C4 and crassulacean acid metabolism within a single leaf: deciphering key components behind a rare photosynthetic adaptation.

Although biochemically related, C4 and crassulacean acid metabolism (CAM) systems are expected to be incompatible. However, Portulaca species, including P. oleracea, operate C4 and CAM within a single leaf, and the mechanisms behind this unique photosynthetic arrangement remain largely unknown. Here, we employed RNA-seq to identify candidate genes involved exclusively or shared by C4 or CAM, and provided an in-depth characterization of their transcript abundance patterns during the drought-induced photosynthetic transitions in P. oleracea. Data revealed fewer candidate CAM-specific genes than those recruited to function in C4 . The putative CAM-specific genes were predominantly involved in night-time primary carboxylation reactions and malate movement across the tonoplast. Analysis of gene transcript-abundance regulation and photosynthetic physiology indicated that C4 and CAM coexist within a single P. oleracea leaf under mild drought conditions. Developmental and environmental cues were shown to regulate CAM expression in stems, whereas the shift from C4 to C4 -CAM hybrid photosynthesis in leaves was strictly under environmental control. Moreover, efficient starch turnover was identified as part of the metabolic adjustments required for CAM operation in both organs. These findings provide insights into C4 /CAM connectivity and compatibility, contributing to a deeper understanding of alternative ways to engineer CAM into C4 crop species.

The contribution of weak CAM to the photosynthetic metabolic activities of a bromeliad species under water deficit.

The Crassulacean acid metabolism (CAM) can be a transitory strategy for saving water during unfavourable conditions, like a dry season. In some cases, CAM can also contribute to the maintenance of photosynthetic integrity, even if carbon gain and growth are impaired. CAM occurs in different intensities, being stronger or weaker depending on the degree of nocturnal malic acid accumulation. For example, Guzmania monostachia is an epiphytic tank bromeliad that shows an increase in its nocturnal organic acid accumulation and a variable CAM behaviour when exposed to water deficit. In this context, this study aimed at investigating whether the weak CAM displayed by this species may mitigate the harmful effects of water limitation on its photosynthetic activity. To this, bromeliads were submitted to well-watered and water deficit conditions. Guzmania monostachia plants under water deficiency conditions showed a reduction on atmospheric carbon assimilation without exhibiting changes in PSII integrity and carbohydrate production while showed an increase in nocturnal malic acid accumulation. Additionally, spots with high PSII efficiency in the leaf portion with a greater nocturnal malic acid accumulation were observed in plants exposed to water shortage conditions. These high-efficiency spots might be associated with a greater malate decarboxylation capacity. Also, the malic acid contributed to approximately 50% of the total carbon assimilated under water deficit. These results suggest that weak CAM may participate in photo-protection and it appears to meaningfully contribute to the overall carbon balance, being an important metabolic strategy to maintain plant fitness during water deficit periods.

Manipulation of a Senescence-Associated Gene Improves Fleshy Fruit Yield.

Senescence is the process that marks the end of a leaf's lifespan. As it progresses, the massive macromolecular catabolism dismantles the chloroplasts and, consequently, decreases the photosynthetic capacity of these organs. Thus, senescence manipulation is a strategy to improve plant yield by extending the leaf's photosynthetically active window of time. However, it remains to be addressed if this approach can improve fleshy fruit production and nutritional quality. One way to delay senescence initiation is by regulating key transcription factors (TFs) involved in triggering this process, such as the NAC TF ORESARA1 (ORE1). Here, three senescence-related NAC TFs from tomato (Solanum lycopersicum) were identified, namely SlORE1S02, SlORE1S03, and SlORE1S06. All three genes were shown to be responsive to senescence-inducing stimuli and posttranscriptionally regulated by the microRNA miR164 Moreover, the encoded proteins interacted physically with the chloroplast maintenance-related TF SlGLKs. This characterization led to the selection of a putative tomato ORE1 as target gene for RNA interference knockdown. Transgenic lines showed delayed senescence and enhanced carbon assimilation that, ultimately, increased the number of fruits and their total soluble solid content. Additionally, the fruit nutraceutical composition was enhanced. In conclusion, these data provide robust evidence that the manipulation of leaf senescence is an effective strategy for yield improvement in fleshy fruit-bearing species.