Axillary Branches Research Articles

The Lateral Suppressor-Like Gene, DgLsL, Alternated the Axillary Branching in Transgenic Chrysanthemum (Chrysanthemum × morifolium) by Modulating IAA and GA Content

The lateral suppressor-like gene DgLsL was transformed by agroinfection into chrysanthemum in both the sense and antisense directions. Sense transformants branched more profusely than the wild-type nontransformant, while branching in the antisense transformants was significantly suppressed. An analysis of DgLsL transcript abundance in the shoot tips revealed that expression was enhanced in the sense transformants and suppressed in the antisense ones. The shoot tip content of indole-3-acetic acid (IAA) was reduced in the sense transformants but enhanced in the antisense ones. The sense transformants had a lower content and the antisense transformants had a higher content of gibberellic acid (GA). Cytokinin content was not affected by the variation in DgLsL expression. We conclude that DgLsL controls shoot branching through its effect on IAA and GA levels.

Nystroemiaceae, a new family of Permian gymnosperms from China with an unusual combination of features

Nystroemiaceae is proposed as a new family of gymnosperms from the Permian of Cathaysia that adds to the diversity of gymnosperms known from this critical time in seed plant evolution. This family is characterized by bifurcating and highly branched pinnate ovuliferous organs bearing bicornute ovules (seeds) and entire leaves with anastomozing veins that are born on complex and modern-looking branching systems with clear axillary branching. The reconstruction is based on numerous large specimens from two localities in North China, in which the different plant parts are attached to each other. The ovulate structures show some apparently plesiomorphic (primitive) character states more typical of early seed plants, whereas the leaves and branches show the clearly apomorphic (derived) character states of broad-leaved gymnosperms.

SHORTENING OF NON-HARVEST PERIOD IN HIGH-BENCH STRAWBERRY FORCING CULTURE BY A SIMPLE CONTROL METHOD OF MEDIUM TEMPERATURE

High temperature of the culture medium in a high-bench culture system of strawberries is apt to induce delay and lack of uniformity in flower bud emergence. These phenomena promote the non-harvest period. We devised an experimental system of heat control using the latent heat of evaporation for the culture medium (a medium-cooling system). We observed that the temperature of the culture medium affects continuous flower bud emergence in high-bench forcing culture. During the post-planting high-temperature period in the beginning of September, the medium-cooling system lowered the temperature of the culture medium by 2.5-3.0°C, compared with the temperature of the control medium. The medium-cooling system increased the duration of an optimum temperature of the medium. In addition, this system accelerated flower bud emergence in cv. 'Benihoppe' by almost 5 days on primary axillary branches and caused more uniform flower bud emergence compared with controls.

Interactions between Auxin and Strigolactone in Shoot Branching Control

In Arabidopsis (Arabidopsis thaliana), the carotenoid cleavage dioxygenases MORE AXILLARY GROWTH3 (MAX3) and MAX4 act together with MAX1 to produce a strigolactone signaling molecule required for the inhibition of axillary bud outgrowth. We show that both MAX3 and MAX4 transcripts are positively auxin regulated in a manner similar to the orthologous genes from pea (Pisum sativum) and rice (Oryza sativa), supporting evolutionary conservation of this regulation in plants. This regulation is important for branching control because large auxin-related reductions in these transcripts are associated with increased axillary branching. Both transcripts are up-regulated in max mutants, and consistent with max mutants having increased auxin in the polar auxin transport stream, this feedback regulation involves auxin signaling. We suggest that both auxin and strigolactone have the capacity to modulate each other's levels and distribution in a dynamic feedback loop required for the coordinated control of axillary branching.

BAC library development for allotetraploid Leymus (Triticeae) wildryes enable comparative genetic analysis of lax- barrenstalk1 orthogene sequences and growth habit QTLs

Tall-caespitose basin wildrye ( Leymus cinereus) and rhizomatous creeping wildrye ( Leymus triticoides) are perennial Triticeae relatives of wheat and barley. Quantitative trait loci (QTLs) controlling rhizome proliferation were previously detected on homoeologous regions of LG3a and LG3b in two full-sib families derived from allotetraploid hybrids. Triticeae homoeologous group 3 aligns to rice chromosome 1, which contains the rice lax panicle and maize barrenstalk1 orthogene responsible for induction of axillary branch meristems, but this gene has not been mapped or sequenced in Triticeae. We developed bacterial artificial chromosome (BAC) libraries representing 6.1 haploid equivalents of the tetraploid Leymus genome (10.7 Mb). Overgo probes designed from the lax- barrenstalk1 orthogene hybridized to 12 Leymus BAC clones. Deduced amino-acid sequences from seven BAC clones were highly conserved with the rice, maize, and sorghum lax- barrenstalk1orthogenes. Gene specific primers designed from two of the most divergent BAC clones map to homoeologous regions of Leymus LG3a and LG3b and align with the lax- barrenstalk1 orthogene on rice 1L. Comparisons of genomic DNA sequences revealed two other conserved regions surrounding the Leymus LG3a, rice, and sorghum lax- barrenstalk1 ortholoci, and one of these regions was also present in maize and Leymus LG3b sequences. Comparisons of Leymus LG3a and LG3b lax- barrenstalk1 coding sequences and flanking genomic regions elucidate molecular differences between subgenomes.

MICROPROPAGATION OF 'TROYER' CITRANGE [PONCIRUS TRIFOLIATA (L.) RAT. × C. SINENSIS (L.) OSBECK

In Citrus, the success of the scion largely depends upon the rootstock on which it has been grafted. 'Troyer' citrange is an important rootstock used worldwide for high density planting. It is also one of the most promising rootstock for several Indian scion varieties such as 'Nagpur' mandarin, 'Kinnow' mandarin, grapefruits, acid lime, 'Satsuma' sweet orange and 'Mosambi' sweet orange. In addition, it has high resistance to tristeza virus, which causes Citrus decline, and has good adaptability for acidic soil. Commercial rootstock cultivars are commonly propagated by seeds. However, seeds of many Citrus varieties are moderately recalcitrant and also face loss of viability within a short period. The plants grown from seeds exhibit extended juvenility (4 to 10 years), thus making vegetative propagation and micropropagation a desirable method of propagation. The present study focuses on developing an efficient micropropagation protocol for the production of clonally uniform plants through axillary branching method of slow growing rootstock variety, 'Troyer' citrange. Multiple shoots can be obtained from single nodes of field grown trees cultured on Murashige and Skoog's (MS; 1962) medium containing Benzylamino purine (BA; 1.11 μM), Kinetin (Kn; 1.1625 μM), and 3% sucrose. A shoot multiplication fold of 3.86 every four weeks was achieved. Proliferated shoots were rooted on half strength MS medium with Naphthalene acetic acid (NAA; 0.5 μM) and plantlets were transferred to a mixture of soil and agropeat. 100% survival was observed during hardening of the rooted plantlets. The comparative analyses of the DNA fingerprinting profiles confirmed the genetic stability of the regenerated plantlets. The results of this study have enormous commercial application for the propagation of healthy disease-free Citrus rootstocks to meet the ever-growing demand for superior quality planting material.

AtMBD9 modulates Arabidopsis development through the dual epigenetic pathways of DNA methylation and histone acetylation

Mutations within the Arabidopsis METHYL-CpG BINDING DOMAIN 9 gene (AtMBD9) cause pleotropic phenotypes including early flowering and multiple lateral branches. Early flowering was previously attributed to the repression of flowering locus C (FLC) due to a reduction in histone acetylation. However, the reasons for other phenotypic variations remained obscure. Recent studies suggest an important functional correlation between DNA methylation and histone modifications. By investigating this relationship, we found that the global genomic DNA of atmbd9 was over-methylated, including the FLC gene region. Recombinant AtMBD9 does not have detectable DNA demethylation activity in vitro, but instead has histone acetylation activity. Ectopic over-expression of AtMBD9 and transient DNA demethylation promotes flowering and causes partial recovery of the normal branching phenotype. Co-immunoprecipitation assays suggest that AtMBD9 interacts in vivo with some regions of the FLC gene and binds to histone 4 (H4). Gene expression profile analysis revealed earlier up-regulation of some flower-specific transcriptional factors and alteration of potential hormonal and signal transducer axillary branching regulatory genes. In accordance with this result, AtMBD9 itself was found to be localized in the nucleus and expressed in the flower and axillary buds. Together, these results suggest that AtMBD9 controls flowering time and axillary branching by modulating gene expression through DNA methylation and histone acetylation, and reveal another component of the epigenetic mechanism controlling gene expression.

Production of Pharmaceuticals from Papaver Cultivars in Vitro

AbstractIranian (Papaver bracteatum Lindl.) and opium poppy (P. somniferum L.) plantlets obtained from germinated seeds grown on a Murashige and Skoog basal medium (BM) readily manifest alkaloids. Temperature had a profound effect on growth and alkaloid production after 8 weeks in culture. Plantlets of poppy cultivars (cvs.) grew best at 18.5 and 20°C compared to 15 or 25°C. An alkaloid survey study with 24 Iranian and 21 opium poppy cvs. revealed that total morphinan alkaloids ranged from 0 to 6.55 mg/g dw. Prolific axillary branching was achieved from poppy cvs. by maintaining shoots on BM containing 1.0 mg/L N6‐benzyladenine and 0.01 mg/L α‐naphthalene acetic acid for an additional 16 weeks. The influence of vessel size on the growth response of established shoot clumps was determined by subculture in a variety of culture vessels for 8 weeks. The tested culture vessels included culture tubes (55 mm3 capacity (cap.)), babyfood jars (143 mm3 cap.), Magenta GA‐7 containers (365 mm3 cap.), and polycarbonate jars (1890 mm3 cap.) employing an in vitro hydroponics system (i.e. an automated plant culture system (APCS)). Highest growth rates occurred employing the APCS. The culture vessel capacity had a significant positive correlation on shoot length, fresh weight, number of leaves, and number of shoots. Shoot length, fresh weight, leaves, and shoots grown in the APCS exhibited increases of 1‐, 21.5‐, 7.8‐, and 8.3‐fold, respectively, compared to shoots grown in culture tubes. Higher culture growth rates that occurred in the larger‐size vessels were correlated with lower alkaloid production (mg alkaloids/g dw). However, the overall total alkaloids/vessel [(mg alkaloid/g dw)×g culture dw] increased because of greater biomass production per vessel. The alkaloid content was found to remain stable for shoots grown over a 6–month evaluation period.

In vitro organogenesis and plant regeneration from unpollinated ovary cultures of Azadirachta indica

A novel method of organogenesis in neem (Azadirachta indica A. Juss.) from unfertilized ovaries is described. The Murashige and Skoog's (MS) medium with 9 % sucrose, 1 µM 2,4-dichlorophenoxyacetic acid (2,4-D) and 5 µM 6-benzylaminopurine (BAP) was the best for callus induction from unfertilized ovaries. However, further proliferation of callus occurred better on MS medium supplemented with 0.5 µM 2,4-D either alone or in combination with 4.5 µM kinetin. Maximum shoot regeneration (78 %) was observed when calli, induced from ovaries of 4 mm size flower buds and proliferating on MS + 0.5 µM 2,4-D, were subcultured to MS medium containing 5 µM BAP. Histological analysis revealed that 4 mm sized flower bud corresponds to a 2-nucleate stage of embryo sac. The shoots were then multiplied by forced axillary branching on MS medium supplemented with 1.0 µM BAP and 250 mg dm-3 casein hydrolysate. The shoots could be rooted on 1/4 strength MS medium supplemented with 0.5 µM indole-3-butyric acid (IBA) at a frequency of 79 %. Cytological analysis by root tip squash preparations revealed that all the plantlets were diploids. These plants were subsequently hardened and established in soil with transplantation rate of 81.8 %.

DWARF27, an Iron-Containing Protein Required for the Biosynthesis of Strigolactones, Regulates Rice Tiller Bud Outgrowth

Tillering in rice (Oryza sativa) is one of the most important agronomic traits that determine grain yields. Previous studies on rice tillering mutants have shown that the outgrowth of tiller buds in rice is regulated by a carotenoid-derived MAX/RMS/D (more axillary branching) pathway, which may be conserved in higher plants. Strigolactones, a group of terpenoid lactones, have been recently identified as products of the MAX/RMS/D pathway that inhibits axillary bud outgrowth. We report here the molecular genetic characterization of d27, a classic rice mutant exhibiting increased tillers and reduced plant height. D27 encodes a novel iron-containing protein that localizes in chloroplasts and is expressed mainly in vascular cells of shoots and roots. The phenotype of d27 is correlated with enhanced polar auxin transport. The phenotypes of the d27 d10 double mutant are similar to those of d10, a mutant defective in the ortholog of MAX4/RMS1 in rice. In addition, 2'-epi-5-deoxystrigol, an identified strigolactone in root exudates of rice seedlings, was undetectable in d27, and the phenotypes of d27 could be rescued by supplementation with GR24, a synthetic strigolactone analog. Our results demonstrate that D27 is involved in the MAX/RMS/D pathway, in which D27 acts as a new member participating in the biosynthesis of strigolactones.

Micropropagation of Gynura procumbens (Lour.) Merr. an important medicinal plant

A rapid micropropagation protocol was established for Gynura procumbens(Lour.) Merr., an important medicinal plant for the treatment of various ailments such as diabetes, hypertension and urinary tract infection. The nodal segments of one year old mature plants was used as the explants for the initiation of axillary branching using Murashige and Skoog (1962) medium (MS) supplemeted with 2mg l–1 BA. Rapid proliferation of shoots was achieved by culturing the in vitroshoots derived from the nodal segments onto MS medium supplemented with 2.0 mg l-1 BA and 0.5 mg l–1 NAA. An average of 18.2 ± 0.6 shoots were produced from each shoot explant. All the micro-shoots produced normal roots within two weeks of culture on the basic MS medium without any plant growth regulators. All the rooted plantlets of G. procumbens (100%) survived when they were planted into Jiffy-7 and placed on a shaded place with a temperature of 27 - 30oC for two weeks. An average of 95 – 98% of the acclimatized plantlets survived after two weeks of transferring into the organic soil: sand mixture (1:1). All the in vitroderived plants displayed normal developement as that of the mother plants when they reached mature stage. Key words: Medicinal plant,micropropagation,nodal segment,shoot proliferation.

Morphology and Genetics of Biomass in Little Bluestem

Little bluestem [Schizachyrium scoparium (Michx.) Nash] was the most important dominant species of uplands in the tallgrass prairie. Presently, it is a dominant species, most significantly on coarse‐textured soils, in the mixed‐grass prairie region of the United States and Canada. This wide geographical adaptation to dry soils suggests little bluestem has potential for biomass production in areas unsuitable for most other grass species in the North American steppe. Primary objectives of this research were to (i) describe the morphology and determine the distribution of biomass among main stem and axillary components of little bluestem, and (ii) determine genetic variation for morphology and biomass production in ‘Camper’ little bluestem. Morphological analysis was conducted on tillers collected from natural populations in South Dakota and Montana and from swards of Camper and ‘Badlands’ ecotype in eastern South Dakota. Sixteen genotypes of Camper were evaluated for three years at two locations for determination of genotypic variation for biomass and biomass components. Quadratic models explained relationships between phytomer position and leaf and internode lengths and mass of individual phytomers of natural and selected populations. Genotypic variation occurred for phytomers per tiller, tillers per plant, mass per tiller, axillary branches per tiller, and biomass (range: 102–387 g plant−1). However, the correlation between locations for biomass was not significant for genotypes due to genetic variation for phenotypic plasticity. High frequency of survival over two winters for all genotypes indicated Camper has adequate winter hardiness to serve as a source population for cultivar development for biomass in the northern Great Plains.

Determination of genetic stability of long-term micropropagated plantlets of Platanus acerifolia using ISSR markers

Inter-simple sequence repeat (ISSR) markers were used to assess the genetic stability of long-term micropropagated plantlets of London plane tree (Platanus acerifolia Willd.). Twenty micropropagated plantlets were chosen from a clonal collection of shoots that originated from a single mother shoot. This clonal collection had been maintained under in vitro culture conditions for at least 8 years, as achieved by axillary branch multiplication. Out of 38 ISSR primers screened, 16 primers were found to produce clear reproducible bands resulting in a total of 103 distinct bands with an average of 6.44 scorable bands per primer. Of these 103 bands, 86 were monomorphic across all 20 of the plants tested and 17 showed polymorphisms (16.5 % polymorphism). Based on the ISSR band data, similarity indices between the plantlets ranged from 0.92 to 1.00. These similarity indices were used to construct an UPGMA dendrogram and demonstrated that all 20 micropropagated plants grouped together in one major cluster with a similarity level of 91 %. A total of 1771 scorable bands were obtained from the full combination of primers and plantlets and only 51 (2.88 %) were polymorphic across the plantlets which indicates that this micropropagated line of P. acerifolia is genetically stable.

In vitro regeneration of clonally uniform plants of Crataeva magna: a high value medicinal tree by axillary branching method

Crataeva magna (Lour.) DC (synonym C. nurvala) is a high-value Indian medicinal tree. The multiple uses of C. magna have resulted in its over-exploitation. Erratic seed germination combined with destructive harvesting and habitat destruction in the form of deforestation has added to the enormity of the problem. Therefore, the need for conservation of this plant is vital. Development of an efficient micropropagation protocol will play a significant role in meeting the requirement of quality planting material for commercial cultivation thereby conserving the species in its natural habitat. In the present study, shoot multiplication was achieved by culturing single node segments derived from a field grown tree on Murashige and Skoog’s (MS) medium supplemented with 2.66 μM N6-benzyladenine, 1.39 μM Kinetin (Kn), 0.57 μM indole-3-acetic acid (IAA), 3% sucrose and 0.2% gelrite. 96% rooting was achieved within 22 days by culturing the in vitro formed shoots on half strength MS medium with 11.42 μM IAA, 9.8 μM indole-3-butyric acid, 0.46 μM Kn and 198.25 μM phloroglucinol. Following a simple hardening procedure involving sequential transfer of plants to a greenhouse, polyhouse, and shade net, the tissue-cultured plants were transferred to the field where the survival rate was 100%. To this date 500 plants have been produced. Inter simple sequence repeat analysis has confirmed genetic uniformity of the tissue-cultured plants.

Regulation of carotenoid composition and shoot branching in Arabidopsis by a chromatin modifying histone methyltransferase, SDG8.

Carotenoid pigments are critical for plant survival, and carotenoid composition is tuned to the developmental stage, tissue, and to environmental stimuli. We report the cloning of the CAROTENOID CHLOROPLAST REGULATORY1 (CCR1) gene. The ccr1 mutant has increased shoot branching and altered carotenoid composition, namely, reduced lutein in leaves and accumulation of cis-carotenes in dark-grown seedlings. The CCR1 gene was previously isolated as EARLY FLOWERING IN SHORT DAYS and encodes a histone methyltransferase (SET DOMAIN GROUP 8) that methylates histone H3 on Lys 4 and/or 36 (H3K4 and H3K36). ccr1 plants show reduced trimethyl-H3K4 and increased dimethyl-H3K4 surrounding the CAROTENOID ISOMERASE (CRTISO) translation start site, which correlates with low levels of CRTISO mRNA. Microarrays of ccr1 revealed the downregulation of 85 genes, including CRTISO and genes associated with signaling and development, and upregulation of just 28 genes. The reduction in CRTISO transcript abundance explains the altered carotenoid profile. The changes in shoot branching are additive with more axillary branching mutants, but the altered carotenoid profile may partially affect shoot branching, potentially by perturbed biosynthesis of the carotenoid substrates of strigolactones. These results are consistent with SDG8 regulating shoot meristem activity and carotenoid biosynthesis by modifying the chromatin surrounding key genes, including CRTISO. Thus, the level of lutein, the most abundant carotenoid in higher plants that is critical for photosynthesis and photoprotection, appears to be regulated by a chromatin modifying enzyme in Arabidopsis thaliana.

Lymphedema Microsurgical Preventive Healing Approach: A New Technique for Primary Prevention of Arm Lymphedema After Mastectomy

The purpose of this manuscript is to assess the efficacy of direct lymphatic venous microsurgery in the prevention of lymphedema following axillary dissection for breast cancer. Nineteen patients with operable breast cancer requiring an axillary dissection underwent surgery, carrying out LVA between the blue lymphatics and an axillary vein branch simultaneously. The follow-up after 6 and 12 months from the operation included circumferential measurements in all cases and lymphangioscintigraphy only in 18 patients out of 19 cases. Blue nodes in relation to lymphatic arm drainage were identified in 18/19 patients. All blue nodes were resected and 2-4 main afferent lymphatics from the arm could be prepared and used for anastomoses. Lymphatic-venous anastomoses allowed to prevent lymphedema in all cases. Lymphangioscintigraphy demonstrated the patency of microvascular anastomoses. Disruption of the blue nodes and closure of arm lymphatics can explain the significantly high risk of lymphedema after axillary dissection. LVA proved to be a safe procedure for patients in order to prevent arm lymphedema.

The ramification of Apinagia riedelii: A key to the understanding of the plant architecture of Podostemaceae, subfamily Podostemoideae

The study of the ramification pattern of Apinagia riedelii results in a new concept of the architecture of this species, with general implications to members of subfamily Podostemoideae with dithecous leaves. The presence of a subtending leaf below the floriferous shoot proves axillary branching also for species with dithecous leaves. Previous opinions of an unusual ramification mode by subfoliar or non-axillary branching or stem bifurcation in combination with dithecous leaves hitherto pleaded for Podostemoideae is refuted. Moreover, the view of the so-called dithecous leaves with one sheath (theca) at the ventral and one at the dorsal side of the leaf, previously regarded as initially connected with branching, has to be changed. The dithecous leaf arises from the branch and not from the mother shoot axis – as previously believed – and represents the addorsed hypsophyll, i.e., the first leaf (prophyll) of the floriferous branch. This finding leads to the conclusion that the lower sheath of the dithecous leaf is the ventral (not dorsal) sheath pointing to the branch and surrounding its flower bud with a ligule or an ochrea and a hood upon the bud. In this way, the branch and its flower bud become seemingly sunk in the leaf base. At the fusion of leaf basis and shoot results this enigmatic common tissue. The wings of the dorsal (upper) sheath of the dithecous leaf point to the mother shoot axis of the branch. Successive floriferous branches along the main stem disclose the shoot axis of A. riedelii as a monopodium (not sympodium) that develops an anthocladial (foliated) inflorescence in the form of a botrys or a compound botrys, respectively. Since it is generally difficult to define cymose or racemose inflorescences if subtending leaves are absent – which occur in most other species of subfamily Podostemoideae with dithecous leaves – the nature of these inflorescences is discussed anew. The findings on A. riedelii have consequences on our comprehension of the shoot architecture of Podostemoideae.

In vitro clonal propagation of bael (Aegle marmelos Corr.) CV. CISH-B1 through enhanced axillary branching

Rapid clonal micropropagation protocol of Aegle marmelos (L.) Corr. cv. CISH-B1 was achieved by nodal stem segment of mature bearing tree. Three centimeter long shoots having one axillary bud excised from 10-15th nodal region of shoots during September gave quick in vitro bud burst (5.33 days) when cultured on MS medium supplemented with BAP, 8.84 μM + IAA 5.7 μM. The maximum number of proliferated shoots (9.0/explant) were obtained on same medium supplemented with BAP 8.84 μM + IAA 5.7 μM. The micro shoots were rooted (100 %) on + IAA 5.7 μM. In vitro rooted plants were acclimatized on autoclaved coconut husk containing plant salt mixture and under shade net house (50 % shade 70-80 % RH). The plants were established in the field after acclimatization. The micropropagated plants were tested for its genetic fidelity using 13 RAPD, 3 ISSR and 2 DAMD primers. Profile obtained by all the three Single Primer Amplification Reaction (SPAR) technique from mother tree and micropropagated plants revealed genetic integrity of micropropagated plants with that of mother tree.

Integrating Molecular Phylogenetic and Paleobotanical Evidence on Origin of the Flower

Inferences on the origin of the angiosperm flower require consideration of other seed plants, especially fossils. Molecular data favor a relationship of Gnetales to conifers rather than to angiosperms, and both alternatives are equally parsimonious in terms of the morphological data set presented here. However, if molecular relationships among extant taxa are accepted, morphology still associates glossopterids, Pentoxylon, Bennettitales, and Caytonia with angiosperms. Bennettitales had flowerlike structures, but if Caytonia is sister to angiosperms, aggregation of fertile parts probably occurred independently in Bennettitales and angiosperms. These results and developmental genetic data are consistent with homology of the angiosperm bitegmic ovule with the cupule of glossopterids and Caytonia, while the carpel could represent a leaf and a cupule‐bearing axillary branch. Origin of an adaxial cross zone could produce a uniovulate, ascidiate carpel, as in living basal angiosperms. Stamens may represent similar units bearing two microsynangia. However, ovulate structures of Pentoxylon and Bennettitales are more difficult to interpret, and any homologue of the carpel wall in Caytonia is unclear. Further progress may require better understanding of homologies in known fossils and/or recognition of closer stem relatives of angiosperms. A proposed Cretaceous stem relative, Archaefructus, is more likely a crown‐group angiosperm related to Hydatellaceae (Nymphaeales).

Influence of Root Zone Calcium on Subapical Necrosis in Potato Shoot Cultures: Localization of Injury at the Tissue and Cellular Levels

Shoot tip necrosis has been attributed to calcium deficiency in in vitro cultures, resulting in death of the stem tip, the loss of apical dominance, and axillary branch development. Using an in vitro shoot culture system with Solanum tuberosum L. cv. Dark Red Norland, we studied the development of injury symptoms at the microscopic and tissue levels at a range of media calcium concentrations varying from 6.8 to 3000 μm. Light and electron microscopic studies revealed that the primary injury due to calcium deficiency was the death and collapse of expanding pith cells below the shoot apex. The structure and organization of the shoot apical meristem was the same when plants were cultured on sufficient or suboptimal media calcium concentrations. However, the apical meristem senesced following subapical shoot tissue collapse. Death of the shoot apical meristem was a secondary effect of calcium deficiency, resulting in loss of apical dominance. Studies with 45Ca indicated that calcium was distributed in a gradient along the shoot, with highest concentration at the base and the lowest at the apex. Shoot tip necrosis developed after 20 days of culture on the suboptimal calcium concentration medium. The development of these symptoms and axillary shoot growth was associated with the lack of calcium accumulation in the shoots. Our results provide evidence that a primary injury of calcium deficiency is localized in the expanding pith cells below the shoot apical meristem and this injury results in the collapse of subapical cells. Death of the shoot apical meristem is a secondary injury resulting from calcium deficiency.