Chlorophyll Cycle Regulates The Light Adaption And Leaf Senescence Of Plants

The chlorophyll a(Chl a)and Chl b pigments are the most important light-harvesting pigments in higher plants.It is known that Chl a is found in both the core complex and the lightharvesting complex(LHC)of the photosystem.Typically,Chl b is only present in the LHC of photosystems,and its synthesis is strictly regulated in response to light conditions in order to control the size of the antenna of the photosystem.It is essential in photochemistry,while Chl b is apparently dispensable for their photosynthesis.Chl b is synthesized from Chl a and is degraded after converting it back to Chl a.The interconversion system between Chl a and Chl b refers to the Chl cycle.Three enzymes are involved in the Chl cycle.They are chlorophyllide a(Chlide a)oxygenase(CAO),Chl b reductase(CBR),and 7-hydroxymethyl-chlorophyll reductase(HCAR),which determine the Chl b level.This study found that the Chl cycle regulation is important for plants’ light adaptation and leaf senescence.1.Tomato Overproduction Chl b Acquisitions a Faster and Slower Growth Rate under LowLight(LL)and Moderate High-Light(HL)Conditions,RespectivelyCAO is the sole enzyme responsible for the biosynthesis of Chl b in plants.It has been shown in previous studies that overexpression of CAO without the regulating domain(A domain)overproduced Chl b in Arabidopsis.However,the growth of the Chl b-overproduced plants has not been well studied regarding how they respond to changes in a light environment.In order to uncover the growth characters,the A domain deleted Arabidopsis CAO fused with the FLAG tag(BCF)was overexpressed in tomatoes to determine the growth characteristics of Chl boverproduced light love plants.This investigated the differences in phenotypical,morphological,and physiological characteristics of WT and BCF overexpressing transgenic tomatoes plant under LL 50-70 μmol photons m-2 s-1 and HL 800-1000 μmol photons m-2 s-1 growth condition.The main findings of this study are as follows.(1)Under the LL growth condition,the growth rate of BCF overexpressing transgene plants was significantly faster than that of WT plants,while under HL conditions,BCF plants grew slower than WT plants.Additionally,the plant height was significantly increased and decreased in BCF overexpressing plants under LL and HL growth conditions,respectively.BCF plants accumulate more biomass under the LL condition and less biomass under the HL condition.(2)Transgenic tomato plants overexpressing BCF accumulated significantly higher levels of Chl b,leading to a significantly lower Chl a/b ratio than WT plants under both LL and HL-grown plants.(3)Furthermore,BCF plants also possessed a lower level of maximum photochemical efficiency of photosystem Ⅱ(Fv/Fm)as well as a lower level of anthocyanin content than WT plants under both LL and HL-grown tomatoes.Additionally,the net photosynthesis rate of BCF plants was significantly higher and lower under LL and HL growth conditions,respectively.(4)More reactive oxygen species(ROS)and heavier photodamages were observed in transgenic BCF overexpressing plants than in WT plants under HL conditions.These result reveals that tomato plants overproducing Chl b could better adapt to LL conditions.On the other hand,they were worse capable of adapting to excess light conditions by accumulating more ROS but fewer anthocyanins.Overproducing Chl b plants can improve the growth rate of light love plants that are grown under LL conditions,indicating the prospect of employing Chl b-overproduced light love crops and ornamental plants for protected or indoor cultivation.2.Overexpressing HCAR Alleviates Non-Programmed Cell Death(non-PCD)during DarkInduced Senescence in Intact Arabidopsis PlantsLeaf senescence is the final stage of the leaf development and is a well-regulated and complex investigation process.To simplify the issue,dark-induced leaf senescence has frequently been used to mimic the natural process of leaf senescence.It is because some of the symptoms associated with natural leaf senescence,such as the degradation of Chl and protein,also occur during dark-induced leaf senescence.In this study,we compared leaf senescence phenotypes that occurred when detached leaves or intact plants were incubated in darkness to induce senescence.We found that the symptoms of non-PCD,which are characterized by the remaining green islands on the senescent leaves,were more prevalent on the senescent leaves of whole plants than on detached leaves.When whole plants were incubated in darkness for a period of time,the contents of pheophorbide a(Pheide a)were found to be much higher in senescent leaves based on the analysis of leaf Chl and its metabolic intermediates.Further,it has been shown that if the soil contains more water for plant growth,more serious non-PCD will occur,and more Pheide a will accumulate in the senescent leaves during dark incubation.As a result of overexpressing HCAR under similar circumstances,the non-PCD phenotype was alleviated,and the accumulation of Pheide a was reduced.We also found that when soil contained a high amount of water,the HCAR overexpression plants grew faster and significantly increased stem length,plant fresh weight,and dry fresh weight compared to WT.Taking together,we concluded that when whole plants were incubated in darkness for senescence,a high soil water content induced non-PCD by decreasing HCAR activity;thus,the investigation of the fundamental aspects of biochemistry and the regulation of leaf senescence is affected.This study implies that it is possible to improve plants’ light adaption and waterlogging tolerance by regulating the Chl cycle.This study will also contribute to creating plants that can better adapt to different growth environments.