2024-09-26 PL-INT-2024-008813 GP 2045 Draft Comments - A. Longo

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factors[107,108].This work in wheat and rice is promising,demonstrating the potential for breeding new varieties better adapted to changing growth conditions,however it is unclear if such strategies will work in horticultural crops.In the case of tomato,there is considerable variation within the wild and elite varieties to suggest that such breeding strategies could be used to enhanced yield and quality[109,1101.See Sharwood et al [u1]for review(Figure 3). In transgenic rice,overproducing Rubisco,increases the biomass production and yield under high N fertilization in paddy fields suggesting that the development of new rice varieties with both high photosynthesis and large sink capacity is essential[99].Furthermore,genes encoding thermostable variants of Rubisco activase(thermos-Rca)have been identified in wild rice relatives.When over-expressed in domesticated rice, thermos-Rca was sufficient to enhance carbohydrate accumulation and improve yields after periodic exposure to elevated temperatures(+45°C) throughout the vegetative phase[112,113].Thermostable Rca have been identified in Thermophilic cyanobacteria,bacteria that thrive in high- temperature environments,making them a potential source of novel genes for engineering crops for growth at higher temperatures[114]. Improving the thermal tolerance of rubisco activase,either through breeding with wild populations or genetic engineering,could aid greenhouse grown crops better tolerate the elevated temperatures that often occur during the growing season(Figure 3). Genetic engineering of photosynthetic traits in crops Increasing the expression of enzymes and/or proteins involved in the regeneration of RuBP,CO2 transport or chloroplast electron transport have previously been shown to enhance photosynthetic efficiency and increases in yield[84,85,115-117].However,once again,it cannot be ignored that much of this work has focused on non-fruiting crops,such as Arabidopsis,tobacco,wheat and rice,(see Simkin et al.[84]for review),grown in controlled conditions,performed in pots,in soil or in the field with controlled irrigation,which is not typical of global agriculture.Furthermore, work carried out in tomato,over-expression of sedoheptulose-1,7-bisphosphatase,involved in RuBP regeneration,did not report on fruit yield [82].These data indicating that more work is required to understand how these manipulations would impact fruiting crops grown in tightly controlled environments. One potential target for genetic manipulation is the starch synthesis enzyme adenosine diphosphate glucose pyrophosphorylase(AGPase); increasing AGPase activity has potential to increase starch accumulation for growth.Increased accumulation of starch has been shown to have little negative feedback on photosynthesis[118]and increased AGPase activity in the chloroplast would increase the strength of the transient starch pool, which acts as a sink in the chloroplast.Reduced sink capacity does induce negative feedback on photosynthesis and can limit photosynthesis even in favourable conditions(e.g.elevated[CO2])[119],suggesting that increasing the sink may allow for greater CO2 assimilation in supplemented[CO2] growth environments. Although genetic manipulation has the potential to further increase yields in crops grown in enriched[CO2]environments,allowing them to take better advantage of supplemental CO2 increasing net photosynthetic rates and associated yields(Figure 3),it should also be noted that some reports have suggested that increases in yield in genetically enhanced photosynthetic crops are likely not uniquely down to increases in carbon assimilation but a combination of factors;for example improvements in carbon uptake allow for an increase in N assimilation[120].Furthermore,it has also been reported that such increase in yield from enhanced photosynthetic efficiency critically rely on the availability and uptake of water and nutrients(for review see[121,122]),therefore,genetic engineering as an approach alone may be limiting if other aspects of crop cultivation,such as irrigation,planting regimes,fertilisation(i.e NUE)and growth media(i.e soil,coir,rockwool),are not taken into account and co-optimised. Non-foliar photosynthesis Leaves are not the only location within the plant where photosynthesis occurs,with evidence of photosynthesis in petioles and stems[123,124],and fruit[124]that may provide significant and alternative sources of photo-assimilates essential for optimal yield.Assimilation of atmospheric CO2 is dependent on the number and behaviour of stomata,and the stems of many plants have stomata distributed along the epidermis[125,126]and an evaluation of the photosynthetic activity in stems of various plants accounted for up to 4%of the total photosynthetic activity[127].Furthermore, Hu et al.demonstrated the importance of stem photosynthesis to yield in cotton;maintaining the stem in darkness reduced seed weight by 16% [128]showing the stem provides photoassimilates for plant development and growth. As previously noted,many fruiting crops produce green fruit containing all the necessary proteins and enzymes to carry out photosynthesis[127, 129,130]that may provide significant and alternative sources of photoassimilates essential for optimal yield and quality[124].Tomato fruit photosynthesis contributes to net sugar accumulation and growth and previous work concluded that tomato fruit photosynthesis contributes between lo%and 15%of the total fixed carbon of the fruit[127,131][132],.It should be noted that,unlike many crops,cucumber fruit remain green through to maturity,have stomata(suggesting they perform gas exchange to drive photosynthesis),and have a similar surface area to an expanded leaf[13o].It has previously been reported that cucumber fruit had high photosynthetic and respiratory rates[133]and contribute approximately 9.4%of their own carbon requirements[13o].It should be noted that in fruit with stomata,such as cucumber,there are two potential major sources of CO2.Firstly,Rubisco assimilates atmospheric[CO2]through the stomatal pores,leading to the production of sugars via the CBC and secondly, CO2 released by mitochondrial respiration is re-fixed(recycling photosynthesis)[125,134].Whilst this confirms that photosynthesis occurs in fruits,the extent and importance is not clear.In e[CO2],it seems plausible that cucumber fruit photosynthesis may contribute directly to fruit size (and therefore yield by weight)and quality through their ability to directly access carbon in an enriched atmosphere via their stomata(for a review fruit photosynthesis,see[124,135].Therefore,increasing carbon capture by non-foliar tissues has the potential to significantly impact yield and combined with an increase N uptake(i.e.slow release fertilizers[136])to balance the increased carbon uptake,and optimised irrigation regimes has the potential to maximise such yield gains. 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