Cysteine (Cys) occupies a central position in plant metabolism due to its biochemical functions. Arabidopsis (Arabidopsis thaliana) cells contain different O-acetylserine(thiol)lyase (OASTL) enzymes that catalyze the biosynthesis of Cys. Because they are localized in the cytosol, plastids, and mitochondria, this results in multiple subcellular Cys pools. Much progress has been made on the most abundant OASTL enzymes; however, information on the less abundant OASTL-like proteins has been scarce. To unequivocally establish the enzymatic reaction catalyzed by the minor cytosolic OASTL isoform CS-LIKE (for Cys synthase-like; At5g28030), we expressed this enzyme in bacteria and characterized the purified recombinant protein. Our results demonstrate that CS-LIKE catalyzes the desulfuration of L-Cys to sulfide plus ammonia and pyruvate. Thus, CS-LIKE is a novel L-Cys desulfhydrase (EC 4.4.1.1), and we propose to designate it DES1. The impact and functionality of DES1 in Cys metabolism was revealed by the phenotype of the T-DNA insertion mutants des1-1 and des1-2. Mutation of the DES1 gene leads to premature leaf senescence, as demonstrated by the increased expression of senescence-associated genes and transcription factors. Also, the absence of DES1 significantly reduces the total Cys desulfuration activity in leaves, and there is a concomitant increase in the total Cys content. As a consequence, the expression levels of sulfur-responsive genes are deregulated, and the mutant plants show enhanced antioxidant defenses and tolerance to conditions that promote oxidative stress. Our results suggest that DES1 from Arabidopsis is an L-Cys desulfhydrase involved in maintaining Cys homeostasis, mainly at late developmental stages or under environmental perturbations.

The leaf surface of a very large number of plant species are covered by trichomes. Non-glandular trichomes are specialized unicellular or multicellular structures that occur in many different plant species and function in xenobiotic detoxification and protecting the plant against pest attack. By analysing the susceptibility of trichome mutants, evidence is provided that indicates the influence of leaf trichomes on foliar fungal infections in Arabidopsis thaliana, probably by facilitating the adhesion of the fungal spores/hyphae to the leaf surface. A decreased trichome number in the hairless Arabidopsis mutant gl1 enhances tolerance against the necrotrophic fungus Botrytis cinerea. By contrast, the try mutant shows an increased susceptibility to both fungal infection and accumulation. Trichome density does not influence infection by the soil-borne pathogen Rhizoctonia solani. In addition, the influence of trichomes on foliar infection is supported by targeting the high-level expression of the Trichoderma harzianum alpha-1,3-glucanase protein to the specialized cell structures. Trichome expression of this anti-fungal hydrolase shows a significant resistance to infection by the foliar pathogen Botrytis cinerea. Resistance to this fungus is not dependent on the constitutive induction of the salicylic or jasmonic defence signalling pathways, but the presence of the alpha-1,3-glucanase protein in trichomes.

A DNA regulatory fragment was isolated from the promoter region of the OASA1 gene, encoding the cytosolic O-acetylserine(thiol)lyase enzyme that is highly expressed in Arabidopsis thaliana trichomes. This DNA fragment has been named an ATP fragment and comprises 1435 bp of the genomic region upstream of the OASA1 gene and 375 bp of the transcriptional initiation start site containing the first intron of the gene. The ATP fragment, fused to the green fluorescent protein (GFP) and b-glucuronidase (GUS) reporter genes, is able to drive high-level gene expression in A. thaliana trichomes. Deletion analysis of the ATP fragment determined that the region from 2266 to 266 contains regulatory elements required for trichome expression. In addition, the region from 1112 to 1375, comprising the first intronic region of the gene, is also essential for trichome gene expression. Expression of the full-length ATP fragment in tobacco and peppermint shows that this fragment is also able to drive expression in glandular trichomes and suggests additional biotechnological applications for this promoter.