Although several glycine-rich protein (GRP) genes were isolated and characterized, very little is known about their function. The primary structure of AtGRP5 from Arabidopsis thaliana has a signal peptide followed by a region with high glycine content. In this work, green fluorescent protein fusions were obtained in order to characterize the sub-cellular localization of the AtGRP5 protein. The results indicated that this protein is the first described vacuolar GRP. Sense, antisense and RNAi transgenic A. thaliana plants were generated and analyzed phenotypically. Plants overexpressing AtGRP5 showed longer roots and an enhanced elongation of the inflorescence axis, while antisense and RNAi plants demonstrated the opposite phenotype. The analysis of a knockout T-DNA line corroborates the phenotypes obtained with the antisense and RNAi plants. Altogether, these results suggest that this vacuolar GRP could be involved in organ growth by promoting cell elongation processes.

AtGRP3 is a glycine-rich protein (GRP) from Arabidopsis thaliana shown to interact with the receptor-like kinase AtWAK1 in yeast, in vitro and in planta. In this work, phenotypic analyses using transgenic plants were performed in order to better characterize this GRP. Plants of two independent knockout alleles of AtGRP3 develop longer roots suggesting its involvement in root size determination. Confocal microscopy analysis showed an abnormal cell division and elongation in grp3-1 knockout mutants. Moreover, we also show that grp3-1 exhibits an enhanced Aluminum (Al) tolerance, a feature also described in AtWAK1 overexpressing plants. Together, these results implicate AtGRP3 function root size determination during development and in Al stress.

AtGRP3 is a glycine-rich protein from Arabidopsis thaliana shown to interact with the extracellular domain of the receptor-like kinase (RLK) AtWAK1. Based on previous functional data for AtWAK1, a model was proposed that AtGRP3 when bound to this RLK would negatively regulate its kinase activity, inhibiting cell expansion. Here, we review recent functional studies on AtGRP3 that corroborate this model and suggest that AtGRP3/AtWAK1 complex regulates also defense signaling pathways. On the other hand, we show new data on AtGRP3-overexpressing plants indicating that its role in aluminum signaling pathways, as previously observed for elicitor signaling, seems to be more complex than a simple negative regulator.