Leaf growth in monocotyledons results from the flux of newly born cells out of the division zone and into the adjacent elongation-only zone, where cells reach their final length. We used a kinematic method to analyze the effect of phosphorus nutrition status on cell division and elongation parameters in the epidermis of Lolium perenne. Phosphorus deficiency reduced the leaf elongation rate by 39% due to decreases in the cell production rate (219%) and final cell length (220%). The former was solely due to a lower average cell division rate (0.028 versus 0.046 cell cell 21 h 21 ) and, thus, a lengthened average cell cycle duration (25 versus 15 h). The number of division cycles of the initial cell progeny (five to six) and, as a result, the number of meristematic cells (32-64) and division zone length were independent of phosphorus status. Accordingly, low-phosphorus cells maintained meristematic activity longer. Lack of effect of phosphorus deficiency on meristematic cell length implies that a lower division rate was matched to a lower elongation rate. Phosphorus deficiency did not affect the elongation-only zone length, thus leading to longer cell elongation duration (99 versus 75 h). However, the substantially reduced postmitotic average relative elongation rate (0.045 versus 0.064 mm mm 21 h 21) resulted in shorter mature cells. In summary, phosphorus deficiency did not affect the general controls of cell morphogenesis, but, by slowing down the rates of cell division and expansion, it slowed down its pace.

BackgroundCrop plants are highly sensitive to ambient temperature, with a 1 ºC difference in temperature sufficient to affect development and yield. Monocot crop plants are particularly vulnerable to higher temperatures during the reproductive and grain-filling phases. The molecular mechanisms by which temperature influences grain development are, however, unknown. In Arabidopsis thaliana, H2A.Z-nucleosomes coordinate transcriptional responses to higher temperature. We therefore investigated whether the effects of high temperature on grain development are mediated by H2A.Z-nucleosomes.ResultsWe have analyzed the thermal responses of the Pooid grass, Brachypodium distachyon, a model system for crops. We find that H2A.Z-nucleosome occupancy is more responsive to increases in ambient temperature in the reproductive tissue of developing grains compared withvegetative seedlings. This difference correlates with strong phenotypic responses of developing grain to increased temperature, including early maturity and reduced yield. Conversely, temperature has limited impact on the timing of transition from the vegetative to generative stage, with increased temperature unable to substitute for long photoperiod induction of flowering. RNAi silencing of components necessary for H2A.Z-nucleosome deposition is sufficient to phenocopythe effects of warmer temperature on grain development.ConclusionsH2A.Z-nucleosomes are important in coordinating the sensitivity of temperate grasses to increased temperature during grain development. Perturbing H2A.Z occupancy, through higher temperature or genetically, strongly reduces yield. Thus, we provide a molecular understanding of the pathways through which high temperature impacts on yield. These findings may be useful for breeding crops resilient to thermal stress.

This study tested whether leaf elongation rate ( LER , mm h) and its components -average relative elemental growth rate ( REGR avg , mm mm) and leaf growth zone length ( L LGZ , mm) -are related to phosphorus (P) concentration in the growth zone ( P LGZ , mg P g− 1 tissue water) of Lolium perenne L. cv. Condesa and whether such relationships are modified by the arbuscular mycorrhizal fungus (AMF) Glomus hoi . Mycorrhizal and non-mycorrhizal plants were grown at a range of P supply rates and analysed at either the same plant age or the same tiller size (defined by the length of the sheath of the youngest fully expanded leaf). Both improved P supply (up to 95%) and AMF (up to 21%) strongly increased LER . In tillers of even-aged plants, this was due to increased REGR avg and L LGZ . In even-sized tillers, it was exclusively due to increased REGR avg . REGR avg was strictly related to P LGZ ( r 2 = = = = 0.95) and independent of tiller size. Conversely, L LGZ strictly depended on tiller size ( r 2 = = = = 0.88) and not on P LGZ . Hence, P status affected leaf growth directly only through effects on relative tissue expansion rates. Symbiosis with AMF did not modify these relationships. Thus, no evidence for P status-independent effects of AMF on LER was found.Key-words : arbuscular mycorrhizal fungi; Glomus hoi ; leaf elongation rate; leaf growth zone; Lolium perenne L.; relative elemental growth rate Abbreviations : AMF, arbuscular mycorrhizal fungus/fungi; LER , leaf elongation rate; L LGZ , length of the leaf growth zone; P LGZ , concentration of P in leaf growth zone; REGR avg , average relative elemental growth rate; WSC, total water soluble carbohydrates.

Summary• Effects of the arbuscular mycorrhizal fungus (AMF) Glomus hoi on the carbon economy of perennial ryegrass ( Lolium perenne ) were investigated by comparing nonmycorrhizal and mycorrhizal plants of the same size, morphology and phosphorus status.• Plants were grown in the presence of CO 2 sources with different C isotope composition ( δ 13 C − 1‰ or − 44‰). Relative respiration and gross photosynthesis rates, and belowground allocation of C assimilated during one light period ('new C'), as well as its contribution to respiration, were quantified by the concerted use of 13 CO 2 / 12 CO 2 steady-state labelling and 13 CO 2 / 12 CO 2 gas-exchange techniques.• AMF ( G. hoi ) enhanced the relative respiration rate of the root + soil system by 16%, inducing an extra C flow amounting to 3% of daily gross photosynthesis. Total C flow into AMF growth and respiration was estimated at < 8% of daily gross photosynthesis. This was associated with a greater amount of new C allocated belowground and respired in mycorrhizal plants. AMF colonization affected the sources supplying belowground respiration, indicating a greater importance of plant C stores in supplying respiration and/or the participation of storage pools within fungal tissues.• When ontogenetic and nutritional effects were accounted for, AMF increased belowground C costs, which were not compensated by increased photosynthesis rates. Therefore the instantaneous relative growth rate was lower in mycorrhizal plants.

Nitrogen deficiency severely inhibits leaf growth. This response was analysed at the cellular level by growing Lolium perenne L. under 7.5 mM (high) or 1 mM (low) nitrate supply, and performing a kinematic analysis to assess the effect of nitrogen status on cell proliferation and cell growth in the leaf blade epidermis. Low nitrogen supply reduced leaf elongation rate (LER) by 43% through a similar decrease in the cell production rate and final cell length. The former was entirely because of a decreased average cell division rate (0.023 versus 0.032 h ). But, nitrogen stress did not affect the position where elongation stopped, and increased cell elongation duration. In conclusion, nitrogen deficiency limited leaf growth by increasing the cell cycle duration and decreasing mitotic and post-mitotic elongation rates, delaying cell maturation.

Summary• The aim of this work was to disentangle phosphorus status-dependent and -independent effects of arbuscular mycorrhizal fungus (AMF) on leaf morphology and carbon allocation in perennial ryegrass ( Lolium perenne ).• To this end, we assessed the P-response function of morphological components in mycorrhizal and nonmycorrhizal plants of similar size.• AMF ( Glomus hoi ) stimulated relative P-uptake rate, decreased leaf mass per area (LMA), and increased shoot mass ratio at low P supply. Lower LMA was caused by both decreased tissue density and thickness. Variation in tissue density was almost entirely caused by variations in soluble C, while that in thickness involved structural changes.• All effects of AMF were indistinguishable from those mediated by increases in relative P-uptake rate through higher P-supply rates. Thus the relationships between relative P-uptake rate, leaf morphology and C allocation were identical in mycorrhizal and nonmycorrhizal plants. No evidence was found for AMF effects not mediated by changes in plant P status. AbbreviationsAMF, arbuscular mycorrhizal fungus; LAR, leaf area ratio; LMR, leaf mass ratio; RGR, relative growth rate; RPUR, relative phosphorus-uptake rate; LMA, leaf mass per area; WSC, water-soluble carbohydrates.

Production of new leaf area was entirely reliant, during the first week after defoliation, on N stores present in the plant. Mobilized N originated mainly from free amino acids and soluble proteins located in roots, and less so from proteins in stubble. Presence of VSP in the roots was not confirmed. The data suggest that rhizomes played an important role in N transport but not in N storage.

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