Signals derived from YABBY gene activities in organ primordia regulate growth and partitioning of Arabidopsis shoot apical meristems

Shoot apical meristems (SAMs) are self-sustaining groups of cells responsible for the ordered initiation of all aerial plant tissues, including stems and lateral organs. The precise coordination of these processes argues for crosstalk between the different SAM domains. The products of YABBY (YAB) genes are limited to the organ primordium domains, which are situated at the periphery of all SAMs and which are separated by a margin of three to seven cells from the central meristem zone marked by WUSCHEL and CLAVATA3 expression. Mutations in the two related YAB1 genes, FILAMENTOUS FLOWER and YABBY3 (YAB3), cause an array of defects, including aberrant phyllotaxis. We show that peripheral YAB1 activity nonautonomously and sequentially affects the phyllotaxis and growth of subsequent primordia and coordinates the expression of SAM central zone markers. These effects support a role for YAB1 genes in short-range signaling. However, no evidence was found that YAB1 gene products are themselves mobile. A screen for suppression of a floral YAB1 overexpression phenotype revealed that the YAB1-born signals are mediated in part by the activity of LATERAL SUPPRESSOR. This GRAS protein is expressed at the boundary of organ primordia and the SAM central zone, distinct from the YAB1 expression domain. Together, these results suggest that YAB1 activity stimulates signals from the organs to the meristem via a secondary message or signal cascade, a process essential for organized growth of the SAM.     

Combining enhanced root and shoot growth reveals cross talk between pathways that control plant organ size in Arabidopsis

Functionally distinct Arabidopsis (Arabidopsis thaliana) genes that positively affect root or shoot growth when ectopically expressed were combined to explore the feasibility of enhanced biomass production. Enhanced root growth resulting from cytokinin deficiency was obtained by overexpressing CYTOKININ OXIDASE/DEHYDROGENASE3 (CKX3) under the control of the root-specific PYK10 promoter. Plants harboring the PYK10-CKX3 construct were crossed with four different transgenic lines showing enhanced leaf growth. For all combinations, the phenotypic traits of the individual lines could be combined, resulting in an overall growth increase. Unexpectedly, three out of four combinations had more than additive effects. Both leaf and root growth were synergistically enhanced in plants ectopically expressing CKX3 and BRASSINOSTEROID INSENSITIVE1, indicating cross talk between cytokinins and brassinosteroids. In agreement, treatment of PYK10-CKX3 plants with brassinolide resulted in a dramatic increase in lateral root growth that could not be observed in wild-type plants. Coexpression of CKX3 and the GROWTH-REGULATING FACTOR5 (GRF5) antagonized the effects of GRF5 overexpression, revealing an interplay between cytokinins and GRF5 during leaf cell proliferation. The combined overexpression of CKX3 and GIBBERELLIN 20-OXIDASE1 led to a synergistic increase in leaf growth, suggesting an antagonistic growth control by cytokinins and gibberellins. Only additive effects on root and shoot growth were visible in plants ectopically expressing both CKX3 and ARABIDOPSIS VACUOLAR PYROPHOSPHATASE1, hinting at an independent action mode. Our results show new interactions and contribute to the molecular and physiological understanding of biomass production at the whole plant level.     

Axillary shoot proliferation and growth of Populus deltoides shoot cultures

Shoot cultures of four genotypes of Populus deltoides Bartr. ex Marsh. were established from adventitious shoots regenerated from internodal stem explants. Stable shoot cultures for all four genotypes were maintained in a continuous culture regime for over one year. The stable shoot cultures were used as explants to investigate the effects of zeatin concentration and genotype on axillary shoot production and growth. The concentration of zeatin significantly affected the production of axillary shoots, with 1.0 mgL^–1 zeatin producing the greatest number of shoots (31.0 shoots per culture vessel) while 0.25 mgL^–1 zeatin produced the greatest growth (5.9 mg per axillary shoot) when measured by dry weight accumulation per shoot. Genotypic differences were significant in the production and growth of axillary shoots.Abbreviations DKW Driver and Kuniyuki Walnut medium - PAR Photosynthetically Active Radiation Journal Series No. 9111, Agricultural Research Division, University of Nebraska     

Continuous shoot growth monitoring in hydroponics

A weighing apparatus for automatic recording of fresh weight of shoots of spinach plants ( Spinacia oleracea L., cv. Subito) growing in nutrient solution is described. The system was tested for 17 days in a controlled environment and enabled the determination of the relative growth rate (RGR) of the shoot fresh weight. Results from three consecutive growth experiments demonstrated diurnal fluctuations in the relative growth rate of the shoot fresh weight. In general, relative growth rates were between 0.32 and 0.36 day⁻¹ 16 days after sowing and decreased to between 0.11 and 0.18 day⁻¹ during the 12 following days. The variance between three replicate growth curves was compared with the variance of a growth function fitted through destructively obtained spinach shoot weight data.     

Determination of floral organ type in cultured Silene shoot apices

Shoot apices of the long day plant, Silene coeli-rosa , were cultured on a basal medium (+3% sucrose) in non-inductive short days (SD) following their excision from plants which had been exposed to long day (LD) treatments in order to examine the period for determination of each floral whorl. In response to the inductive LD treatments, the pattern of whorl formation in vitro reflected their normal appearance in Silene : sepals, stamens 1–5, petals, stamens 6–10 and carpels, although the number of apices initiating each whorl was lower in vitro compared with apices in vivo. However, supplementing the medium with 7 instead of 3% sucrose corrected this deficiency and, for the first time, resulted in apices initiating floral whorls in SD. The interval between the shortest treatment to result in whorl initiation in vitro, 4 LD (which also resulted in 50% flowering in vivo), and the treatment which gave 50% initiation of the corresponding whorl in vitro, was taken to be the period for determination of that whorl. The determination times on the 3% medium were: sepals (2 days), stamens 1–5 (3 days), petals (3 days), stamens 6–10 (4 days) and carpels (4 days); all of these periods shortened to about 1 day on the 7% medium. Tissue culture did not perturb the pattern of initiation of each whorl since apices excised and cultured from plants which had received 7 LD + 2 SD, exhibited each whorl over the same time scale as those of intact plants which received the same treatment. The data are consistent with a sequential determination and initiation of each whorl in the order that they appear normally in Silene . Synchronisation of cell division, as represented by peaks of the mitotic index and G₂/G₁ ratios on day 8 (7 LD + 2 SD), did not occur in vitro but the mitotic index did not descend to zero, further emphasising that tissue culture did not perturb the Silene apex.     

Bidder's organ extract induced anaphylaxis in experimental animals

Bidder's organ (BO, a vestigeal organ), present in toad Bufo melanostictus (Schenider), is a characteristic feature of all male bufo. Its possible anaphylactic properties are investigated on experimental animals. BO extract produced both in vivo and in vitro anaphylactic reaction in guineapig. Dyspnoea and bronchoconstriction was a major cause of anaphylactic death. Blood histamine level was significantly increased in the anaphylactic animals. BO extract significantly released histamine from chopped lung preparation, an action antagonised by disodium chromoglycate. BO extract degranulated peritoneal mast cell in vitro. Passive cutaneous anaphylactic reactions were enhanced by BO extract and were significantly inhibited by disodium chromoglycate. Anaphylotoxin (identity not known) present in bidder's organ is probably involved in toad defence.     

Promotion of asparagus shoot and root growth by growth retardants

Plantlets regenerated from shoot-tip culture of Asparagus officinalis L. possessed weak shoots and roots. Various combinations of auxins and cytokinins did not improve the plantlets. Incorporation of a number of growth retardants, viz. ancymidol, B-995, phosfon, Amo 1618, cycocel and paclobutrazol, promoted growth of stronger shoots and roots. The effectiveness of the growth retardants varied, with ancymidol being most effective and cycocel least effective.The response to ancymidol was prevented by exogenous GA₃ and GA_4/7. GA_1/3 and GA_4/7-like activities were detected in asparagus shoot-tip culture and these activities were reduced by the presence of the growth retardants ancymidol, Amo-1618, and cycocel.     

Phloroglucinol enhances growth and rate of axillary shoot proliferation in potato shoot tip cultures in vitro

Efficacy of phloroglucinol in promoting growth and development of in vitro-derived shoot tips was studied in six potato (Solanum tuberosum L.) genotypes. Different concentrations of phloroglucinol (0, 0.08, 0.4, 0.8, 1.2 and 1.6 mM) were tested in combination with either 0.1 or 0.2 M sucrose in shoot tip proliferation medium based on MS (Murashige and Skoog, 1962) medium supplemented with 5.8 μM GA₃ (gibberellic acid), 1.1 μM BA (N⁶-benzyladenine) and 8.39 μM D-calcium pantothenate. Phloroglucinol fostered multiple shoot formation, promoted axillary shoot proliferation in terms of shoot tip fresh weight and shoot length, and stimulated root formation on the shoot tips. There was significant phloroglucinol × sucrose interaction for number of shoots developed per shoot tip, shoot tip fresh weight and number of roots induced per shoot tip. The beneficial effect of phloroglucinol on shoot tip survival was conspicuous only in genotypes that showed poor survival in the control proliferation medium. There were significant differences in response between the two sucrose levels with regard to shoot tip fresh weight and number of roots per shoot tip. Phloroglucinol in combination with 0.2 M sucrose induced maximum number of roots per shoot tip. Optimum shoot tip growth was fostered in medium containing 0.8 mM phloroglucinol and 0.2 M sucrose. High frequency multiple shoot formation in this medium ensures a faster rate of potato shoot tip multiplication within a limited time and space. This revised version was published online in June 2006 with corrections to the Cover Date.     

A novel method of supplying nutrients permits predictable shoot growth and root : shoot ratios of pre-transplant bedding plants

BACKGROUND AND AIMS: Growth of bedding plants, in small peat plugs, relies on nutrients in the irrigation solution. The object of the study was to find a way of modifying the nutrient supply so that good-quality seedlings can be grown rapidly and yet have the high root : shoot ratios essential for efficient transplanting. METHODS: A new procedure was devised in which the concentrations of nutrients in the irrigation solution were modified during growth according to changing plant demand, instead of maintaining the same concentrations throughout growth. The new procedure depends on published algorithms for the dependence of growth rate and optimal plant nutrient concentrations on shoot dry weight W(s) (g m(-2)), and on measuring evapotranspiration rates and shoot dry weights at weekly intervals. Pansy, Viola tricola 'Universal plus yellow' and petunia, Petunia hybrida 'Multiflora light salmon vein' were grown in four independent experiments with the expected optimum nutrient concentration and fractions of the optimum. Root and shoot weights were measured during growth. KEY RESULTS: For each level of nutrient supply W(s) increased with time (t) in days, according to the equation DeltaW(s)/Deltat=K(2)W(s)/(100+W(s)) in which the growth rate coefficient (K(2)) remained approximately constant throughout growth. The value of K(2) for the optimum treatment was defined by incoming radiation and temperature. The value of K(2) for each sub-optimum treatment relative to that for the optimum treatment was logarithmically related to the sub-optimal nutrient supply. Provided the aerial environment was optimal, R(sb)/R(o) approximately W(o)/W(sb) where R is the root : shoot ratio, W is the shoot dry weight, and sb and o indicate sub-optimum and optimum nutrient supplies, respectively. Sub-optimal nutrient concentrations also depressed shoot growth without appreciably affecting root growth when the aerial environment was non-limiting. CONCLUSION: The new procedure can predict the effects of nutrient supply, incoming radiation and temperature on the time course of shoot growth and the root : shoot ratio for a range of growing conditions.     

Arabidopsis CUP-SHAPED COTYLEDON3 regulates postembryonic shoot meristem and organ boundary formation

Overall shoot architecture in higher plants is highly dependent on the activity of embryonic and axillary shoot meristems, which are produced from the basal adaxial boundaries of cotyledons and leaves, respectively. In Arabidopsis thaliana, redundant functions of the CUP-SHAPED COTYLEDON genes CUC1, CUC2, and CUC3 regulate embryonic shoot meristem formation and cotyledon boundary specification. Their functional importance and relationship in postembryonic development, however, is poorly understood. Here, we performed extensive analyses of the embryonic and postembryonic functions of the three CUC genes using multiple combinations of newly isolated mutant alleles. We found significant roles of CUC2 and CUC3, but not CUC1, in axillary meristem formation and boundary specification of various postembryonic shoot organs, such as leaves, stems, and pedicels. In embryogenesis, all three genes make significant contributions, although CUC3 appears to possess, at least partially, a distinct function from that of CUC1 and CUC2. The function of CUC3 and CUC2 overlaps that of LATERAL SUPPRESSOR, which was previously shown to be required for axillary meristem formation. Our results reveal that redundant but partially distinct functions of CUC1, CUC2, and CUC3 are responsible for shoot organ boundary and meristem formation throughout the life cycle in Arabidopsis.     

Aphid-induced reduction of shoot and root growth in Douglas-fir seedlings

1. The short‐ and long‐term effects of photosynthate removal by sap‐sucking herbivores on plant growth were examined by experimentally manipulating densities of an aphid Cinara pseudotsugae (Wilson) on 2‐year‐old Douglas‐fir seedlings Pseudotsuga menziesii (Mirb.) Franco under greenhouse conditions. An 18‐week test was conducted to determine short‐term effects. Effects of long‐term aphid feeding were examined by exposing seedlings to aphid feeding for two consecutive growing seasons. A third experiment evaluated the extent of recovery during 1 year following a single season of aphid feeding. At least 35 seedlings were used in each test. 2. Volume and dry weight of both shoots and roots decreased significantly with increasing aphid feeding in all three experiments. 3. The most significant and severe aphid effect was reduced root tissue density, suggesting carbohydrate depletion due to translocation from roots to shoots. 4. There was no sign of recovery, of either root or shoot growth, during the year following one season of feeding. 5. The results of this study indicate that short‐term feeding by aphids can affect plant growth and structure for a relatively long time.     

Connective tissue growth factor (CCN2, CTGF) and organ fibrosis: lessons from transgenic animals

In recent months, four different systems have been reported in the literature in which CCN2 transgenes were individually expressed in podocytes, hepatocytes, cardiomyocytes or respiratory epithelial cells to achieve overexpression in, respectively, the kidney, liver, heart, or lung. These transgenic systems have provided valuable information about the contribution of CCN2 to fibrosis in vivo and have begun to reveal the complexities of the underlying mechanisms involved. On the one hand, studies of these animals have revealed that CCN2 overexpression does not necessarily lead directly to fibrotic pathology but may cause severe non-fibrotic tissue damage due to its other effects on cell function (e.g. heart). On the other hand, overexpression of CCN2 in concert with signaling pathways associated with development (e.g. lung) or fibrosing injuries (e.g. kidney, liver) can lead to the initiation or exacerbation of fibrosis. The significance of these studies is discussed in the context of the requirement for interactions between CCN2 and co-stimulatory factors in the microenvironment for the manifestation of CCN2-dependent fibrosis.     

Mitochondrial activity during shoot formation and growth in tobacco callus

Mitochondria isolated from tobacco ( Nicotiana tabacum L. cv. Wisconsin 38) callus growing on either shoot-forming or non-shoot forming medium show an increase in state 3 and state 4 respiration and a drop in respiratory control and ADP/O ratios after subculture. the protein content of the mitochondria fraction and the activity of succinate dehydrogenase, malate dehydrogenase, cytochrome c oxidase and catalase also increase after subculture but there is no apparent difference between shoot-forming and non-shoot-forming tissue. For mitochondria assayed at their native osmolarities, a trend of higher respiration rates and respiratory control as well as lower levels of cyanide-resistant respiration was observed for shoot-forming tissue. Generally, differences were greatest after day 9 in culture, the time during which primordia formation occurred in the shoot-forming callus. These patterns are in concert with the view that the shoot-forming process has a high energy requirement which must be realized during the time of primordia formation.     

Shoot orientation affects vessel size, shoot hydraulic conductivity and shoot growth rate in Vitis vinifera L.

Vitis vinifera L. plants were grown in containers and each plant's single shoot was orientated upwards or downwards. Some plants were trained first upwards, then downwards, then again upwards (N-shaped plants). Vegetative growth was reduced in plants trained downwards compared to that in upward and N-shaped plants. Shoot growth rate slowed in downward shoot portions, but only after the apex had grown downwards for at least 10 internodes. Shoot hydraulic conductivity k _h, measured after elimination of xylem embolisms, was lower in downward than in upward plants. In N-shaped plants k _h was higher in the upward-growing shoot portions, and lower in the central, downward-growing portion. Shoot- and leaf-specific conductivities were also lower in downward than in upward shoot portions. Xylem cross-sectional area and xylem structure (number of wedges, number of vessels per unit xylem area) differed little in the three orientations. In contrast, vessel diameter and the sum of vessel cross-sectional areas were significantly smaller in downward than in upward shoot portions. These differences could explain the reduction in conductivity observed in the downward-orientated shoot portions. The measurements taken on N-shaped plants showed that the decreases in k _h and in vessel size were a result of shoot orientation, not shoot bending.     

Root growth and gravireaction: Influence of the shoot

The kinetics of root gravicurvature of intact maize ( Zea mays L., cv. LG 11) seedlings can be separated into two steps. From 0 to 2 h, a rapid downward bending, enhanced by light, occurred. During the next 4 h (2 to 6 h) no further curvature appeared in the dark, whereas it continued in light. The final root gravicurvature was greater for light-than dark-treated seedlings. Growth was particulary inhibited during the first 2 h.
When intact seedlings were placed horizontally and returned to the dark after a 4 h light pretreatment in a vertical position, only the second step (2 to 6 h) was changed. A negative gravitropism, associated with a stimulation of growth, appeared. Thus, when gravireaction took place in darkness, the final root gravicurvature was similar for the light-pretreated intact seedlings and the dark control. Using apical root segments, this phase of negative gravicurvature was not observed although the stimulation of gravicurvature of light-pretreated roots remained. Similar experiments were performed with seedlings having coleoptile or endosperm removed, after intact seedlings had been exposed to a light pretreatment, and gravireaction took place in the dark. Results indicated that these two organs could play an essential role in the development of gravicurvature and in the regulation of the light effects, respectively.