To exclude or to accumulate? Revealing the role of the sodium HKT1;5 transporter in plant adaptive responses to varying soil salinity

Arid/semi-arid and coastal agricultural areas of the world are especially vulnerable to climate change-driven soil salinity. Salinity tolerance in plants is a complex trait, with salinity negatively affecting crop yield. Plants adopt a range of mechanisms to combat salinity, with many transporter genes being implicated in Na⁺-partitioning processes. Within these, the high-affinity K⁺ (HKT) family of transporters play a critical role in K⁺ and Na⁺ homeostasis in plants. Among HKT transporters, Type I transporters are Na⁺-specific. While Arabidopsis has only one Na ⁺ -specific HKT (AtHKT1;1), cereal crops have a multiplicity of Type I and II HKT transporters. AtHKT1; 1 (Arabidopsis thaliana) and HKT1; 5 (cereal crops) 'exclude' Na⁺ from the xylem into xylem parenchyma in the root, reducing shoot Na⁺ and hence, confer sodium tolerance. However, more recent data from Arabidopsis and crop species show that AtHKT1;1/HKT1;5 alleles have a strong genetic association with 'shoot sodium accumulation' and concomitant salt tolerance. The review tries to resolve these two seemingly contradictory effects of AtHKT1;1/HKT1;5 operation (shoot exclusion vs shoot accumulation), both conferring salinity tolerance and suggests that contrasting phenotypes are attributable to either hyper-functional or weak AtHKT1;1/HKT1;5 alleles/haplotypes and are under strong selection by soil salinity levels. It also suggests that opposite balancing mechanisms involving xylem ion loading in these contrasting phenotypes exist that require transporters such as SOS1 and CCC. While HKT1; 5 is a crucial but not sole determinant of salinity tolerance, investigation of the adaptive benefit(s) conferred by naturally occurring intermediate HKT1;5 alleles will be important under a climate change scenario.

Characterization of a non-reducing polyketide synthase gene from lichen Dirinaria applanata

Lichens are known to produce a variety of secondary metabolites including polyketides that have diverse biological role(s). The biosynthesis of fungal polyketides is governed by type I polyketide synthases (PKS), enzymes with a multidomain structure, including the beta-ketoacyl synthase (KS), acyl transferase (AT), ketoreductase (KR), dehydratase (DH), enoyl reductase (ER) and acyl carrier protein (ACP) domains. Established soredial cultures of Dirinaria applanata (Fée) producing atranorin and divaricatic acid were used to characterize a polyketide synthase gene (DnPKS). A 743bp fragment corresponding to the ketosynthase domain (KS) was isolated using degenerate primers. Complete sequence information for DnPKS (8162bp) was obtained by walking in the 5'and 3' directions of the isolated KS domain using TAIL PCR. A translation of the DnPKS sequence identified the presence of KS, AT, two ACP and TE domains with eight intervening introns. TBLASTX analysis and comparison with other PKS sequences suggest that the coding region of DnPKS sequence is complete with the identification of putative start and stop codons and a stretch of 1226 upstream of the start codon corresponding to the putative promoter. This sequence shows the presence of putative binding sites for fungal transcription factors such as AflR, AreA and PacC. Southern blot analysis suggests that additional DnPKS-like genes may be present in the D. applanata genome. Additionally, expression of a DnPKS-like transcript was examined under different culture conditions and found to be down-regulated by sucrose and up-regulated by mannitol, UV and neutral pH.