Programmed cell death, antioxidant response and oxidative stress in wheat f lag leaves induced by chemical hybridization agent SQ-1

Male sterility induced by a chemical hybridization agent （CHA） is an important tool for utilizing crop heterosis. Leaves, especially the flag leaves, as CHA initial recipients play a decisive role in inducing male sterility. To investigate effects of different treatment times of CHA-SQ-1 used, morphological, biochemical and physiological responses of wheat flag leaves were detected in thistudy. CHA induced programmed cell death （PCD） as shown in terminal deoxynucleotidyl transferase-mediated dUTP nick end-labelling （TUNEL） and DNA laddering analysis. In the early phase, CHA-SQ-1 trig- gered organelle changes arid PCD in wheat leaves accompanied by excess production of reactive oxygen species （O2- and H202） and down-regulation of the activities of superoxide dismutase （SOD）, catalase （CAT） and guaiacol peroxidase （POD）. Meanwhile, leaf cell DNAs showed ladder-like patterns on agarose gel, indicating that CHA-SQ-1 led to the activation of the responsible endonuclease. The oxidative stress assays showed that lipid peroxidation was strongly activated and photosynthesis was obviously inhibited in SQ-l-induced leaves. However, CHA contents in wheat leaves gradually reduced along with the time CHA-SQ-1 applied. Young flags returned to an oxidative/antioxidative balance and ultimately developed into mature green leaves. These results provide explanation of the relations between PCD and anther abortion and practical application of CHA for hybrid breeding.

Screening and Analysis of Proteins Interacting with TaPDK from Physiological Male Sterility Induced by CHA in Wheat

To further research the regulatory network of pyruvate dehydrogenase kinase （designated as TaPDK） in physiological male-sterility （PHYMS） of wheat induced by chemical hybridizing agent （CHA） SQ-1, an anther cDNA library was constructed, and the proteins interacting with TaPDK were screened via yeast two-hybrid technique. Subsequently, a few candidate proteins in nucleotide expression levels were detected by real-time quantitative PCR. Yeast-two hybrid screening was performed by mating yeast strain Y2HGold containing BD-TaPDK bait plasmid with yeast strain Y187 including anther cDNA library plasmid. Diploid yeast cells were plated on synthetic dropout nutrient medium （SD/-Ade/-His/-Leu/-Trp） （QDO）, and further were incubated on QDO medium containing AbA and X-α-Gal. The interactions between TaPDK and the proteins obtained from positive colonies were further confirmed by co-transformation validation. After plasmids DNA were extracted from blue colonies and sequenced, the sequences results were analyzed by bioinformatic methods. Finally, 24 colonies were obtained, including eight genes, namely non-specific lipid-transfer protein precursor （TanLTP）, polyubiquitin （TaPUbi）, glyceraldehyde-3-phosphate dehydrogenase, proliferating cell nuclear antigen （TaPCNA）, CBS domain containing protein （TaCBS）, actin, guanine nucleotide-binding protein beta subunit, chalcone synthase, and three new genes with unknown function. The results of quantitative RT-PCR showed that the expression levels of TanLTP, TaPUbi, and TaPCNA were obviously up-regulated in PHYMS anther, and TaCBS expression was only increased at the tricellular stage in PHYMS anther compared with in fertile lines. Whereas, the expression of TaPDK was obviously down-regulated in PHYMS lines. Collectively, these datas indicated that the majority of candidate proteins might be related to pollen abortion in PHYMS lines, which further suggested that TaPDK plays multiple roles in pollen development, besides participating in regulating pyruvate dehydrogenase complex activity.

Tribenuron-Methyl Induces Male Sterility through Anther-Specific Inhibition of Acetolactate Synthase Leading to Autophagic Cell Death

Tribenuron-methyl （TM） is a powerful sulfonylurea herbicide that inhibits branched-chain amino acid （BCAA） biosynthesis by targeting the catalytic subunit （CSR1） of acetolactate synthase （ALS）. Selective in- duction of male sterility by foliar spraying of TM at low doses has been widely used for hybrid seed produc- tion in rapeseed （Brassica napus）; however, the underlying mechanism remains unknown. Here, we report greater TM accumulation and subsequent stronger ALS inhibition and BCAA starvation in anthers than in leaves and stems after TM application. Constitutive or anther-specific expression of csrl-lD （a CSR1 mutant） eliminated anther-selective ALS inhibition and reversed the TM-induced male sterile phenotype in both rapeseed and Arabidopsis. The results of TM daub-stem experiments, combined with the observations of little TM accumulation in anthers and reversion of TM-induced male sterility by targeted expression of the TM metabolism gene Bel in either the mesophyll or phloem, suggested that foliar-sprayed TM was polar-transported to anthers mainly through the mesophyll and phloem. Microscopy and immunoblotting revealed that autophagy, a bulk degradation process induced during cell death, was elevated in TM-induced male sterile anthers and by anther-specific knockdown of ALS. Moreover, TM-induced pollen abortion was significantly inhibited by the autophagy inhibitor 3-MA. These data suggested that TM was polar-transported to anthers, resulting in BCAA starvation via anther-specific ALS inhibition and, ulti- mately, autophagic cell death in anthers.