SUMMARY The Arabidopsis thaliana aldehyde oxidase 3 (AAO3) catalyzes the oxidation of abscisic aldehyde (ABal) to abscisic acid (ABA). Besides ABal, plants generate other aldehydes that can be toxic above a certain threshold. AAO3 knockout mutants (aao3) exhibited earlier senescence but equivalent relative water content compared with wild‐type (WT) during normal growth or upon application of UV‐C irradiation. Aldehyde profiling in leaves of 24‐day‐old plants revealed higher accumulation of acrolein, crotonaldehyde, 3Z‐hexenal, hexanal and acetaldehyde in aao3 mutants compared with WT leaves. Similarly, higher levels of acrolein, benzaldehyde, crotonaldehyde, propionaldehyde, trans‐2‐hexenal and acetaldehyde were accumulated in aao3 mutants upon UV‐C irradiation. Aldehydes application to plants hastened profuse senescence symptoms and higher accumulation of aldehydes, such as acrolein, benzaldehyde and 4‐hydroxy‐2‐nonenal, in aao3 mutant leaves as compared with WT. The senescence symptoms included greater decrease in chlorophyll content and increase in transcript expression of the early senescence marker genes, Senescence‐Related‐Gene1, Stay‐Green‐Protein2 as well as NAC‐LIKE, ACTIVATED‐BY AP3/P1. Notably, although aao3 had lower ABA content than WT, members of the ABA‐responding genes SnRKs were expressed at similar levels in aao3 and WT. Moreover, the other ABA‐deficient mutants [aba2 and 9‐cis‐poxycarotenoid dioxygenase3‐2 (nced3‐2), that has functional AAO3] exhibited similar aldehydes accumulation and chlorophyll content like WT under normal growth conditions or UV‐C irradiation. These results indicate that the absence of AAO3 oxidation activity and not the lower ABA and its associated function is responsible for the earlier senescence symptoms in aao3 mutant.

No abstract

The roles of cytosolic O-acetylserine-(thiol)-lyase A (OASTLA), chloroplast-localized OASTLB and mitochondrion OASTLC in plant selenate resistance were studied in Arabidopsis. Impairment in OASTLA and OASTLB resulted in reduced biomass, chlorophyll and soluble protein levels compared with selenate treated impaired OASTLC and Wild-Type plants. The general lower total selenium (Se), protein-Se, organic-sulfur and protein-sulfur (S) in oastlA and oastlB levels compared to Wild-Type and oastlC leaves indicate that Se accumulation levels are not the main cause for the stress symptoms in these mutants. Notably, the application of selenate positively induced the S-starvation transcript markers: low-sulfur-induced1 (LSU1), LSU2 and ChaC like protein. Additionally, the activity of S-starvation enzymes markers, the adenosine-5′-phosphosulfate reductase and the OASTLs was increased followed by increased sulfite reductase and sulfite oxidase activities, as well as increased sulfite and sulfide levels. Taken together, the results indicate a futile anabolic S-starvation response that resulted in lower glutathione and increased oxidative stress symptoms in leaves of oastlA and oastlB mutants. Employing in-gel assays of L-cysteine and L-seleno-cysteine desulfhydrase activities revealed that two of the three OASTLs activity bands in each of the oastls single mutants were enhanced in response to selenate, whereas the impaired proteins exhibited a missing activity band in stressed and unstressed plants. The absence of differently migrated activity band in each of the three oastls indicates that these OASTLs are major components in Arabidopsis desulfhydrase activity, degrading not only L-cysteine but also L-Seleno-cysteine.

Molybdenum cofactor containing sulfite oxidase (SO) enzyme is an important player in protecting plants against exogenous toxic sulfite. It was also demonstrated that SO activity is essential to cope with rising dark-induced endogenous sulfite levels and maintain optimal carbon and sulfur metabolism in tomato plants exposed to extended dark stress. The response of SO and sulfite reductase to direct exposure of low and high levels of sulfate and carbon was rarely shown. By employing Arabidopsis wild-type, sulfite reductase, and SO-modulated plants supplied with excess or limited carbon or sulfur supply, the current study demonstrates the important role of SO in carbon and sulfur metabolism. Application of low and excess sucrose, or sulfate levels, led to lower biomass accumulation rates, followed by enhanced sulfite accumulation in SO impaired mutant compared with wild-type. SO-impairment resulted in the channeling of sulfite to the sulfate reduction pathway, resulting in an overflow of organic S accumulation. In addition, sulfite enhancement was followed by oxidative stress contributing as well to the lower biomass accumulation in SO-modulated plants. These results indicate that the role of SO is not limited to protection against elevated sulfite toxicity but to maintaining optimal carbon and sulfur metabolism in Arabidopsis plants.

To examine a role of purine degraded metabolites in response to wounding or UV-C stress, the Arabidopsis wild-type (WT) and Atxdh1 KO mutants, defective in xanthine dehydrogenase1 (XDH1), were exposed to wounding and UV-C irradiation stress. In Atxdh1 mutant, wounding or UV-C stresses resulted in lower fresh-weight, increased senescence symptoms and higher tissue cell death rate compared to WT plants. Additionally, WT plants exhibited lower levels of oxidative stress indicators; reactive oxygen species and malondialdehyde than Atxdh1 mutant leaves. Notably, Transcripts and Proteins functioning in purine degradation pathway were orchestrated to lead to enhanced ureide levels in WT leaves 24 h after applying UV-C or wound stress. Yet, different remobilization of the accumulated ureides was noticed 72 h after stresses application. In plants treated with UV-C the allantoin level was highest in young leaves, whereas in wounded plants it was lowest in the young leaves, accumulated mainly in the middle and wounded leaves. The results indicate that in UV-C treated WT plants, during the recovery period from stress, ureides are remobilized from the lower older leaves to support young leaf growth. In contrast, after wounding, the ureides are remobilized to the young leaves, yet more to the middle wounded leaves, to function as antioxidants and/or healing agents.