Early colonization pattern of maize (Zea mays L. Poales, Poaceae) roots by Herbaspirillum seropedicae (Burkholderiales, Oxalobacteraceae)

Abstract

The bacterium Herbaspirillum seropedicae is an endophytic diazotroph found in several plants, including economically important poaceous species. However, the mechanisms involved in the interaction between H. seropedicae and these plants are not completely characterized. We investigated the attachment of Herbaspirillum to maize roots and the invasion of the roots by this bacterium using H. seropedicae strain SMR1 transformed with the suicide plasmid pUTKandsRed, which carries a mini-Tn5 transposon containing the gene for the Discosoma red fluorescent protein (Dsred) constitutively expressed together with the kanamycin resistance gene. Integration of the mini-Tn5 into the bacterial chromosome yielded the mutant H. seropedicae strain RAM4 which was capable of expressing Dsred and could be observed on and inside fresh maize root samples. Confocal microscopy of maize roots inoculated with H. seropedicae three days after germination showed that H. seropedicae cell were attached to the root surface 30 min after inoculation, were visible in the internal tissues after twenty-four hours and in the endodermis, the central cylinder and xylem after three days.

Evidence for the endophytic colonization of Phaseolus vulgaris(common bean) roots by the diazotroph Herbaspirillum seropedicae

“…The H. seropedicae colonization process on and in P. vulgaris roots appears to occur in a pattern similar to that of Poaceae (9,10): the bacteria invade the intercellular spaces and disperse in the cortex, eventually reaching the xylem vessels. A notable difference was a lower number of bacterial cells visualized at the time tested, when compared with that of maize (10).…”

“…A notable difference was a lower number of bacterial cells visualized at the time tested, when compared with that of maize (10). We also evaluated the effect of H. seropedicae inoculation on the growth of common bean seedlings ( Table 1).…”

“…seropedicae SMR1 is a spontaneous streptomycinresistant derivative of the wild-type strain Z78 (13). RAM4 is a strain tagged with the dsred gene, which expresses the red fluorescent protein, to allow the monitoring of single bacterial cells in P. vulgaris roots (10 (14). After incubation, the plantlets were placed in wells of a 96-well block containing filter paper soaked with plant medium (15) and incubated for a 14-h light period at 25°C.…”

“…The bacterial association with poaceous crops apparently initiates with attachment to root surfaces followed by proliferation at the emergence points of secondary roots and penetration through discontinuities in the epidermis. Rapid occupation of root intercellular spaces then occurs, and the bacteria spread to aerenchyma, xylem vessels and aerial portions (8)(9)(10).…”

Structural analysis of Herbaspirillum seropedicae lipid-A and of two mutants defective to colonize maize roots

“…The interaction process of H. seropedicae with poaceous crops initiates with attachment of the bacteria to root surfaces, followed by colonization of the emergence points in secondary roots and penetration through discontinuities of the epidermic tissue. Intercellular spaces are then rapidly occupied, along with the colonization of root xylem, aerenchyma end aerial parts [2,3]. Even though large number of H. seropedicae cells are found inside plant tissues [4,5], the molecular mechanism for colonization is not yet fully understood.…”