The NSP-interacting kinase, NIK, belongs to the five leucine-rich repeats-containing receptor-like serine/threonine kinase subfamily that includes members involved in plant development and defence. NIK was first identified by its capacity to interact with the geminivirus nuclear shuttle protein (NSP) and has been strongly associated with plant defence against geminivirus. Recent studies corroborate its function in transducing a defence signal against virus infection and describe components of the NIK-mediated antiviral signalling pathway. This mini-review describes the role of NIK as a transducer of a novel layer of plant innate defence, presents new data on NIK function, and discusses its possible involvement in plant development.

Optimizing in vitro shoot regeneration conditions in wheat is one of the important steps in successful micropropagation and gene transformation. Various factors such as genotypes, explants, and phytohormones affect in vitro regeneration of wheat, hindering the ability to tailor genotype-independent protocols. Novel computational approaches such as artificial neural networks (ANNs) can facilitate modeling and predicting outcomes of tissue culture experiments and thereby reduce large experimental treatments and combinations. In this study, generalized regression neural network (GRNN) were used to model and forecast in vitro shoot regeneration outcomes of wheat on the basis of 10 factors including genotypes, explants, and different concentrations of 6-benzylaminopurine (BAP), kinetin (Kin), 2,4-dichlorophenoxyacetic acid (2,4-D), indole-3-acetic acid (IAA), indole-3-butyric acid (IBA), 1-naphthaleneacetic acid (NAA), zeatin, and CuSO₄. In addition, GRNN was linked to a genetic algorithm (GA) to identify an optimized solution for maximum shoot regeneration. Results indicated that GRNN could accurately predict the shoot regeneration frequency in the validation set with a coefficient determination of 0.78. Sensitivity analysis demonstrated that shoot regeneration frequency was more sensitive to variables in the order of 2,4-D > explant > genotype < zeatin < NAA. Results of this study suggest that GRNN-GA can be used as a tool, besides experimental approaches, to develop and optimize in vitro genotype-independent regeneration protocols.

Summary: Begomovirus‐associated epidemics currently threaten tomato production worldwide due to the emergence of highly pathogenic virus species and the proliferation of a whitefly B biotype vector that is adapted to tomato. To generate an efficient defence against begomovirus, we modulated the activity of the immune defence receptor nuclear shuttle protein (NSP)‐interacting kinase (NIK) in tomato plants; NIK is a virulence target of the begomovirus NSP during infection. Mutation of T474 within the kinase activation loop promoted the constitutive activation of NIK‐mediated defences, resulting in the down‐regulation of translation‐related genes and the suppression of global translation. Consistent with these findings, transgenic lines harbouring an activating mutation (T474D) were tolerant to the tomato‐infecting begomoviruses ToYSV and ToSRV. This phenotype was associated with reduced loading of coat protein viral mRNA in actively translating polysomes, lower infection efficiency and reduced accumulation of viral DNA in systemic leaves. Our results also add some relevant insights into the mechanism underlying the NIK‐mediated defence. We observed that the mock‐inoculated T474D‐overexpressing lines showed a constitutively infected wild‐type transcriptome, indicating that the activation of the NIK‐mediated signalling pathway triggers a typical response to begomovirus infection. In addition, the gain‐of‐function mutant T474D could sustain an activated NIK‐mediated antiviral response in the absence of the virus, further confirming that phosphorylation of Thr‐474 is the crucial event that leads to the activation of the kinase.

Thioredoxins (Trxs) modulate metabolic responses during stress conditions; however, the mechanisms governing the responses of plants subjected to multiple drought events and the role of Trxs under these conditions are not well understood. Here we explored the significance of the mitochondrial Trx system in Arabidopsis following exposure to single and repeated drought events. We analyzed the previously characterized NADPH-dependent Trx reductase A and B double mutant (ntra ntrb) and two independent mitochondrial thioredoxin o1 (trxo1) mutant lines. Following similar reductions in relative water content (∼50%), Trx mutants subjected to two drought cycles displayed a significantly higher maximum quantum efficiency (Fv/Fm) and were less sensitive to drought than their wild-type counterparts and than all genotypes subjected to a single drought event. Trx mutant plants displayed a faster recovery after two cycles of drought, as observed by the higher accumulation of secondary metabolites and higher stomatal conductance. Our results indicate that plants exposed to multiple drought cycles are able to modulate their subsequent metabolic and physiological response, suggesting the occurrence of an exquisite acclimation in stressed Arabidopsis plants. Moreover, this differential acclimation involves the participation of a set of metabolic changes as well as redox poise alteration following stress recovery.

Summary: Nicotinamide adenine dinucleotide (NAD⁺) is an essential coenzyme required for all living organisms. In eukaryotic cells, the final step of NAD⁺ biosynthesis is exclusively cytosolic. Hence, NAD⁺ must be imported into organelles to support their metabolic functions. Three NAD⁺ transporters belonging to the mitochondrial carrier family (MCF) have been biochemically characterized in plants. AtNDT1 (At2g47490), focus of the current study, AtNDT2 (At1g25380), targeted to the inner mitochondrial membrane, and AtPXN (At2g39970), located in the peroxisomal membrane. Although AtNDT1 was presumed to reside in the chloroplast membrane, subcellular localization experiments with green fluorescent protein (GFP) fusions revealed that AtNDT1 locates exclusively in the mitochondrial membrane in stably transformed Arabidopsis plants. To understand the biological function of AtNDT1 in Arabidopsis, three transgenic lines containing an antisense construct of AtNDT1 under the control of the 35S promoter alongside a T‐DNA insertional line were evaluated. Plants with reduced AtNDT1 expression displayed lower pollen viability, silique length, and higher rate of seed abortion. Furthermore, these plants also exhibited an increased leaf number and leaf area concomitant with higher photosynthetic rates and higher levels of sucrose and starch. Therefore, lower expression of AtNDT1 was associated with enhanced vegetative growth but severe impairment of the reproductive stage. These results are discussed in the context of the mitochondrial localization of AtNDT1 and its important role in the cellular NAD⁺ homeostasis for both metabolic and developmental processes in plants.