Biological outliers: essential elements to understand the causes and consequences of reductions in maximum photochemical efficiency of PSII in plants.

MAIN CONCLUSION: By studying Cistus albidus shrubs in their natural habitat, we show that biological outliers can help us to understand the causes and consequences of maximum photochemical efficiency decreases in plants, thus reinforcing the importance of integrating these often-neglected data into scientific practice. Outliers are individuals with exceptional traits that are often excluded of data analysis. However, this may result in very important mistakes not accurately capturing the true trajectory of the population, thereby limiting our understanding of a given biological process. Here, we studied the role of biological outliers in understanding the causes and consequences of maximum photochemical efficiency decreases in plants, using the semi-deciduous shrub C. albidus growing in a Mediterranean-type ecosystem. We assessed interindividual variability in winter, spring and summer maximum PSII photochemical efficiency in a population of C. albidus growing under Mediterranean conditions. A strong correlation was observed between maximum PSII photochemical efficiency (Fv/Fm ratio) and leaf water desiccation. While decreases in maximum PSII photochemical efficiency did not result in any damage at the organ level during winter, reductions in the Fv/Fm ratio were associated to leaf mortality during summer. However, all plants could recover after rainfalls, thus maximum PSII photochemical efficiency decreases did not result in an increased mortality at the organism level, despite extreme water deficit and temperatures exceeding 40ºC during the summer. We conclude that, once methodological outliers are excluded, not only biological outliers must not be excluded from data analysis, but focusing on them is crucial to understand the causes and consequences of maximum PSII photochemical efficiency decreases in plants.

Photo-oxidative stress leading to oxylipin accumulation may trigger drought-induced leaf abscission in olive trees.

Olive trees shed their leaves under severe drought as a defence mechanism. Foliar drought-induced abscission is a programmed process occurring in a differentiated cell layer at the base of the petiole. Considering the antioxidant properties of vitamin E and its interplay with lipid peroxidation-derived jasmonates in abiotic stress responses, we hypothesized about their possible role in abscission signaling by forming a jasmonates-increasing basipetal gradient along the leaf up to the abscission zone. We exposed young olives trees to water withdrawal for 21 days, after which five leaf sections, from the apex to the petiole, were sampled on both attached and detached leaves of irrigated and water-stressed trees. We found that prolonged drought stress resulted in a sharp reduction in the photosystem II efficiency, chlorophyll and vitamin E contents in leaves, leading to photo-oxidative stress, reflected by the increase in lipid peroxidation. In addition, the content of chloroplast-derived oxylipins and phytohormones, such as jasmonoyl-isoleucine and salicylic acid, increased. At the same time, α-tocopherol decreased in the petiole of water-stressed attached leaves, suggesting a conditioning for the abscission process to occur. Although no differences were observed in petioles from attached and detached leaves, the dropped ones showed higher oxidative stress in the leaf blade. It is concluded that redox signaling through oxylipins accumulation may trigger leaf abscission in drought-stressed olive trees. Mechanical stress is, however, additionally needed to execute leaf abscission once the abscission zone is properly conditioned.

Cheap, cost-effective, and quick stress biomarkers for drought stress detection and monitoring in plants.

The detection and monitoring of drought stress in plants growing in their natural habitat are essential for the study of plant stress physiology. However, with the advent of plant phenotyping and new -omics technologies, the application of simple, cheap, cost-effective, quick, and practical methods to assess drought stress in plants seems more challenging than ever, particularly in low-income countries. Here, currently available methods that do not require specialized equipment, but reliably detect and monitor drought stress in plants at low cost will be discussed. This will not only boost research on plant stress physiology in low-income countries but will also help several laboratories with very limited resources around the globe to perform high-quality research.

Acclimation to a combination of water deficit and nutrient deprivation through simultaneous increases in abscisic acid and bioactive jasmonates in the succulent plant Sempervivum tectorum L.

Activation of hormonal responses defines the drought acclimation ability of plants and may condition their survival. However, aside ABA, little is known about the possible contribution of other phytohormones, such as jasmonates and salicylates, in the response of CAM plants to water deficit. Here, we aimed to study the physiological mechanisms underlying the stress tolerance of house leek (Sempervivum tectorum L.), a CAM plant adapted to survive harsh environments, to a combination of water deficit and nutrient deprivation. We exposed plants to the combination of these two abiotic stresses by withholding nutrient solution for 10 weeks and monitored their physiological response every two weeks by measuring various stress makers together with the accumulation of stress-related phytohormones and photoprotective molecules, such as tocopherols (vitamin E). Results showed that ABA content increased by 4.2-fold after four weeks of water deficit to keep later constant up to 10 weeks of stress, variations that occurred concomitantly with reductions in the relative leaf water content, which decreased by up to 20% only. The bioactive jasmonate, jasmonoyl-isoleucine was the other stress-related phytohormone that simultaneously increased under stress together with ABA. While contents of salicylic acid and the jasmonoyl-isoleucine precursors, 12-oxo-phytodienoic acid and jasmonic acid decreased with water deficit, those of jasmonoyl-isoleucine increased 3.6-fold at four weeks of stress. The contents of ABA and jasmonoyl-isoleucine correlated positively between them and with the content of α-tocopherol per unit of chlorophyll, thus suggesting a photoprotective activation role. It is concluded that S. tectorum not only withstands a combination of water deficit and nutrient deprivation for 10 weeks without any symptom of damage but also activates effective defense strategies through the simultaneous accumulation of ABA and the bioactive jasmonate form, jasmonoyl-isoleucine.

Mechanism of DNA primer synthesis by human PrimPol.

PrimPol is the second primase discovered in eukaryotic cells, whose function is to restart the stalled replication forks during both mitochondrial and nuclear DNA replication. This chapter revises our current knowledge about the mechanism of synthesis of DNA primers by human PrimPol, and the importance of its distinctive Zn-finger domain (ZnFD). After PrimPol forms a binary complex with ssDNA, the formation of the pre-ternary complex strictly requires the presence of Mn2+ ions to stabilize the interaction of the incoming deoxynucleotide at the 3'-site. The capacity to bind both ssDNA template and 3'-deoxynucleotide was shown to reside in the AEP core of PrimPol, with ZnFD being dispensable at these two early steps of the primase reaction. Sugar selection favoring dNTPs versus NTPs at the 3' site is mediated by a specific tyrosine (Tyr100) acting as a steric gate. Besides, a specific glutamate residue (Glu116) conforming a singular A motif (DxE) promotes the use of Mn2+ to stabilize the pre-ternary complex. Mirroring the function of the PriL subunit of dimeric AEP primases, the ZnFD of PrimPol is crucial to stabilize the initiating 5'-nucleotide, specifically interacting with the gamma-phosphate. Such an interaction is crucial to optimize dimer formation and the subsequent translocation events leading to the processive synthesis of a mature DNA primer. Finally, the capacity of PrimPol to tolerate lesions is discussed in the context of its DNA primase function, and its potential as a TLS primase.