Physiological Mechanism Of Different Maize Varieties In Response To Drought Stress

Drought stress is a crucial factor that limits maize productivity in the Northwest of China.Increasingly accumulating knowledge has shown that drought stress occurring in the seedling stage of maize will impact greatly on growth and yield,and different maize varieties adopt different strategies when responding to drought stress.The rapid advances of high-throughput proteomics technology enabled us to explore differentially expressed proteins（DEPs）under drought stress at the whole proteome level.Therefore,studying how different maize varieties respond to drought stress by proteomics technology is of great significance for deciphering the mechanism of drought stress.In this work,we took two hybrids maize cultivars,drought-tolerant（Shaandan 609）and drought-sensitive（Shaandan 902）,as materials and systematically compared their differences in physiological level when responding moderate drought stress by measuring leaf chlorophyll content,photosynthetic energy efficiency and antioxidant enzyme activities.Subsequently,the protein expression levels between the two maize varieties were compared with iTRAQ tandem mass spectrometry technology and further experimentally verified by q RT-PCR.These findings will help us to better understand the drought tolerance mechanisms and would be useful for future crop improvement in maize.The main findings are as follows:（1）To be specific,the leaf edge curl of Shaandan 609 was lighter than that of Shaandan902;the growth of Shaandan 609 was stronger than that of Shaandan 902;the decrease in chlorophyll content was smaller than that of Shaandan 902.Additionally,we found that the net photosynthetic rate and stomatal conductance of Shaandan 609 were higher than those of in Shaandan 902.The concentration of intercellular CO₂ increased in the two varieties,indicating that the decline in the net photosynthetic rate of the two varieties may be caused by non-stomatal factors.（2）By comparing their photosynthetic physiological index,we found both Y（II）and Y（I）in the leaves of Shaandan 902 were significantly lower than those of in Shaandan 609,indicating that drought stress reduced the ability of linear electron transfer of the two varieties.In addition,compared with Shaandan 609,Y（NO）and Y（NA）in Shaandan 902 were significantly increased.We speculate that the photosystem I and photosystem II occurred light suppression of Shaandan 902.Meanwhile,the Y（CEF）in Shaandan 609 is significantly higher than that in Shaandan 902.These results showed that Shaandan 609 can coordinate the changes in electron transfer between PSII and PSI better than Shaandan 902,maintain high PSII and PSI activity,and further stimulate the heat dissipation and the photoprotective defense mechanism of the ring electron transfer pathway to dissipate excess light.These photoprotective defense mechanism cooperatively weakened the photoinhibition of the photosynthetic mechanism and ensured the normal transmission of electrons.This is an important photosynthetic regulatory mechanism for the strong drought resistance of Shaandan609.（3）We also found that the antioxidant enzyme system in Shaandan 609 was activated in moderate drought stress,the activities of SOD,POD and GR were significantly increased,and the free radical scavenging ability was enhanced.These results indicated that the ability to activate the antioxidant system to resist drought stress of Shaandan 609 is higher than that of Shaandan 902.（4）The DEPs of two maize varieties under moderate drought stress were identified by iTRAQ proteomics technology and experimentally verified by q RT-PCR.Based on iTRAQ results,198（108 up-regulated;90 down-regulated）and 102 DEPs（66 up-regulated;36 down-regulated）were identified for Shaandan 609 and Shaandan 902,respectively.Gene ontology enrichment analysis showed that up-regulated DEPs in Shaandan 609 were mainly involved in the photosynthesis-photoreaction stage,oxidative phosphorylation,and protein processing.Moreover,the number of up-regulated DEPs in Shaandan 609 is higher than that in Shaandan902.These results indicated that Shaandan 609 can improve its drought resistance by regulating the transmission of photosynthetic electron chain,enhancing the ability to remove active oxygen,and coordinating protein metabolism.To verify the results from iTRAQ proteomics technology,we used q RT-PCR experiments to quantify the expression level of 13random selected genes,their expression level was consistent with iTRAQ results.Under moderate drought stress,compared with Shaandan 902,Shaandan 609 can activate multiple antioxidant enzyme systems by coordinating the transmission of photosynthetic electron chain and energy distribution in terms of physiological index.Therefore,Shaandan609 is more drought resistant.In terms of protein expression,the DEPs in Shaandan 609 were involved in all photosynthesis processes.It can be speculated that the photosynthesis process may play an important role in resisting moderate drought stress.In summary,optimizing the photosynthetic mechanism by repairing damaged and protecting the photosynthetic system,and establishing multiple antioxidant systems to maintain the balance of ROS in cells are important physiological mechanisms for Shaandan 609 to adapt to moderate drought stress.