Sucrose synthase and enolase expression in actinorhizal nodules ofAlnus glutinosa: comparison with legume nodules

Two different types of nitrogen-fixing root nodules are known — actinorhizal nodules induced byFrankia and legume nodules induced by rhizobia. While legume nodules show a stem-like structure with peripheral vascular bundles, actinorhizal nodule lobes resemble modified lateral roots with a central vascular bundle. To compare carbon metabolism in legume and actinorhizal nodules, sucrose synthase and enolase cDNA clones were isolated from a cDNA library, obtained from actinorhizal nodules ofAlnus glutinosa. The expression of the corresponding genes was markedly enhanced in nodules compared to roots. In situ hybridization showed that, in nodules, both sucrose synthase and enolase were expressed at high levels in the infected cortical cells as well as in the pericycle of the central vascular bundle of a nodule lobe. Legume sucrose synthase expression was studied in indeterminate nodules from pea and determinate nodules fromPhaseolus vulgaris by usingin situ hybridization.     

Control of root architecture and nodulation by the LATD/NIP transporter

The Medicago truncatula LATD/NIP gene is essential for the development of lateral and primary root and nitrogen-fixing nodule meristems as well as for rhizobial invasion of nodules. LATD/NIP encodes a member of the NRT1(PTR1) nitrate and di-and tri-peptide transporter family, suggesting that its function is to transport one of these or another compound(s). Because latd/nip mutants can have their lateral and primary root defects rescued by ABA, ABA is a potential substrate for transport. LATD/NIP expression in the root meristem was demonstrated to be regulated by auxin, cytokinin and abscisic acid, but not by nitrate. LATD/NIP''s potential function and its role in coordinating root architecture and nodule formation are discussed.Key words: nodule development, lateral root development, root architecture, symbiotic nitrogen fixation, Medicago truncatula, NRT1(PTR) gene familyUnlike most other plants, legumes form two kinds of lateral root organs: lateral roots and nitrogen-fixing root nodules that form in conjunction with compatible symbiotic rhizobium bacteria. Although the morphology and function of these two root organs is distinct, both require the function of the LATD/NIP gene, indicating shared genetic components for these two developmental processes and providing support for a model in which legume nodules evolved from a lateral root blueprint. Both lateral roots and nodules initiate in previously differentiated root cells in response to environmental and developmental cues mediated by hormones. Interestingly, regulation of nodules and lateral roots by hormones is often opposite, allowing formation of one organ or another depending on the conditions.     

Sucrose synthase and enolase expression in actinorhizal nodules ofAlnus glutinosa: comparison with legume nodules

Two different types of nitrogen-fixing root nodules are known — actinorhizal nodules induced byFrankia and legume nodules induced by rhizobia. While legume nodules show a stem-like structure with peripheral vascular bundles, actinorhizal nodule lobes resemble modified lateral roots with a central vascular bundle. To compare carbon metabolism in legume and actinorhizal nodules, sucrose synthase and enolase cDNA clones were isolated from a cDNA library, obtained from actinorhizal nodules ofAlnus glutinosa. The expression of the corresponding genes was markedly enhanced in nodules compared to roots. In situ hybridization showed that, in nodules, both sucrose synthase and enolase were expressed at high levels in the infected cortical cells as well as in the pericycle of the central vascular bundle of a nodule lobe. Legume sucrose synthase expression was studied in indeterminate nodules from pea and determinate nodules fromPhaseolus vulgaris by usingin situ hybridization.     

尼泊尔马桑根瘤维管束形态发育研究

以生物制片和光学显微技术研究了尼泊尔马桑根瘤维管束的形态结构和发育特点。尼泊尔马桑根瘤维管束呈多重二叉分枝的树状结构,这是由根瘤顶端分生组织不断分裂形成的。它在基部与植物维管系统联成一体,在本质上是由植物侧根原基衍生而来。马桑根瘤维管束的这种形态结构和发育特点不同于豆科植物根瘤,也不同于杨梅属、木麻黄属等非豆科植物的根瘤。     

nip, a symbiotic Medicago truncatula mutant that forms root nodules with aberrant infection threads and plant defense-like response

To investigate the legume-Rhizobium symbiosis, we isolated and studied a novel symbiotic mutant of the model legume Medicago truncatula, designated nip (numerous infections and polyphenolics). When grown on nitrogen-free media in the presence of the compatible bacterium Sinorhizobium meliloti, the nip mutant showed nitrogen deficiency symptoms. The mutant failed to form pink nitrogen-fixing nodules that occur in the wild-type symbiosis, but instead developed small bump-like nodules on its roots that were blocked at an early stage of development. Examination of the nip nodules by light microscopy after staining with X-Gal for S. meliloti expressing a constitutive GUS gene, by confocal microscopy following staining with SYTO-13, and by electron microscopy revealed that nip initiated symbiotic interactions and formed nodule primordia and infection threads. The infection threads in nip proliferated abnormally and very rarely deposited rhizobia into plant host cells; rhizobia failed to differentiate further in these cases. nip nodules contained autofluorescent cells and accumulated a brown pigment. Histochemical staining of nip nodules revealed this pigment to be polyphenolic accumulation. RNA blot analyses demonstrated that nip nodules expressed only a subset of genes associated with nodule organogenesis, as well as elevated expression of a host defense-associated phenylalanine ammonia lyase gene. nip plants were observed to have abnormal lateral roots. nip plant root growth and nodulation responded normally to ethylene inhibitors and precursors. Allelism tests showed that nip complements 14 other M. truncatula nodulation mutants but not latd, a mutant with a more severe nodulation phenotype as well as primary and lateral root defects. Thus, the nip mutant defines a new locus, NIP, required for appropriate infection thread development during invasion of the nascent nodule by rhizobia, normal lateral root elongation, and normal regulation of host defense-like responses during symbiotic interactions.     

MIR166基因家族在陆生植物中的进化模式分析

MicroRNA（miRNA）是一类广泛存在于真核生物中的具有转录后水平调控功能的内源非编码小分子RNA。在植物中．miRNA通过对靶基因的剪切或沉默来实现对植物生命活动的调控,它是基因表达调控网络的重要组成部分。miR165／166（miR166）是陆生植物中最为古老的MIRNA家族之一,它通过对3型同源异域型-亮氨酸拉链（1id—ZIPⅢ）等靶标的调控,在植物的众多发育时期起着关键的调控作用。本文分析了MIR166基因在陆生植物中的进化关系,并对MIR166在基部陆生植物小立碗藓（Physcomitrella patens）中的复制及进化进行了研究。此外,HD—ZIPⅢ蛋白是植物中重要的一类转录因子,miR166对HD-ZIP Ⅲ基因的调控作用在陆地植物保守的存在,本文对HD—ZIP Ⅲ基因和miR166在进化中的相互作用进行了初步的探讨。     

The LATD gene of Medicago truncatula is required for both nodule and root development

The evolutionary origins of legume root nodules are largely unknown. We have identified a gene, LATD, of the model legume Medicago truncatula, that is required for both nodule and root development, suggesting that these two developmental processes may share a common evolutionary origin. The latd mutant plants initiate nodule formation but do not complete it, resulting in immature, non-nitrogen-fixing nodules. Similarly, lateral roots initiate, but remain short stumps. The primary root, which initially appears to be wild type, gradually ceases growth and forms an abnormal tip that resembles that of the mutant lateral roots. Infection by the rhizobial partner, Sinorhizobium meliloti, can occur, although infection is rarely completed. Once inside latd mutant nodules, S. meliloti fails to express rhizobial genes associated with the developmental transition from free-living bacterium to endosymbiont, such as bacA and nex38. The infecting rhizobia also fail to express nifH and fix nitrogen. Thus, both plant and bacterial development are blocked in latd mutant roots. Based on the latd mutant phenotype, we propose that the wild-type function of the LATD gene is to maintain root meristems. The strong requirement of both nodules and lateral roots for wild-type LATD gene function supports lateral roots as a possible evolutionary origin for legume nodules.