Abstract
Several members of the Yellow Stripe-Like (YSL) family of proteins are transporters of metals that are bound to the metal chelator nicotianamine or the related set of mugineic acid family chelators known as phytosiderophores. Here, we examine the physiological functions of three closely related Arabidopsis (Arabidopsis thaliana) YSL family members, AtYSL1, AtYSL2, and AtYSL3, to elucidate their role(s) in the allocation of metals into various organs of Arabidopsis. We show that AtYSL3 and AtYSL1 are localized to the plasma membrane and function as iron transporters in yeast functional complementation assays. By using inflorescence grafting, we show that AtYSL1 and AtYSL3 have dual roles in reproduction: their activity in the leaves is required for normal fertility and normal seed development, while activity in the inflorescences themselves is required for proper loading of metals into the seeds. We further demonstrate that the AtYSL1 and AtYSL2 proteins, when expressed from the AtYSL3 promoter, can only partially rescue the phenotypes of a ysl1ysl3 double mutant, suggesting that although these three YSL transporters are closely related and have similar patterns of expression, they have distinct activities in planta. In particular, neither AtYSL1 nor AtYSL2 is able to functionally complement the reproductive defects exhibited by ysl1ysl3 double mutant plants.
Highlights
Several members of the Yellow Stripe-Like (YSL) family of proteins are transporters of metals that are bound to the metal chelator nicotianamine or the related set of mugineic acid family chelators known as phytosiderophores
We have further examined the effect of overexpressing AtYSL3, which resulted in a small increase in Cu in shoots, and have demonstrated that the AtYSL1 protein, when expressed from the AtYSL3 promoter, can only partially rescue the phenotypes of the ysl1ysl3 double mutant, indicating that these proteins have distinct biochemical activities
We conclude that even low levels of expression of AtYSL3 in Escherichia coli are causing lethality; the only clones we obtained in E. coli were ones that do not encode AtYSL3 protein
Summary
Several members of the Yellow Stripe-Like (YSL) family of proteins are transporters of metals that are bound to the metal chelator nicotianamine or the related set of mugineic acid family chelators known as phytosiderophores. In response to Fe starvation, grasses secrete phytosiderophores (PS): derivatives of the mugineic acid family that are structurally similar to NA and that form stable Fe(III) chelates in soil (Tagaki et al, 1984) This accomplishes solubilization of the otherwise nearly insoluble soil Fe. The YS1 protein, located at the root surface, moves the Fe(III)-PS complexes from the rhizosphere into root cells (Romheld and Marchner, 1986; Curie et al, 2001; Roberts et al, 2004). AtYSL1 single mutant plants have subtle phenotypes, the most striking of which is a decrease in both NA and Fe in seeds (Le Jean et al, 2005) Leaves of these mutants contain excess NA, while Fe levels are normal. These observations are consistent with the more obvious and extensive phenotypes exhibited by the ysl1ysl double mutant and highlight the idea that AtYSL proteins affect the homeostasis of both Fe and NA
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