Introgression from local cultivars is a driver of agricultural adaptation in Argentinian weedy rice.

Weedy rice, a pervasive and troublesome weed found across the globe, has often evolved through fertilization of rice cultivars with little importance of crop-weed gene flow. In Argentina, weedy rice has been reported as an important constraint since the early 1970s, and, in the last few years, strains with herbicide-resistance are suspected to evolve. Despite their importance, the origin and genetic composition of Argentinian weedy rice as well its adaptation to agricultural environments has not been explored so far. To study this, we conducted genotyping-by-sequencing on samples of Argentinian weedy and cultivated rice and compared them with published data from weedy, cultivated and wild rice accessions distributed worldwide. In addition, we conducted a phenotypic characterization for weedy-related traits, a herbicide resistance screening and genotyped accessions for known mutations in the acetolactate synthase (ALS) gene, which confers herbicide resistance. Our results revealed large phenotypic variability in Argentinian weedy rice. Most strains were resistant to ALS-inhibiting herbicides with a high frequency of the ALS mutation (A122T) present in Argentinian rice cultivars. Argentinian cultivars belonged to the three major genetic groups of rice: japonica, indica and aus while weeds were mostly aus or aus-indica admixed, resembling weedy rice strains from the Southern Cone region. Phylogenetic analysis supports a single origin for aus-like South American weeds, likely as seed contaminants from the United States, and then admixture with local indica cultivars. Our findings demonstrate that crop to weed introgression can facilitate rapid adaptation to agriculture environments.

Boosting underwater germination in Echinochloa colona seeds: the impact of high amplitude alternating temperatures and potassium nitrate osmopriming.

Underwater germination could risk seedling survival, suggesting the need for control through seed perception of environmental cues. These cues include diurnally alternating temperatures tied to drained soils or shallow water tables. We examined high-amplitude alternating temperatures impact on underwater germination. Besides, the conditions experimented by seeds in the soil (e.g. hydration/dehydration phases) change their germinability so we tested if osmopriming could affect underwater germination. We worked with Echinochloa colona seedlots from extensive crop fields, exposing seeds to sequential submergence and drained treatments in combination with cues that promote germination. While a 10°C difference between maximum and minimum daily temperatures maximised germination in drained conditions, higher amplitudes (>15°C) alternating temperatures promoted E. colona underwater germination under hypoxic water (pO2 <4.1kPa). KNO3 osmopriming in drained conditions promoted later underwater germination even under hypoxic water; however, PEG 6000 osmopriming induced seeds to enter secondary dormancy inhibiting underwater germination. KNO3 improved E. colona underwater germination under air-equilibrated floodwater (pO2 : 16.5-17.4kPa) yet not under hypoxic conditions. This suggests that germination can proceed in flooded nitrate-fertile soils as long as it remains aerobic. Hypoxic submergence did not inhibit the induction of hypersensitivity to light in E. colona seeds. This research expands our understanding of wetland seed germination ecophysiology, shedding light on the inducible nature of underwater germination in hydrophyte weeds.

Photoreceptor-mediated kin recognition in plants.

Although cooperative interactions among kin have been established in a variety of biological systems, their occurrence in plants remains controversial. Plants of Arabidopsis thaliana were grown in rows of either a single or multiple accessions. Plants recognized kin neighbours and horizontally reoriented leaf growth, a response not observed when plants were grown with nonkin. Plant kin recognition involved the perception of the vertical red/far-red light and blue light profiles. Disruption of the light profiles, mutations at the PHYTOCHROME B, CRYPTOCHROME 1 or 2, or PHOTOTROPIN 1 or 2 photoreceptor genes or mutations at the TRYPTOPHAN AMINOTRANSFERASE OF ARABIDOPSIS1 gene required for auxin (growth hormone) synthesis impaired the response. The leaf-position response increases plant self-shading, decreases mutual shading between neighbours and increases fitness. Light signals from neighbours are known to shape a more competitive plant body. Here we show that photosensory receptors mediate cooperative rather than competitive interactions among kin neighbours by reducing the competition for local pools of resources.

Balancing forces in the photoperiodic control of flowering.

In many plant species, the duration of the daily exposure to light (photoperiod) provides a seasonal cue that helps to adjust flowering time to the most favourable time of the year. In Arabidopsis thaliana, the core mechanism of acceleration of flowering by long days involves the stabilisation of the CONSTANS (CO) protein by light reaching the leaves, the direct induction of the expression of FLOWERING LOCUS T (FT) by CO and the migration of FT to the apex to promote flowering. In rice (Oryza sativa), the promotion of flowering by short days depends on the interplay between light conditions, and the genes Grain number, plant height and heading date locus 7 (Ghd7) and Early heading date 1 (Ehd1). In both cases, other day length-induced changes reinforce the core photoperiodic pathway of promotion of flowering. However, there are regulators of flowering time, quantitatively less important than the core pathways but still significant, which impact in the opposite direction, i.e. favouring rice flowering under long days or Arabidopsis flowering under short days. We show, for instance, that short days enhance leaf expression of SQUAMOSA PROMOTER BINDING PROTEIN-LIKE 3 (SPL3), which stimulates Arabidopsis flowering under these conditions. We propose that fine tuning of flowering time depends on the balance of a hierarchy of multiple points of action of photoperiod on the network controlling flowering.

Cryptochrome as a sensor of the blue/green ratio of natural radiation in Arabidopsis.

Green light added to blue light has been proposed to shift cryptochromes from their semireduced active form to the reduced, inactive state. Whether the increased proportion of green light observed under leaf canopies compared to open places reduces cryptochrome-mediated effects remained to be elucidated. Here we report that the length of the hypocotyl of Arabidopsis (Arabidopsis thaliana) seedlings grown under controlled conditions decreased linearly with increasing blue/green ratios of the light within the range of ratios found in natural environments. This effect was stronger under higher irradiances. We developed a model, parameterized on the basis of field experiments including photoreceptor mutants, where hypocotyl growth of seedlings exposed to different natural radiation environments was related to the action and interaction of phytochromes and cryptochromes. Adding the blue/green ratio of the light in the term involving cryptochrome activity improved the goodness of fit of the model, thus supporting a role of the blue/green ratio under natural radiation. The blue/green ratio decreased sharply with increasing shade by green grass leaves to one-half of the values observed in open places. The impact of blue/green ratio on cryptochrome-mediated inhibition of hypocotyl growth was at least as large as that of irradiance. We conclude that cryptochrome is a sensor of blue irradiance and blue/green ratio.

GIGANTEA regulates phytochrome A-mediated photomorphogenesis independently of its role in the circadian clock.

GIGANTEA (GI) is a nuclear protein involved in the promotion of flowering by long days, in light input to the circadian clock, and in seedling photomorphogenesis under continuous red light but not far-red light (FR). Here, we report that in Arabidopsis (Arabidopsis thaliana) different alleles of gi have defects in the hypocotyl-growth and cotyledon-unfolding responses to hourly pulses of FR, a treatment perceived by phytochrome A (phyA). This phenotype is rescued by overexpression of GI. The very-low-fluence response of seed germination was also reduced in gi. Since the circadian clock modulates many light responses, we investigated whether these gi phenotypes were due to alterations in the circadian system or light signaling per se. In experiments where FR pulses were given to dark-incubated seeds or seedlings at different times of the day, gi showed reduced seed germination, cotyledon unfolding, and activity of a luciferase reporter fused to the promoter of a chlorophyll a/b-binding protein gene; however, rhythmic sensitivity was normal in these plants. We conclude that while GI does not affect the high-irradiance responses of phyA, it does affect phyA-mediated very-low-fluence responses via mechanisms that do not obviously involve its circadian functions.

Blue Rhythms Between GIGANTEA and Phytochromes.

GIGANTEA (GI) is involved in the promotion of flowering by long days, in light input to the circadian clock, and in seedling de-etiolation under continuous red light or blue light but not under continuous far-red light (FR). Since red-light effects are mediated largely by phytochrome B and those of FR by phytochrome A (phyA), GI was considered not to affect phyA signaling. However, recent observations using brief FR pulses indicate that GI does affect the very-low-fluence response (VLFR) and not the high-irradiance response (HIR) pathway of phyA. Seed germination, seedling de-etiolation and gene expression showed rhythmic sensitivity to FR pulses. GI affected the magnitude but not the rhythmic pattern of the responses, indicating that GI regulation of phyA signaling does not derive from its effects on the clock. Here we show that despite de above divergence between VLFR and HIR, both pathways require the bZip transcription factor HY5. Furthermore, the blue-light phenotype of gi is not mediated by phyA, indicating a role of GI in the control of cryptochrome signaling. These findings illustrate features of plant signaling networks.