Lowered sulfite reduction affects metabolism and seed composition in Arabidopsis thaliana

Abstract

Sulfite reductase (SiR) plays an exclusive role in the assimilatory sulfur reduction pathway by catalyzing the reduction of sulfite to sulfide. The T-DNA insertion mutant line sir1-1 shows lower amounts of SiR transcript, protein and lower activity in vegetative and generative tissues and is severely affected in growth. However, sir1-1 plants flower and set viable seeds, albeit later than wild-type plants and with lower yield. A T-DNA insertion with even less SiR transcript (sir1-2) causes early seedling lethality. It was found that sir1-1 is less tolerant to sulfite treatment. Death of sir1-2 plants and the severe growth phenotype of sir1-1 could be caused by the lack of reduced downstream products (cysteine, GSH) or/and the accumulation of toxic sulfite. Mutant plants could be partially complemented by cysteine and GSH. Despite the earlier observation of reduced flux of sulfur into cysteine and GSH the steady state concentration of sulfide was found to be unchanged in sir1-1. In addition to chlorosis observed in sir1-1 the contents of sugars and starch were reduced. The large scale quantification of metabolites in sir1-1 leaf and root compared to wild-type revealed that sir1-1 resides in a sulfur deprivation stage. However, the response of sir1-1 in leaves to sulfur deprivation was different from wild-type, whereas in roots the response of both sir1-1 and wild-type plants were similar. The consequences of the severe changes in leaf growth and metabolism prompted the investigation of the transcriptome using Affymetrix arrays. sir1-1 plants of the same age and the same size as compared wild-type revealed numerous changes in expression of several functional groups of genes. Gene set enrichment analysis revealed up-regulation of pathways related to DNA damage. In seeds of sir1-1 the transcription of the SiR gene and levels and activity of the SiR protein were even more reduced compared to leaves, resulting in severe effects on seed production. With respect to seed composition protein and free amino acid contents were increased while oil contents were decreased. The seeds were found to respond to this limitation of sulfate assimilation by comprehensive changes in their proteome. Precursors of sulfur-poor globulins accumulated but not the mature forms, while a sulfur-rich albumin decreased in content. Thus, reduced expression of sulfite reductase has profound consequences for primary metabolism of vegetative and generative organs.