Does variable epigenetic inheritance fuel plant evolution?

Epigenetic changes influence gene expression and contribute to the modulation of biological processes in response to the environment. Transgenerational epigenetic changes in gene expression have been described in many eukaryotes. However, plants appear to have a stronger propensity for inheriting novel epialleles. This mini-review discusses how plant traits, such as meristematic growth, totipotency, and incomplete epigenetic erasure in gametes promote epiallele inheritance. Additionally, we highlight how plant biology may be inherently tailored to reap the benefits of epigenetic metastability. Importantly, environmentally triggered small RNA expression and subsequent epigenetic changes may allow immobile plants to adapt themselves, and possibly their progeny, to thrive in local environments. The change of epigenetic states through the passage of generations has ramifications for evolution in the natural and agricultural world. In populations containing little genetic diversity, such as elite crop germplasm or habitually self-reproducing species, epigenetics may provide an important source of heritable phenotypic variation. Basic understanding of the processes that direct epigenetic shifts in the genome may allow for breeding or bioengineering for improved plant traits that do not require changes to DNA sequence.

Chemical Potential-Induced Wall State Transitions in Plant Cell Growth

The pH/T duality of acidic pH and temperature (T) action for the growth of grass shoots was examined in order to derive the phenomenological equation of wall properties for living plants. By considering non-meristematic growth as a dynamic series of state transitions (STs) in the extending primary wall, the critical exponents were identified, which exhibit a singular behaviour at a critical temperature, critical pH and critical chemical potential (μ) in the form of four power laws: $$f_{\pi } \left( \tau \right) \propto \left| \tau \right|^{\beta - 1}$$ f π τ ∝ τ β - 1 , $$f_{\tau } (\pi ) \propto \left| \pi \right|^{1 - \alpha }$$ f τ ( π ) ∝ π 1 - α , $$g_{\mu } (\tau ) \propto \left| \tau \right|^{ - 2 - \alpha + 2\beta }$$ g μ ( τ ) ∝ τ - 2 - α + 2 β and $$g_{\tau } (\mu ) \propto \left| \mu \right|^{2 - \alpha }$$ g τ ( μ ) ∝ μ 2 - α . The indices α and β are constants, while π and τ represent a reduced pH and reduced temperature, respectively. The convexity relation α + β ≥ 2 for practical pH-based analysis and β ≡ 2 “mean-field” value in microscopic (μ) representation were derived. In this scenario, the magnitude that is decisive is the chemical potential of the H+ ions, which force subsequent STs and growth. Furthermore, observation that the growth rate is generally proportional to the product of the Euler beta functions of T and pH, allowed to determine the hidden content of the Lockhart constant Ф. It turned out that the pH-dependent time evolution equation explains either the monotonic growth or periodic extension that is usually observed—like the one detected in pollen tubes—in a unified account.

FACTORS CONTROLLING INITIATION AND ORIENTATION OF THE MACROCOTYLEDON IN ANISOCOTYLOUS GESNERIACEAE

Anisocotyly, the prolonged meristematic growth of one of the two cotyledons, is a distinctive feature of Gesneriaceae subfam. Cyrtandroideae. The larger cotyledon, the macrocotyledon, often grows to resemble a normal foliage leaf, and in some taxa may be the only foliar organ of the plant. This raises a number of questions. Which cotyledon becomes the macrocotyledon? Is this pre-determined in the embryo or differentiated after germination? Which external factors such as gravity and light are involved? The observation that the macrocotyledons of unifoliate Gesneriaceae growing on steep rocks mostly point downwards suggests that gravity is involved, but it is not clear whether it plays an initial determinant role or merely later influences orientation. In order to identify possible controlling factors, several experiments were performed, mostly on material of Chirita lavandulacea and Streptocarpus rexii. All seedlings responded significantly to light, the cotyledon further from the light source becoming the macrocotyledon. Seedlings growing in inclined pots with the light source below the pots mostly developed an upper macrocotyledon. The explanation proposed is that this cotyledon receives more light when the two cotyledons unfold after germination. Later on, apparently due to gravity, Chirita seedlings showed re-orientation with the macrocotyledons ultimately pointing downwards, though in Streptocarpus no such downwards re-orientation was observed. This difference is probably correlated to a difference in hypocotyl structure. Our conclusion is that while light is the initial factor controlling macrocotyledon development, gravity may cause re-orientation in some species.

Morphological reconstruction ofMiaohephyton bifurcatum, a possible brown alga from the Neoproterozoic Doushantuo Formation, South China

On the basis of morphological and taphonomic study of a large sample population,Miaohephyton bifurcatumSteiner, emend. from the terminal Proterozoic Doushantuo Formation (600-550 Ma), South China, is interpreted as algal fragments shed from their parent thalli for reproductive or environmental reasons. Characters such as regularly dichotomous, multicellular thalli with forked tips, apical and intercalary meristematic growth, abscission structures, and possible conceptacles collectively suggest an affinity with the brown algae, in particular the order Fucales. In conjunction with reports of xanthophyte fossils in older Neoproterozoic rocks, this reinterpretation ofMiaohephyton bifurcatumindicates that photosynthetic stramenopiles (chrysophytes, synurophytes, xanthophytes, phaeophytes, and diatoms; or chromophytes sensu stricto) diversified during the Neoproterozoic Era along with the red and green algae. This, in turn, suggests that the secondary endosymbiosis that gave rise to the photosynthetic stramenopiles took place relatively soon after the evolutionary transformation of cyanobacteria to rhodophyte plastids.

Effects of Illuminance and Time on Accumulation of Glyphosate in Johnsongrass (Sorghum halepense)

Translocation of14C-glyphosate [N-(phosphonomethyl) glycine] in johnsongrass [Sorghum halepense(L.) Pers.] was examined in relation to time and illuminance. Plants were treated with14C-glyphosate and harvested 3 days and 6 days after treatment. Translocation increased significantly between the 3 day and 6 day harvest. The greatest accumulation of14C occurred in areas of meristematic growth. The concentration of14C recovered in rhizomes nearly doubled between the 3 day and 6 day harvest. Plants harvested after 3 days of treatment in the dark translocated 1.3% of the applied14C, while plants exposed to full light translocated 4.7%. Plants harvested after 6 days of treatment in the dark translocated 2.1% of the applied14C, while plants exposed to full light translocated 5.8%. Translocation data indicated that movement of glyphosate in johnsongrass was via the phloem. Accumulation of14C in roots and rhizomes was significantly greater in plants exposed to full light compared with plants exposed to either darkness or shade.

FURTHER STUDIES ON THE BALANCE BETWEEN Adh1 AND Adh2 IN MAIZE: GENE COMPETITIVE PROGRAMS

ABSTRACT Two unlinked genes which specify alcohol dehydrogenase (ADH) enzymes in maize are coordinately regulated by competition for a factor which limits the rate of total ADH expression during anaerobiosis. The "gene competition hypothesis" and the existence of organ-specific competitive programs, as proposed by Schwartz (1971), is further supported. The potential balance of expression between these two genes appears to be organ-specific and may be "locked-in" and inherited via meristematic growth. The actual expression of these two genes is dependent on the mode of ADH induction. The discussion examines alternative hypotheses explaining gene competition and reinterprets extant data on the adaptive significance of maize ADH polymorphism.