Release of cotyledonary shoots of pea (<Emphasis Type="Italic">Pisum sativum</Emphasis>) seedlings from inhibition as related to endogenous zeatin and ethylene and exogenous IAA and benzyladenine

This paper deals with apical dominance using a dicotylar model obtained after decapitation of pea seedlings with two shoots — one dominant and the other inhibited. When the dominant shoot was decapitated the inhibited one is released from inhibition and after 24 to 72 h begins to grow. However, the levels of trans-zeatin and production of ethylene increase within 4 and 6 hours respectively after release from inhibition, and within an interval of 72 h the levels of both phytohormones begin gradually to decrease. This indicates that also in this model, the release from apical dominance is associated with an increase in the level of cytokinin zeatin and, thereafter, also with an increased production of ethylene. If indolyl-3-acetic acid (IAA) is applied on the decapitated main stem after decapitation of the dominant shoot, the growth of the initially inhibited one is very strongly retarded; if, however, IAA is applied on the decapitated dominant shoot, this inhibition is significantly weaker. This means that the inhibiting effect of IAA on the inhibited shoot originates to a greater degree from the main stem rather than from the dominant shoot. The effect of benzyladenine (BA) is transferred equally from the decapitated main stem and from the decapitated dominant shoot because the initially inhibited shoot begins to grow as well as also other shoots from serial cotyledonary buds.     

Regulation of cytokinin biosynthesis, compartmentalization and translocation

Cytokinins, a group of mobile phytohormones, play an important role in plant growth and development, and their activity is finely controlled by environmental factors in the control of morphogenic and metabolic adaptations. Inorganic nitrogen sources, such as nitrate, are a major factor regulating gene expression of adenosine phosphate-isopentenyltransferase (IPT), a key enzyme of cytokinin biosynthesis. Modulation of IPT and macronutrient transporter gene expression in response to nitrate, sulphate and phosphate, and cytokinin-dependent repression of the transporter genes suggest that cytokinins play a critical role in balancing acquisition and distribution of macronutrients. Biased distribution of trans-zeatin (tZ)-type cytokinins in xylem and N(6)-(Delta(2)-isopentenyl)adenine (iP)-type cytokinins in phloem saps suggest that, in addition to acting as local signals, cytokinins communicate acropetal and systemic long-distance signals, and that structural side chain variations mediate different biological messages. The compartmentalization of tZ- and iP-type cytokinins implies the involvement of a selective transport system. Recent studies have raised the possibility of subsets of the purine permease family as a transporter of cytokinin nucleobases and equilibrative nucleoside transporters (ENT) for cytokinin nucleosides. These biochemical and transgenic data suggest that AtENT6, an Arabidopsis ENT, could also participate in cytokinin nucleoside transport with a preference for iP riboside in vascular tissue.     

Rhizobial communication with rice roots: Induction of phenotypic changes,mode of invasion and extent of colonization

Legume-rhizobial interactions culminate in the formation of structures known as nodules. In this specialized niche, rhizobia are insulated from microbial competition and fix nitrogen which becomes directly available to the legume plant. It has been a long-standing goal in the field of biological nitrogen fixation to extend the nitrogen-fixing symbiosis to non-nodulated cereal plants, such as rice. To achieve this goal, extensive knowledge of the legume-rhizobia symbioses should help in formulating strategies for developing potential rice-rhizobia symbioses or endophytic interactions. As a first step to assess opportunities for developing a rice-rhizobia symbiosis, we evaluated certain aspects of rice-rhizobia associations to determine the extent of predisposition of rice roots for forming an intimate association with rhizobia. Our studies indicate that: a. Rice root exudates do not activate the expression of nodulation genes such as nodY of Bradyrhizobium japonicum USDA110, nodA of R. leguminosarum bv. trifolii, or nodSU of Rhizobium. sp. NGR234; b. Neither viable wild-type rhizobia, nor purified chitolipooligosaccharide (CLOS) Nod factors elicit root hair deformation or true nodule formation in rice; c. Rhizobia-produced indole-3-acetic acid, but neither trans-zeatin nor CLOS Nod factors, seem to promote the formation of thick, short lateral roots in rice; d. Rhizobia develop neither the symbiont-specific pattern of root hair attachment nor extensive cellulose microfibril production on the rice root epidermis; e. A primary mode of rhizobial invasion of rice roots is through cracks in the epidermis and fissures created during emergence of lateral roots; f. This infection process is nod-gene independent, nonspecific, and does not involve the formation of infection threads; g. Endophytic colonization observed so far is restricted to intercellular spaces or within host cells undergoing lysis. h. The cortical sclerenchymatous layer containing tightly packed, thick walled fibers appears to be a significant barrier that restricts rhizobial invasion into deeper layers of the root cortex. Therefore, we conclude that the molecular and cell biology of the Rhizobium-rice association differs in many respects from the biology underlying the development of root nodules in the Rhizobium-legume symbiosis.     

农杆菌GV3101中反式玉米素合成基因促进烟草再生

胭脂碱型农杆菌GV3101已经被广泛用于植物遗传转化研究。已有的研究结果证明,农杆菌GV3101株系含有的反式玉米素合成 (trans-zeatin synthesizing,tzs)基因编码产物会影响烟草细胞器的形态及细胞的生理状态。然而,有关tzs基因对遗传转化过程外植体再生的影响研究却少有报道。本文在前期研究工作的基础上,以2种烟草、4个农杆菌株系为组培实验材料,验证了胭脂碱型农杆菌tzs基因产物的生理活性。结果表明：以外源添加生长调节物质的外植体为阳性对照,在不添加任何生长调节剂的培养基上,与GV3101菌株共培养的烟草外植体能分化再生,并发育成完整植株;外植体再生与GV3101携带的质粒种类无关;外植体与农杆菌GV3101培养液共培养24 h,烟草再生效果较好;与GV3101株系共培养24 h,将外植体烟草叶片匀浆,经亲和柱分离纯化后,检测出烟草外植体叶片中高达0.78 ng/g FW-1的反式玉米素含量。菌落PCR扩增结果证实,农杆菌GV3101株系有tzs基因序列。以上结果表明,农杆菌GV3101株系内的tzs基因的表达产物有生理学活性,能够促进烟草外植体再生,调节细胞生长。