Alfonso Muñoz

The symbiotic association of legumes with rhizobia involves bacterially derived Nod factor, which is sufficient to activate the formation of nodules on the roots of the host plant. Perception of Nod factor by root hair cells induces calcium oscillations that are a component of the Nod factor signal transduction pathway. Perception of the calcium oscillations is a function of a calcium- and calmodulin-dependent protein kinase, and this activates nodulation gene expression via two GRAS domain transcriptional regulators, Nodulation Signaling Pathway1 (NSP1) and NSP2, and an ERF transcription factor required for nodulation. Here, we show that NSP1 and NSP2 form a complex that is associated with the promoters of early nodulin genes. We show that NSP1 binds directly to ENOD promoters through the novel cis-element AATTT. While NSP1 shows direct binding to the ENOD11 promoter in vitro, this association in vivo requires NSP2. The NSP1-NSP2 association with the ENOD11 promoter is enhanced following Nod factor elicitation. Mutations in the domain of NSP2 responsible for its interaction with NSP1 highlight the significance of the NSP1-NSP2 heteropolymer for nodulation signaling. Our work reveals direct binding of a GRAS protein complex to DNA and highlights the importance of the NSP1-NSP2 complex for efficient nodulation in the model legume Medicago truncatula.

Two different gene families have been proposed to act as sorting receptors for vacuolar storage cargo in plants: the vacuolar sorting receptors (VSRs) and the receptor homology-transmembrane-RING H2 domain proteins (RMRs). However, functional data on these genes is scarce and the identity of the sorting receptor for storage proteins remains controversial. Through a genetic screen we have identified the mtv2 mutant, which is defective in vacuolar transport of the storage cargo VAC2 in shoot apices. Map-based cloning revealed that mtv2 is a loss of function allele of the VSR4 gene. We show that VSR1, VSR3 and VSR4, but not the remaining VSRs or RMRs, participate in vacuolar sorting of VAC2 in vegetative tissues, and 12S globulins and 2S albumins in seeds, an activity that is essential for seedling germination vigor. Finally, we demonstrate that the functional diversification in the VSR family results from divergent expression patterns and also from distinct sorting activities of the family members.

For years, the study of gene expression regulation of plants in response to stress conditions has been focused mainly on the analysis of transcriptional changes. However, the knowledge on translational regulation is very scarce in these organisms, despite in plants, as in the rest of the eukaryotes, translational regulation has been proven to play a pivotal role in the response to different stresses. Regulation of protein synthesis under abiotic stress was thought to be a conserved process, since, in general, both the translation factors and the translation process are basically similar in eukaryotes. However, this conservation is not so clear in plants as the knowledge of the mechanisms that control translation is very poor. Indeed, some of the basic regulators of translation initiation, well characterised in other systems, are still to be identified in plants. In this paper we will focus on both the regulation of different initiation factors and the mechanisms that cellular mRNAs use to bypass the translational repression established under abiotic stresses. For this purpose, we will review the knowledge from different eukaryotes but paying special attention to the information that has been recently published in plants.