Understanding nitrate assimilation and its regulation in microalgae

Abstract

Nitrate assimilation is a key process for nitrogen (N) acquisition in green microalgae. Among Chlorophyte algae, Chlamydomonas reinhardtii has resulted to be a good model system to unravel important facts of this process, and has provided important insights for agriculturally relevant plants. In this work, the recent findings on nitrate transport, nitrate reduction and the regulation of nitrate assimilation are presented in this and several other algae. Latest data have shown nitric oxide (NO) as an important signal molecule in the transcriptional and posttranslational regulation of nitrate reductase and inorganic N transport. Participation of regulatory genes and proteins in positive and negative signaling of the pathway and the mechanisms involved in the regulation of nitrate assimilation, as well as those involved in Molybdenum cofactor synthesis required to nitrate assimilation, are critically reviewed.

Keywords: Chlamydomonas; green algae; nitrate assimilation; nitrate/nitrite uptake; nitric oxide; nitrogen metabolism.

Figure 1

General scheme of proteins involved in nitrate assimilation and molybdenum cofactor biosynthesis in Chlamydomonas. Those steps symbolized in gray lines are not empirically demonstrated. Proteins that mediate these hypothetic steps are represented in the most probable localization. Different colors are used for each family of transporters. Numbers on the transporters identify each family member.

Figure 2

Clusters of nitrate assimilation genes in microalgae. NIA1 and NII1 (genes encoding NR and NiR) are the only genes conserved in all clusters represented. All clusters have been arranged to show NIA1 gene fixed in the middle of the figure. All the genes represented are involved in nitrate assimilation and molybdenum metabolism with exception of Cytb5R whose relationship with nitrate metabolism is unknown. ^*In Micromonas RCC299 NIA1 and NAR1 are overlapped.

Figure 3

Scheme of transcriptional and posttranslational negative regulation of nitrate assimilation in Chlamydomonas. Red and blue lines indicate inhibition and activation, respectively. Dashed lines represent hypothetic steps. NOS-like protein is an unidentified player that symbolizes the NOS activity reported in photosynthetic organisms. NO represses gene expression by activating a soluble guanylate cyclase (sGC) and inhibits NR activity and high affinity nitrate uptake. Two different sources of NO are represented, NR and a putative NOS enzyme. NO is scavenged by the complex NR/THB.