Root hydrotropism is the phenomenon of directional root growth toward moisture under water-deficient conditions. Although physiological and genetic studies have revealed the involvement of the root cap in the sensing of moisture gradients, and those of auxin and abscisic acid (ABA) in the signal transduction for asymmetric root elongation, the overall mechanism of root hydrotropism is still unclear. We found that the promoter activity of the Arabidopsis phospholipase Dzeta2 gene (PLDzeta2) was localized to epidermal cells in the distal root elongation zone and lateral root cap cells adjacent to them, and that exogenous ABA enhanced the activity and extended its area to the entire root cap. Although pldzeta2 mutant root caps did not exhibit a morphological phenotype in either the absence or presence of exogenous ABA, the inhibitory effect of ABA on gravitropism, which was significant in wild-type roots, was not observed in pldzeta2 mutant roots. In root hydrotropism experiments, pldzeta2 mutations significantly retarded or disturbed root hydrotropic responses. A drought condition similar to that used in a hydrotropism experiment enhanced the PLDzeta2 promoter activity in the root cap, as did exogenous ABA. These results suggest that PLDzeta2 responds to drought through ABA signaling in the root cap and accelerates root hydrotropism through the suppression of root gravitropism.

C4 photosynthesis is present in approximately 7,500 species classified into 19 families, including monocots and eudicots. In the majority of documented cases, a two-celled CO2-concentrating system that uses a metabolic cycle of four-carbon compounds is employed. C4 photosynthesis repeatedly evolved from C3 photosynthesis, possibly driven by the survival advantages it bestows in the hot, often dry, and nutrient-poor soils of the tropics and subtropics. The development of the C4 metabolic cycle greatly increased the ATP demand in chloroplasts during the evolution of malic enzyme-type C4 photosynthesis, and the additional ATP required for C4 metabolism may be produced by the cyclic electron transport around PSI. Recent studies have revealed the nature of cyclic electron transport and the elevation of its components during C4 evolution. In this review, we discuss the energy requirements of C3 and C4 photosynthesis, the current model of cyclic electron transport around PSI and how cyclic electron transport is promoted during C4 evolution using studies on the genus Flaveria, which contains a number of closely related C3, C4 and C3-C4 intermediate species.

C 4 plants are believed to have evolved from C 3 plants through various C 3 -C 4 intermediate stages in which a photorespiration-dependent CO 2 concentration system known as C 2 photosynthesis operates. Genes involved in the C 4 cycle were thought to be recruited from orthologs present in C 3 species and developed cell-specific expression during C 4 evolution. To understand the process of establishing C 4 photosynthesis, we performed whole-genome sequencing and investigated expression and mesophyll-or bundle-sheath-cell-specific localization of phosphoenolpyruvate carboxylase (PEPC), NADP-malic enzyme (NADP-ME), pyruvate, orthophosphate dikinase (PPDK) in C 3 , C 3 -C 4 intermediate, C 4 -like, and C 4 Flaveria species. While genome sizes vary greatly, the number of predicted protein-coding genes was similar among C 3 , C 3 -C 4 intermediate, C 4 -like, and C 4 Flaveria species. Cell-specific localization of the PEPC, NADP-ME, and PPDK transcripts was insignificant or weak in C 3 -C 4 intermediate species, whereas these transcripts were expressed celltype specific in C 4 -like species. These results showed that elevation of gene expression and cell-specific control of pre-existing C 4 cycle genes in C 3 species was involved in C 4 evolution. Gene expression was gradually enhanced during C 4 evolution, whereas cell-specific control was gained independently of quantitative transcriptional activation during evolution from C 3 -C 4 intermediate to C 4 photosynthesis in genus Flaveria.

C4 plants assimilate CO2 more efficiently than C3 plants because of their C4 cycle that concentrates CO2. However, the C4 cycle requires additional ATP molecules, which may be supplied by cyclic electron flow around photosystem I. One cyclic electron flow route, which depends on a chloroplast NADH dehydrogenase-like (NDH) complex, is suggested to be crucial for C4 plants despite the low activity in C3 plants. The other route depends on proton gradient regulation 5 (PGR5) and PGR5-like photosynthetic phenotype 1 (PGRL1) and is considered a major cyclic electron flow route to generate the proton gradient across the thylakoid membrane in C3 plants. However, its contribution to C4 photosynthesis is still unclear. In this study, we investigated the contribution of the two cyclic electron flow routes to the NADP-malic enzyme subtype of C4 photosynthesis in Flaveria bidentis. We observed that suppressing the NDH-dependent route drastically delayed growth and decreased the CO2 assimilation rate to approximately 30% of the wild-type rate. On the other hand, suppressing the PGR5/PGRL1-dependent route did not affect plant growth and resulted in a CO2 assimilation rate that was approximately 80% of the wild-type rate. Our data indicate that the NDH-dependent cyclic electron flow substantially contributes to the NADP-malic enzyme subtype of C4 photosynthesis and that the PGR5/PGRL1-dependent route cannot complement the NDH-dependent route in F. bidentis. These findings support the fact that during C4 evolution, photosynthetic electron flow may have been optimized to provide the energy required for C4 photosynthesis.