Revealing the Genetic Components Responsible for the Unique Photosynthetic Stem Capability of the Wild Almond Prunus arabica (Olivier) Meikle

Abstract

Almond [Prunus dulcis (Mill.) D. A. Webb] is a major deciduous fruit tree crop worldwide. During dormancy, under warmer temperatures and inadequate chilling hours, the plant metabolic activity increases and may lead to carbohydrate deficiency. Prunus arabica (Olivier) Meikle is a bushy wild almond species known for its green, unbarked stem, which stays green even during the dormancy period. Our study revealed that P. arabica green stems assimilate significantly high rates of CO2 during the winter as compared to P. dulcis cv. Um el Fahem (U.E.F.) and may improve carbohydrate status throughout dormancy. To uncover the genetic inheritance and mechanism behind the P. arabica stem photosynthetic capability (SPC), a segregated F1 population was generated by crossing P. arabica to U.E.F. Both parent’s whole genome was sequenced, and SNP calling identified 4,887 informative SNPs for genotyping. A robust genetic map for U.E.F. and P. arabica was constructed (971 and 571 markers, respectively). QTL mapping and association study for the SPC phenotype revealed major QTL [log of odd (LOD) = 20.8] on chromosome 7 and another minor but significant QTL on chromosome 1 (LOD = 3.9). As expected, the P. arabica allele in the current loci significantly increased the SPC phenotype. Finally, a list of 64 candidate genes was generated. This work sets the stage for future research to investigate the mechanism regulating the SPC trait, how it affects the tree’s physiology, and its importance for breeding new cultivars better adapted to high winter temperatures.

Physiological characterization of the wild almond Prunus arabica stem photosynthetic capability

“…This similarity raised the assumption that P. arabica green stems have a photosynthetic capability (Sorkheh et al, 2009). Indeed, in a recent detailed genetic and physiological study, we have provided for the first time experimental evidence supporting P. arabica stem photosynthesis capability [SPC, (Brukental et al, 2021)]. Our study showed that P. arabica green stems are photosynthetically active all year round, including during the winter months after the tree sheds its leaves and enters its dormancy period.…”

“…With no photosynthetic organs (leaves) during dormancy, to regain carbon loss, these conditions challenge the almond tree's carbon reserves, and, eventually, its productivity (Zwieniecki and Secchi, 2015;Sperling et al, 2019). Photosynthetic stems, which have never been described before in deciduous fruit crop trees (until our work by Brukental et al, 2021), can thus contribute to the net carbon gain, and increase the tree resilience to extreme environmental conditions.…”

“…(a) Stem Recycling Photosynthesis (SRP), also termed corticular photosynthesis, which occurs in a wide variety of plants (Schaedle, 1975) and involves the reassimilation of CO 2 from respiration (Nilsen, 1995;Ávila et al, 2014), in suberized, stomata-less stem tissue (Saveyn et al, 2010;Ávila et al, 2014;Ávila-Lovera et al, 2019). (b) Stem Photosynthetic Capability [SPC, Brukental et al (2021)], also known as Stem Net Photosynthesis (SNP) (Ávila et al, 2014)], which includes gas exchange and assimilation of atmospheric CO 2 via stomatal pores within stem epidermis. On a whole tissue basis, both SRP and SPC contribute to the carbon economy of the plant.…”

“…Indeed, SRP can recycle 7 to 123% of the respired CO 2 , but its net photosynthetic rate is negligible compared to leaves (Ávila-Lovera et al, 2019). On the other hand, SPC can reach up to 60% of leaves' photosynthetic rates (Ávila et al, 2014), and even more (Brukental et al, 2021).…”

“…On the other hand, the wild almond, P. arabica, uses both its leaves and its stems for efficient carbon gain. Its stems assimilate remarkable levels of CO 2 via Stem Photosynthetic Capability (SPC) (Brukental et al, 2021), which involves the uptake of atmospheric CO 2 via stomata pores within stem epidermis.…”