Fig 1.
A model for auxin biosynthesis, transport, and signaling in Chlorella.
Putative pathways of IAA biosynthesis in C. sorokiniana include (1.) the indole-3-pyruvic acid (IPyA) pathway: A tryptophan transaminase first converts Trp to IPyA and flavin monooxygenase (Yucca) converts IPyA to indole-3-acetic acid (IAA); (2.) The indole-3-acetamide (IAM) pathway: Primarily a pathway in phytopathogenic bacteria, IAM has been detected in all plants tested; furthermore, plants encode an amidase that converts IAM to IAA; (3.) The indole-3-acetaldehyde (IAAld) pathway. In the IAAld pathway, IPyA is converted to IAAld by a decarboxylase, and indole-3-acetaldehyde oxidase converts IAAld to IAA. Efflux and import functions are associated with the presence of specific transporters and the ABP1 receptor and associated downstream signaling components are present, as specified in Table 1.
Table 1.
The Chlorella sorokiniana genome encodes putative orthologs for the biosynthesis, transport, sensing, and signal transduction of auxin.
Fig 2.
Phylogenetic analysis indicates sequence conservation of PILS.
PILS (PIN-like) transporters, similar to but distinct from PIN proteins, mediate intracellular auxin transport. Putative orthologs and outgroups were identified by reciprocal BLAST searches, aligned with Clustal Omega and analyzed using the LG+I substitution model in MEGA 6.06.
Fig 3.
Phylogenetic analysis indicates sequence conservation of AUX1.
AUX1, an AAP-like transporter specific for auxin transport, is compared with ANT1, an aromatic amino acid transporter in plants. Putative orthologs and outgroups were identified by reciprocal BLAST searches, aligned with Clustal Omega and analyzed using the LG+I substitution model in MEGA 6.06.
Fig 4.
Phylogenetic analysis indicates sequence conservation of ABCB4.
An ABCB transporter, is compared with a p-coumaric acid transporter which also transports a small organic molecule. Putative orthologs and outgroups were identified by reciprocal BLAST searches, aligned with Clustal Omega and analyzed using the LG+I substitution model in MEGA 6.06.
Fig 5.
Phylogenetic analysis indicates sequence conservation of ABP1.
ABP1, a putative IAA receptor, compared with GLP1, a similarly short (145 AA) cupin domain protein. Putative ABP1 ortholog and outgroups were identified by reciprocal BLAST searches, aligned with Expresso and analyzed using the LG+I substitution model in MEGA 6.06.
Fig 6.
Multiple sequence and structural alignment reveal a high degree of structural conservation between Z. mays ABP1 and C. sorokiniana ABP1.
A SWISSMODEL structural alignment on automated mode aligned CsABP1 with the ZmaABP1 crystal structure co-crystallized with the synthetic auxin naphthalenacetic acid (NAA). The resulting model revealed almost complete conservation of the auxin binding pocket (A), and complete conservation of the histidines and and glutamic acid residues that coordinate the zinc atom (B) that stabilizes the carboxylate moiety of IAA/NAA. The red structure represents Zea mays, and the blue structure represents Chlorella sorokiniana. Z. mays residues are indicated first, with C. sorokiniana residues following the slash.
Fig 7.
Phylogenetic analysis indicates sequence conservation, suggesting bona fide IAA signaling orthologs in Chlorella.
IBR5, identified as a MAP Kinase Phosphatase with roles in auxin signaling, is compared with MKP2, which also contains a MAP kinase phosphatase domain and participates in other phytohormone signaling pathways. Putative auxin signaling orthologs and outgroups were identified by reciprocal BLAST searches, aligned with Clustal Omega (except for ABP1, aligned with Expresso) and analyzed using the LG+I substitution model in MEGA 6.06.
Fig 8.
IAA production in Chlorella sorokiniana.
Chlorella sorokiniana produces IAA within the cell pellet and secretes it into the culture medium. For each 50 ml culture, approximately 2 ng is retained within the cell pellet, while 8–9 ng is secreted into the culture medium.