DEVELOPMENTAL CHANGES IN THE VASCULAR CAMBIUM OF POLYGONUM LAPATHIFOLIUM

Developmental changes in the vascular cambium of Polygonum lapathifolium were determined primarily by an analysis of the secondary xylem. The cambium and xylem consist of fascicular and interfascicular regions in this herbaceous dicotyledon. Near the pith vessels are restricted to the fascicular regions of the xylem. During secondary growth vessels are formed in some radial files in the interfascicular regions. Anticlinal divisions are of two types, oblique and lateral. In interfascicular files consisting of fibers only, about two-thirds of the anticlinal divisions are oblique. The oblique partition averages 31% of the length of the dividing initials. In interfascicular files consisting of vessel elements and fibers, there are almost equal numbers of oblique and lateral divisions. The oblique partition averages 37% of the length of the dividing initials in these files. Lateral divisions account for approximately three-fifths of the anticlinal divisions in the fascicular regions, consisting of vessel elements and fibers. The partitions formed in oblique anticlinal divisions average 64% of the length of the dividing cells in the fascicular regions. The frequency of anticlinal division is much higher in files consisting of vessel elements and fibers than in those consisting of fibers only. There is no loss of fusiform initials, except by ray formation. Ray initiation occurs by simple subdivision of fusiform initials. The findings are discussed in relation to the developmental changes in the vascular cambium in plants of different habits.     

Cloning and characterisation of JAZ gene family in Hevea brasiliensis

Mechanical wounding or treatment with exogenous jasmonates (JA) induces differentiation of the laticifer in Hevea brasiliensis. JA is a key signal for latex biosynthesis and wounding response in the rubber tree. Identification of JAZ (jasmonate ZIM‐domain) family of proteins that repress JA responses has facilitated rapid progress in understanding how this lipid‐derived hormone controls gene expression and related physiological processes in plants. In this work, the full‐length cDNAs of six JAZ genes were cloned from H. brasiliensis (termed HbJAZ). These HbJAZ have different lengths and sequence diversity, but all of them contain Jas and ZIM domains, and two of them contain an ERF‐associated amphiphilic repression (EAR) motif in the N‐terminal. Real‐time RT‐PCR analyses revealed that HbJAZ have different expression patterns and tissue specificity. Four HbJAZ were up‐regulated, one was down‐regulated, while two were less effected by rubber tapping treatment, suggesting that they might play distinct roles in the wounding response. A yeast two‐hybrid assay revealed that HbJAZ proteins interact with each other to form homologous or heterogeneous dimer complexes, indicating that the HbJAZ proteins may expand their function through diverse JAZ–JAZ interactions. This work lays a foundation for identification of the JA signalling pathway and molecular mechanisms of latex biosynthesis in rubber trees.     

Role of ethylene signalling in growth and systemic resistance induction by the plant growth‐promoting fungus Penicillium viridicatum in Arabidopsis

The plant growth‐promoting fungi (PGPF) have long been known to improve plant growth and suppress plant diseases. The PGPF Penicillium viridicatum GP15‐1 elicited plant growth and induced systemic resistance (ISR) in Arabidopsis thaliana against Pseudomonas syringae pv. tomato DC3000 (Pst), leading to a restriction of pathogen growth and disease development. Examination of local and systemic genes indicated that GP15‐1 did not modulate the expression of any of the tested defence‐related marker genes involved in salicylic acid (SA), jasmonic acid (JA) and ethylene signalling pathways. Subsequent challenge of GP15‐1‐colonized plants with Pst bacterium primed Arabidopsis plants for enhanced activation of the JA‐inducible Atvsp (vegetative storage protein) gene at a later stage of infection. To assess the contribution of different signalling pathways in GP15‐1‐elicited plant growth and ISR, Arabidopsis genotypes implicated in SA signalling expressing the nahG transgene (NahG) or carrying disruption in NPR1 (npr1), JA signalling (jar1) and ethylene signalling (ein2) were tested. The GP15‐1‐induced plant growth and ISR were fully compromised in an ein2 mutation. Root colonization assay revealed that the inability of the ein2 mutant to express GP15‐1‐induced plant growth and ISR was not associated with reduced root colonization by GP15‐1. In conclusion, our results demonstrate the ethylene signalling pathway is involved in plant growth promotion and ISR elicitation by the PGPF P. viridicatum GP15‐1 in Arabidopsis. These results provide evidence that ethylene signalling has a substantial role in plant growth and disease resistance.     

Ectopic Expression of Erigeron breviscapus JAZ1 Affects JA-Induced Development Processes in Transgenic Arabidopsis

The jasmonate ZIM-domain (JAZ) proteins are repressors that function in the regulation of plant growth, development, and response to stimulation of different signals in the JA signaling pathway. Erigeron breviscapus is characteristic of sporophyte self-incompatibility (SSI). However, whether JA signaling is involved in regulation of development processes in E. breviscapus is unclear. In this study, the JAZ homolog EbJAZ1 was isolated and characterized from E. breviscapus. EbJAZ1 was localized to the nucleus, and expressed in roots, stems, leaves and flowers. Ectopic expression of EbJAZ1 in Arabidopsis resulted in shorter filament and silique length, and lower seed fertility. In addition, MeJA-induced root growth inhibition was compromised in transgenic plants. Further qRT-PCR analysis indicated that expression patterns of marker genes for VSP1, VSP2, JAZ1, JAZ5, JAZ8, JAZ10, MYC2, and bHLH17 were downregulated in transgenic plants compared to wild-type, suggesting that EbJAZ regulates the development of flower organs, seed fertility, and primary root growth through the JA signaling pathway. Thus, our results indicate that EbJAZ1 is one of the important regulators possibly involved in SSI and other developmental processes in Erigeron breviscapus.

     

ANATOMY OF NODES VS. INTERNODES IN COLEUS: THE NODAL CAMBIUM

Secondary growth begins in the nodal regions before the internodal regions in Coleus, so that longitudinally discontinuous vascular cambia are formed in the 6th through the 9th or 10th nodes, where the internodal cambium becomes continuous between nodal cambia. The nodal cambia are identifiable by radial seriation in interfascicular regions, typical cytology of fusiform initials, and the presence of a ray system. Anatomical features distinct from the primary plant body are shared by the nodal and internodal cambia. Branching of primary vascular strands, restricted to procambium and phloem, is virtually confined to nodal regions. In secondary growth, vascular branching of xylem and phloem occurs in both nodes and internodes. Xylem strand branches are formed only from derivatives of vascular cambia. It is proposed that the cambium provides the secondary plant body an efficient channel for lateral auxin transport, by which branching across interfascicular regions is facilitated.     

ANOMALOUS SECONDARY GROWTH IN LIANAS OF THE BIGNONIACEAE IS CORRELATED WITH THE VASCULAR PATTERN

Vascular pattern and anomalous secondary growth were studied in shoots of Clytostoma callistegoides, a liana having two types of phyllotaxy, one decussate and the other whorled. In shoots with decussate phyllotaxy, typical of bignoniaceous lianas, the vascular pattern has four major vascular strands that extend continuously from internode to internode, whereas in shoots having a whorled phyllotaxy the pattern has six major vascular strands. The first unidirectional cambium segments which result in the anomalous secondary growth were initiated precisely opposite each of the major vascular strands in both types of shoots. It is concluded that positioning of unidirectional cambium segments responsible for anomalous growth is correlated morphogenetically with the vascular pattern.     

Different mechanisms of Trichoderma virens‐mediated resistance in tomato against Fusarium wilt involve the jasmonic and salicylic acid pathways

In the present study, we investigated the role of Trichoderma virens (TriV_JSB100) spores or cell‐free culture filtrate in the regulation of growth and activation of the defence responses of tomato (Solanum lycopersicum) plants against Fusarium oxysporum f. sp. lycopersici by the development of a biocontrol–plant–pathogen interaction system. Two‐week‐old tomato seedlings primed with TriV_JSB100 spores cultured on barley grains (BGS) or with cell‐free culture filtrate (CF) were inoculated with Fusarium pathogen under glasshouse conditions; this resulted in significantly lower disease incidence in tomato Oogata‐Fukuju plants treated with BGS than in those treated with CF. To dissect the pathways associated with this response, jasmonic acid (JA) and salicylic acid (SA) signalling in BGS‐ and CF‐induced resistance was evaluated using JA‐ and SA‐impaired tomato lines. We observed that JA‐deficient mutant def1 plants were susceptible to Fusarium pathogen when they were treated with BGS. However, wild‐type (WT) BGS‐treated tomato plants showed a higher JA level and significantly lower disease incidence. SA‐deficient mutant NahG plants treated with CF were also found to be susceptible to Fusarium pathogen and displayed low SA levels, whereas WT CF‐treated tomato plants exhibited moderately lower disease levels and substantially higher SA levels. Expression of the JA‐responsive defensin gene PDF1 was induced in WT tomato plants treated with BGS, whereas the SA‐inducible pathogenesis‐related protein 1 acidic (PR1a) gene was up‐regulated in WT tomato plants treated with CF. These results suggest that TriV_JSB100 BGS and CF differentially induce JA and SA signalling cascades for the elicitation of Fusarium oxysporum resistance in tomato.     

Rays,intrusive growth,and storied cambium in the inflorescence stems of <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> (L.) Heynh

Arabidopsis thaliana is a model plant used in analysis of different aspects of plant growth and development. Under suitable conditions, secondary growth takes place in the hypocotyl of Arabidopsis plants, a finding which helps in understanding many aspects of xylogenesis. However, not all developmental processes of secondary tissue can be studied here, as no secondary rays and intrusive growth have been detected in hypocotyl. However, results presented here concerning the secondary growth in inflorescence stems of Arabidopsis shows that both secondary rays and intrusive growth of cambial cells can be detected, and that, in the interfascicular regions, a storied cambium can be developed.     

Ontogeny of the interfascicular cambium in the petiole ofTabebuia rosea DC. (Bignoniaceae)

In order to elucidate the origin of interfascicular cambium in the petiole ofTabebuia rosea, transverse and tangential views of the cells in the interfascicular region during the different developmental stages of the petiole have been traced. Interfascicular cambium originates from the interfascicular parenchyma, which has been differentiated from interfascicular vascular meristem cells. Interfascicular parenchyma cells divide periclinally differentiating into the interfascicular metacambial cells and then into the cambium. Tangentially the homogenous structure of interfascicular parenchyma cells in the early stage gradually changes into a heterogenous structure with long and short cells from which fusiform and ray initials are derived, respectively. Ontogenetic pattern of the interfascicular cambium is similar to that of the fascicular one with interfascicular metacambium as an intermediate stage.