A. C. M. M. Gomes

Concentration of β-ecydisone (20E) in susceptible and resistant accessions of Pfaffia glomerata infected with Meloidogyne incognita and histological characterisation of resistance

Pfaffia glomerata is a medicinal plant widely distributed in Brazil, which is considered the worlds greatest supplier of P. glomerata roots. Among active ingredients contained in this plant, the steroid β -ecydisone (20E) is the most important compound extracted from roots. This steroid presents therapeutic properties for the treatment of diabetes and haemorrhoids, besides having bioenergy, tonic and aphrodisiac effects. The root-knot nematode Meloidogyne spp. is a major limiting factor in root production. Recent studies showed resistance of accessions of P. glomerata to Meloidogyne incognita . The aims of this work were: i ) to correlate the concentration of 20E with resistance and susceptibility of P. glomerata accessions to M. incognita in inoculated and non-inoculated plants; ii ) to study the effect of the parasitism of M. incognita on the concentration of the steroid 20E in the roots; and iii ) to clarify resistance mechanisms by comparing the response of a highly resistant UFV with a highly susceptible accession (Farm) to nematode infection. The concentration of 20E in the healthy susceptible Farm accession was significantly higher than in the healthy resistant UFV accession, showing that the resistance mechanism was not related to 20E concentrations. Plants of the Farm accession infected with M. incognita showed higher levels of 20E than the non-infected control. A positive and significant Pearson correlation coefficient was observed between 20E concentrations and gall indexes. Resistance of UFV to the root-knot nematode M. incognita was associated with unidentified factors that limited nematode penetration or emigration of second-stage juveniles and with post-penetration responses, including the hypersensitive response. Giant cells were sometimes found in the resistant cultivar, but displayed a highly vacuolated and degraded cytoplasm with thinner cell walls than those induced in the susceptible accession. Microscope observations under UV light showed a strong autofluorescence, suggesting that phenolic compounds may be involved in ginseng UFV resistance.

Additional information on an atypical population of Meloidogyne exigua Göldi, 1887 (Tylenchida: Meloidogynidae) parasitising rubber tree in Brazil

Females, males and second-stage juveniles of an unusual population of Meloidogyne exigua race 3 that only parasitises the rubber tree were examined by light and scanning electron microscopy. Previous molecular characterisation, using SCAR markers, showed consistency with other M. exigua populations from coffee. However, RAPD analysis showed high polymorphism (43.1-57.8%) but with 100% bootstrap support among populations from coffee and rubber tree. In addition, this population showed the main M. exigua esterase band (Rm: 1.5) and an additional band (Rm: 1.6) not detected in the coffee population. The morphology of this atypical population was very close to M. exigua from coffee in major diagnostic characters such as perineal pattern and male head region. However, minor differences occurred in morphological and morphometric features, particularly in the female and male stylet morphology. Cytogenetic studies confirmed the occurrence of meiotic parthenogenesis with haploid chromosome number (n = 18).

GID1 expression is associated with ovule development of sexual and apomictic plants

Key messageBbrizGID1is expressed in the nucellus of apomicticBrachiaria brizantha, previous to aposporous initial differentiation. AtGID1aoverexpression triggers differentiation ofArabidopsis thalianaMMC-like cells, suggesting its involvement in ovule development.AbstractGIBBERELLIN-INSENSITIVE DWARF1 (GID1) is a gibberellin receptor previously identified in plants and associated with reproductive development, including ovule formation. In this work, we characterized the Brachiaria brizantha GID1 gene (BbrizGID1). BbrizGID1 showed up to 92% similarity to GID1-like gibberellin receptors of other plants of the Poaceae family and around 58% to GID1-like gibberellin receptors of Arabidopsis thaliana. BbrizGID1 was more expressed in ovaries at megasporogenesis than in ovaries at megagametogenesis of both sexual and apomictic plants. In ovules, BbrizGID1 transcripts were detected in the megaspore mother cell (MMC) of sexual and apomictic B. brizantha. Only in the apomictic plants, expression was also observed in the surrounding nucellar cells, a region in which aposporous initial cells differentiate to form the aposporic embryo sac. AtGID1a ectopic expression in Arabidopsis determines the formation of MMC-like cells in the nucellus, close to the MMC, that did not own MMC identity. Our results suggest that GID1 might be involved in the proper differentiation of a single MMC during ovule development and provide valuable information on the role of GID1 in sexual and apomictic reproduction.