Physiological and molecular mechanisms regulated mesophyll conductance under severe drought in water-saving drought-resistant rice

Water-saving and drought-resistant rice (WDR) is a new type of rice varieties. It plays an important role in responding to drought with high yield and has been widely planted in central China at present. High photosynthetic production potential caused by high mesophyll conductance (gm) is the main factor promoted high yield formation in drought for WDR. But little is known about physiological and molecular mechanisms regulated gm in drought for WDR. Therefore, WDR cultivar HY73 and drought-sensitive cultivar HLY898 were used for comparative studies with three irrigation regimes before applying severe drought treatment at heading to create different differential individuals of photosynthetic potential and gm. The results showed that cultivar HY73 had lower up-regulation different expression genes (DEGs) than cultivar HLY898 in drought at transcriptional level. Conversely, DEGs of down-regulation was higher in cultivar HY73 than cultivar HLY898. In addition, 3071 DEGs were clustered in 3 modules named Midnightblue (734 DEGs), Blue (921 DEGs), and Turquoise (1416 DEGs) in severe drought merged three irrigation regimes and both cultivars, which the modules had significant correlational relationship with gm based on weighted gene co-expression network analysis (P<0.05). Only DEGs in midnightblue module were enriched in photosynthesis process and positively regulated gm (P<0.05). The main biological process were photosynthesis (GO:0015979), light harvesting in photosystem I (GO:0009768), reductive pentose-phosphate cycle (GO:0019253), protein-chromophore linkage (GO:0018298), photosynthetic electron transport in photosystem I (GO:0009773), and photosystem II repair (GO:0010206). These results indicate that gm and energy distribution in PSI and PSII systems could synergistic effect photosynthetic production potential in severe drought for rice plants. In the modules, the 18 most highly connected hub genes were screened using co-expression networks method. RT-PCR analysis indicated that CSP41B, PGLP1A, LHCA5, and GSTU6 genes had a similar variation trend with gm among treatments for both cultivar. LHCA5 and CSP41B genes were significantly up-regulated in HY73 compared with HLY898 in drought (P<0.05). And the both genes locates in thylakoid membrane in photosystems. Therefore, LHCA5 and CSP41B genes could be key genes to synergistically manage gm and energy distribution in photosystems. Our results provide some new physiological and molecular mechanisms regulated gm in severe drought for WDR.