Physiological Mechanism Of Leaf Senescence Formation And Its Metabolic Regulation In Premature Senescence Rice(psf)Mutant Leaves

The premature senescence of functional leaves may cause a serious shortage of carbohydrates supply from leave to filling grains for rice plants,therby resulting in the significantly lowered rice yield.Therefore,studies on delaying the start of functional rice leaves senescence will be beneficial to improve rice production and the nutritional quality of grains.In this respect,we employed a rice mutant with premature senescence at filling stage(psf)derived by radiation mutagenesis and its corresponding wild type as the experimental materials,to research the characteristic of seed setting at different sowing seasons.At the same time,the regulation of ABA to photodamage photosystem II(PSII)D1 protein,the relationship between leaf senescence and the change of antioxidant defense enzyme system and the endogenous reactive oxygen species,and the regulation of IAA signal to transcription factor NACs were analyzed and discussed from the perspective of photosynthetic physiology,the changes of antioxidant protective enzyme system and hormone regulation during leaf senescence.The main results are as follows.1.The premature senescence mutation of rice leaves not only caused significant decrease in filled grains per panicle,seed setting rate and 1000-grain weight,but also resulted in a significant decreased of brown rice rate,milled rice rate and head rice rate,especially for the head rice rate.Chalky grain rate and chalkiness degree of premature senescence mutant were increased in different degrees.Compare to its wild type,the amylose content in the premature senescence mutant was significant lower,but with a notable increase in the gelatinization enthalpy(?H).In addition,the amylopectin blue value and the maximum absorption wavelength of the psf mutant were significantly higher than that of it wildtype.The premature senescence of psf mutant could lead to significant increase in total protein,phytic acid(PA),and Zn content than its wild type,but the the bioavailability of Zn was notably decreased at the same time.2.D1 protein in the PSII reaction center is the major target of photodamage,and it exhibits the highest turnover rate among all the thylakoid proteins.The appearance and the degree of leaves senescence were induced by illumination under field cultivation conditions.Compared to the wild type,the psf displayed significantly higher levels of ABA accumulation in senescent leaves.However,the premature senescence lesion of psf leaves could be alleviated by shaded treatment,concomitantly with the strikingly suppressed ABA level in the shaded areas of flag leaves.The change in ABA concentration contributed to the regulation of shade-delayed leaf senescence.The participation of ABA in the timing of senescence initiationand in the subsequent rate of leaf senescence was closely associated with PSII photodamage and D1 protein turnover during leaf senescence,in which the transcriptional expression of several key genes(psbA,psbB,psbC and OsFtsH2)involved in D1 protein biosynthesis and PSII repair cycle was seriously suppressed by the significantly increased ABA level.This response resulted in the low rate of D1 protein synthesis and impaired repair recovery in the presence of ABA.The psf showed evidently decreased D1 protein amount in the senescent leaves.Both the inhibition of de novo synthesized D1 protein and the slow rate of proteolytic removal for the photodamaged D1 protein was among the most crucial steps for the linkage between light-dependent leaf senescence and the varying ABA concentration in psf mutant leaves.OsFtsH2 transcriptional expression possibly played an important role in the regulation of D1 protein turnover and PSII repair cycle in relation to ABA mediated leaf senescence.3.Antioxidant protective enzymes can effectively scavenge ROS,and thus keep blade from oxidative damage in senescence leaves.We investigated the genotype-dependent alteration in temporal patterns of the O2·-production rate,H2O2 content and antioxidant protective enzymes during leaf senescence in mutant and its wild type.The O2·-generation rate and H20 accumulation of occurred earlier than the initial decline in Pn rate and Chla/b ratio in senescent mutant leaves,the psf mutant differed obviously from its wild type in leaf O2·-generation rate and H2O2 contentaccumulation,and the lower activities of SOD,CAT and APX in the psf leaves was strongly responsible for the increased ROS level and the accelerated leaf senescence,whereas the increase in POD activity was positively correlated with the senescence-related enhancement in O2·-generation in rice leaves.Among various SOD isoforms,Mn-SOD responded sensitively to the increasing O2·-generation rate,whereas Cu/Zn-SOD remained stable with the progression of leaf senescence.These findings suggested that the senescence-related decline in total SOD activity was mostly attributable to the downregulation of both the translation and transcription of Mn-SOD isoform.This occurrence finally resulted in the collapse of SOD defense system and accelerated leaf senescence for psf mutant.4.Sugars play dual roles in response to the external environment and internal adjustment factor signal during leaf senescence.Compared to the wild type,the soluble sugar,sucrose and hexose contents in the psf mutant decreased significantly during leaf senescence.Leaf senescence could be accelerated by sugar starvation treatment for detached leaves,at the same time,the endogenous ABA content increased significantly.NCED1 and ABA8ox3 were the key genes that regulated the synthesis and degradation of ABA and showed highly expressions during leaf senescence,and both of them performed significantly down-regulated expression in either wild type or in the mutant during leaf senescence.The expressions of NCEDI and ABA8ox3 were significantly down-regulated along with the intensification of leaf senescence induced by sugar starvation treatment for detached leaves.Exogenous sucrose treatment could significantly delay detached leaf senescence and suppress the rise of endogenous ABA content at the same time.Therefore,sugar starvation-induced leaf senescence was positively correlated the endogenous ABA content.The expressions of NCEDI and ABA8ox3 were significantly up-regulated with the increasing concentrations of sucrose treatments.Thus,the rise of ABA induced by sugar starvation is mediated by the suppression of ABA catabolism rather than an enhancement of ABA biosynthesis during leaf senescence.5.Auxin(IAA)play an important role in the formation and differentiation of plant tissue and organand the signal exchange between plant and environment,but the regulation of IAA in leaf senescence is stillunclear.The IAA contents in the psf mutant were significantly lower than its wild type during the flag leaves senescence,the IAA-ASP contents just exhibited an opposite trend.The result of transcriptional expression of several key genes which included IAA signal transduction,IAA biosynthesis and IAA cojugation metabolic pathway in premature senescence rice leaves showed that the key gene GH3s,which regulated the conjugation of IAA,was down-regulated in early leaf senescence,but it was up-regulated with the progress of leaf senescence.Exogenous NAA treatment demonstrated that IAA signal could regulate the transcriptional expressions of NACs by up and down to induce or delay leaf senescence.