NaCl markedly increases the seed number and quality of S. salsa primarily via increasing flower number and fertility and S. salsa develops strategy to maintain ion homeostasis in reproductive organs for the generation. These factors play a pivotal role in setting up plant populations in saline environment.

Eutrema bulbiferum (Brassicaceae), a new species with bulbils from Hunan, China

Eutrema bulbiferum, a unique new species of Brassicaceae found in the limestone areas of Longshan and Jishou, Hunan, China, is described and illustrated. This species is most similar to E. tenue and E. yunnanense, but can be easily distinguished by its rosulate fleshy bulbils in the leaf axil or near the stem base, fewer ovules per ovary, slightly 4-angled short wand-like silique, and bended silique apex with a beak. E. bulbiferum is categorized into Eutrema by phylogenetic analysis based on the nuclear internal transcribed spacer (ITS). It is also clearly separated from E. tenue and E. yunnanense by the results of both phylogenetic analysis and Principal Component Analysis (PCA) based on morphometric characters.