Role for the shoot apical meristem in the specification of juvenile leaf identity in Arabidopsis

The extent to which the shoot apical meristem (SAM) controls developmental decisions, rather than interpreting them, is a longstanding issue in plant development. Previous work suggests that vegetative phase change is regulated by signals intrinsic and extrinsic to the SAM, but the relative importance of these signals for this process is unknown. We investigated this question by examining the effect of meristem-deficient mutations on vegetative phase change and on the expression of key regulators of this process, miR156 and its targets, SPL transcription factors. We found that the precocious phenotypes of meristem-deficient mutants are a consequence of reduced miR156 accumulation. Tissue-specific manipulation of miR156 levels revealed that the SAM functions as an essential pool of miR156 early in shoot development, but that its effect on leaf identity declines with age. We also found that SPL genes control meristem size by repressing WUSCHEL expression via a novel genetic pathway.

Ecological strategies of Al-accumulating and non-accumulating functional groups from the cerrado sensu stricto

The cerrado's flora comprises aluminum-(Al) accumulating and non-accumulating plants, which coexist on acidic and Al-rich soils with low fertility. Despite their existence, the ecological importance or biological strategies of these functional groups have been little explored. We evaluated the leaf flushing patterns of both groups throughout a year; leaf concentrations of N, P, K, Ca, Mg, S, Al, total flavonoids and polyphenols; as well as the specific leaf area (SLA) on young and mature leaves within and between the groups. In Al-accumulating plants, leaf flushed throughout the year, mainly in May and September; for non-accumulating plants, leaf flushing peaked at the dry-wet seasons transition. However, these behaviors could not be associated with strategies for building up concentrations of defense compounds in leaves of any functional groups. Al-accumulating plants showed low leaf nutrient concentrations, while non-accumulating plants accumulated more macronutrients and produced leaves with high SLA since the juvenile leaf phase. This demonstrates that the increase in SLA is slower in Al-accumulating plants that are likely to achieve SLA values comparable to the rest of the plant community only in the wet season, when sunlight capture is important for the growth of new branches.

Genetic Control of Reproductive and Vegetative Phase Change in the Eucalyptus risdonii - E. tenuiramis Complex

Eucalyptus risdonii Hook.f. is believed to be a juvenilised form of its sister species, E. tenuiramis Miq., differing largely in the retention of the juvenile leaf type at reproductive maturity. The genetic basis of this ontogenetic variation was examined by monitoring reproductive and vegetative phase changes in 1201 open-pollinated progeny from 40 E. risdonii–E. tenuiramis populations in a field trial over 6 years. Vegetative and reproductive phase changes were highly heritable and genetically independent within populations. Estimates of individual narrow-sense heritabilities for height and timing of vegetative phase change ranged from 0.46–0.67 and 0.19–0.23 respectively, and for time of first flowering from 0.31–0.41. Variation in the height of vegetative phase change amongst progeny grown in a common environment was very similar to that observed in the natural populations from different environments, demonstrating a genetic basis to a stepped cline in the retention of the juvenile leaf form (neoteny). However, a separate pattern of variability in the time to flowering was evident, with precocious flowering found in a number of phenetic groups. This independent variation of reproductive and vegetative phase changes may allow dramatic heterochronic alterations in morphology and physiology with minimal genetic change. The continuous nature of the neotenic variation suggests that speciation by this mode of evolution is not yet complete in the E. risdonii–E. tenuiramis complex, but has presumably operated to produce many other neotenous eucalypt species.