Expression of <i>T4HR1</i>, a 1,3,6,8-Tetrahydroxynaphthalene Reductase Gene Involved in Melanin Biosynthesis, Is Enhanced by Near-Ultraviolet Irradiation in <i>Bipolaris oryzae</i>

Bipolaris oryzae is the causal agent of brown spot disease in rice and produces the dark pigment melanin. We isolated and characterized T4HR1 gene encoding 1,3,6,8-tetrahydroxynaphthalene (1,3,6,8-THN) reductase, which converted 1,3,6,8-THN to scytalone in the melanin biosynthesis from B. oryzae. A sequence analysis showed that the T4HR1 gene encoded a putative protein of 268 amino acids showing 50% - 99% sequence identity to other fungal 1,3,6,8-THN reductases. Targeted disruption of the T4HR1 gene showed a different phenotype of mycelial color due to an accumulation of shunt products compared to those of wild-type on PDA plates using tricyclazole as a melanin biosynthesis inhibitor. A quantitative real-time PCR analysis showed that the expression of T4HR1 transcripts was enhanced by near-ultraviolet (NUV) irradiation and regulated by transcriptional factor BMR1, similar to three other melanin biosynthesis genes (polyketide synthase gene [PKS1], scytalone dehydratase gene [SCD1], and 1,3,8-THN reductase gene [THR1]) in the melanin biosynthesis of B. oryzae. These results suggested that common transcriptional mechanisms could regulate the enhanced gene expression of these melanin biosynthesis genes by NUV irradiation in B. oryzae.

N^(6)-methyladenosine on the natural antisense transcript of NIA1 stabilizes its mRNA to boost NO biosynthesis and modulate stomatal movement

Nitric oxide(NO)is a crucial signaling molecule that regulates a wide range of metabolic pathways in different strata of organisms.In plants,nitrate reductase(NR)is a key enzyme for NO biosynthesis.There are two NR-encoding genes in Arabidopsis genome,NIA1 and NIA2,which are precisely regulated and expressed in a tissue-specific manner.In this study,we found that the natural antisense transcript as-NIA1,transcribed from the 30 UTR of NIA1,stabilizes NIA1 mRNA to maintain its circadian oscillation in plants grown under the light/dark cycle.Importantly,as-NIA1-dependent NIA1 mRNA stability is indispensable for NIA1-mediated NO biosynthesis in guard cells and natural stomatal closure.Moreover,we revealed that polypyrimidine tract-binding 3(PTB3)regulates the stabilization of NIA1 mRNA by directly binding to UC-rich elements of as-NIA1.We further found that MTA deposits N^(6)-methyladenosine(m6 A)on as-NIA1,facilitating the as-NIA1-PTB3 interaction in vivo,in agreement with RNA structure prediction in that m6 Amediated structural alterations expose the UC-rich elements to enhance the accessibility of PTB3.Taken together,these findings reveal a novel molecular mechanism by which plants precisely manipulate NO biosynthesis to modulate light/dark-regulated stomatal movement,highlighting the coupling of RNA epigenetic modifications and structures shaping RNA-protein interactions in the regulation of hormone biosynthesis.

Cotton BLH1 and KNOX6 antagonistically modulate fiber elongation via regulation of linolenic acid biosynthesis

BEL1-LIKE HOMEODOMAIN(BLH)proteins are known to function in various plant developmental processes.However,the role of BLHs in regulating plant cell elongation is still unknown.Here,we identify a BLH gene,GhBLH1,that positively regulates fiber cell elongation.Combined transcriptomic and biochemical analyses reveal that GhBLH1 enhances linolenic acid accumulation to promote cotton fiber cell elongation by activating the transcription of GhFAD7A-1 via binding of the POX domain of GhBLH1 to the TGGA cis-element in the GhFAD7A-1 promoter.Knockout of GhFAD7A-1 in cotton significantly reduces fiber length,whereas overexpression of GhFAD7A-1 results in longer fibers.The K2 domain of GhKNOX6 directly interacts with the POX domain of GhBLH1 to form a functional heterodimer,which interferes with the transcriptional activation of GhFAD7A-1 via the POX domain of GhBLH1.Overexpression of GhKNOX6 leads to a significant reduction in cotton fiber length,whereas knockout of GhKNOX6 results in longer cotton fibers.An examination of the hybrid progeny of GhBLH1 and GhKNOX6 transgenic cotton lines provides evidence that GhKNOX6 negatively regulates GhBLH1-mediated cotton fiber elongation.Our results show that the interplay between GhBLH1 and GhKNOX6 modulates regulation of linolenic acid synthesis and thus contributes to plant cell elongation.