PLANT PHYSIOLOGY , Vol 103, Issue 1 21-30, Copyright © 1993 by American Society of Plant Biologists
|
PLANT-MICROBE INTERACTIONS |
A Gene That Encodes a Proline-Rich Nodulin with Limited Homology to PsENOD12 Is Expressed in the Invasion Zone of Rhizobium meliloti-Induced Alfalfa Root Nodules
M. Lobler and A. M. Hirsch
Department of Biology, University of California, Los Angeles, 405 Hilgard Avenue, Los Angeles, California 90024-1606
To define the early stages of the interaction between Rhizobium and host
legumes, we have cloned and characterized three early nodulin-encoding
sequences from an alfalfa (Medicago sativa L.) cDNA library by probing with
a fragment of a cDNA clone for PsENOD12, an infection-related nodulin from
pea (Pisum sativum L.). Although the coding regions of the three clones are
95 to 98% homologous to each other, they are only 43% homologous to the pea
clone. However, the putative signal peptide encoded by the alfalfa cDNA
clones is 100% homologous to the PsENOD12 signal peptide. The spatial and
temporal expression patterns of PsENOD12 and the alfalfa clones were
compared. In situ hybridization experiments detected RNA transcripts in the
invasion zone of mature nitrogen-fixing nodules, the same site where
PsENOD12 mRNAs are found. Transcripts were also found by in situ
hybridization in cells of Rhizobium meliloti exoH mutant-induced nodules
penetrated by infection threads, but northern analysis did not detect
transcripts in Inf- (infection thread minus) nodules elicited by R.
meliloti exoB nodules or in pseudonodules elicited by treatment with the
auxin transport inhibitor N-1-(naphthyl)phthalamic acid. In addition, the
alfalfa gene represented by these cDNA clones exhibited a temporal
expression pattern that differed from that of PsENOD12, which is
transiently expressed. These data, plus information derived from Southern
blot analysis, indicate that we have isolated cDNA clones for a novel early
nodulin, which we have designated MsENOD10 (Medicago sativa Early Nodulin
10).
