Compartmentation of photosynthesis gene expression in C4 maize depends on time of day.

Compared with the ancestral C3 state, C4 photosynthesis occurs at higher rates with improved water and nitrogen use efficiencies. In both C3 and C4 plants, rates of photosynthesis increase with light intensity and are maximal around midday. We determined that in the absence of light or temperature fluctuations, photosynthesis in maize (Zea mays) peaks in the middle of the subjective photoperiod. To investigate the molecular processes associated with these temporal changes, we performed RNA sequencing of maize mesophyll and bundle sheath strands over a 24-h time course. Preferential expression of C4 cycle genes in these cell types was strongest between 6 and 10 h after dawn when rates of photosynthesis were highest. For the bundle sheath, DNA motif enrichment and gene coexpression analyses suggested members of the DNA binding with one finger (DOF) and MADS (MINICHROMOSOME MAINTENANCE FACTOR 1/AGAMOUS/DEFICIENS/Serum Response Factor)-domain transcription factor families mediate diurnal fluctuations in C4 gene expression, while trans-activation assays in planta confirmed their ability to activate promoter fragments from bundle sheath expressed genes. The work thus identifies transcriptional regulators and peaks in cell-specific C4 gene expression coincident with maximum rates of photosynthesis in the maize leaf at midday.

C4 gene induction during de-etiolation evolved through changes in cis to allow integration with ancestral C3 gene regulatory networks.

C4 photosynthesis has evolved by repurposing enzymes found in C3 plants. Compared with the ancestral C3 state, accumulation of C4 cycle proteins is enhanced. We used de-etiolation of C4 Gynandropsis gynandra and C3 Arabidopsis thaliana to understand this process. C4 gene expression and chloroplast biogenesis in G. gynandra were tightly coordinated. Although C3 and C4 photosynthesis genes showed similar induction patterns, in G. gynandra, C4 genes were more strongly induced than orthologs from A. thaliana. In vivo binding of TGA and homeodomain as well as light-responsive elements such as G- and I-box motifs were associated with the rapid increase in transcripts of C4 genes. Deletion analysis confirmed that regions containing G- and I-boxes were necessary for high expression. The data support a model in which accumulation of transcripts derived from C4 photosynthesis genes in C4 leaves is enhanced because modifications in cis allowed integration into ancestral transcriptional networks.

A bipartite transcription factor module controlling expression in the bundle sheath of Arabidopsis thaliana.

C4 photosynthesis evolved repeatedly from the ancestral C3 state, improving photosynthetic efficiency by ~50%. In most C4 lineages, photosynthesis is compartmented between mesophyll and bundle sheath cells, but how gene expression is restricted to these cell types is poorly understood. Using the C3 model Arabidopsis thaliana, we identified cis-elements and transcription factors driving expression in bundle sheath strands. Upstream of the bundle sheath preferentially expressed MYB76 gene, we identified a region necessary and sufficient for expression containing two cis-elements associated with the MYC and MYB families of transcription factors. MYB76 expression is reduced in mutant alleles for these transcription factors. Moreover, downregulated genes shared by both mutants are preferentially expressed in the bundle sheath. Our findings are broadly relevant for understanding the spatial patterning of gene expression, provide specific insights into mechanisms associated with the evolution of C4 photosynthesis and identify a short tuneable sequence for manipulating gene expression in the bundle sheath.

Chloroplast Signaling Gates Thermotolerance in Arabidopsis.

Temperature is a key environmental variable influencing plant growth and survival. Protection against high temperature stress in eukaryotes is coordinated by heat shock factors (HSFs), transcription factors that activate the expression of protective chaperones such as HEAT SHOCK PROTEIN 70 (HSP70); however, the pathway by which temperature is sensed and integrated with other environmental signals into adaptive responses is not well understood. Plants are exposed to considerable diurnal variation in temperature, and we have found that there is diurnal variation in thermotolerance in Arabidopsis thaliana, with maximal thermotolerance coinciding with higher HSP70 expression during the day. In a forward genetic screen, we identified a key role for the chloroplast in controlling this response, suggesting that light-induced chloroplast signaling plays a key role. Consistent with this, we are able to globally activate binding of HSFA1a to its targets by altering redox status in planta independently of a heat shock.