SummaryGenetic evidence links the Arabidopsis MONOPTEROS (MP) and PIN-FORMED1 (PIN1) genes to the patterning of leaf veins. To elucidate their potential functions and interactions in this process, we have assessed the dynamics of MP and PIN1 expression during vascular patterning in Arabidopsis leaf primordia. Both genes undergo a dynamic process of gradual refinement of expression into files one to two cells wide before overt vascular differentiation. The subcellular distribution of PIN1 is also gradually refined from a non-polar distribution in isodiametric cells to strongly polarized in elongated procambial cells and provides an indication of overall directions of auxin flow. We found evidence that MP expression can be activated by auxin exposure and that PIN1 as well as DR5::GUS expression is defective in mp mutant leaves. Taken together the results suggest a feedback regulatory loop that involves auxin, MP and PIN1 and provide novel experimental support for the canalization-of-auxin-flow hypothesis.

The framework of the root system of a mature, field-grown corn plant of variety Seneca Chief consists of about 70 axile roots. One of these is the primary root. The others develop on the stem, a single tier at each of the seven basal nodes. Just over half of these roots grow out at or above ground level from nodes 6 and 7 late in the development of the plant, with those of node 7 entering the soil just before flowering. The mean diameter of the root produced at successively higher nodes increases, as does also the mean number of large metaxylem elements seen in a cross section, so that about 75% of the large xylem conduits between the root system and the stem are in the roots of the two uppermost tiers. Nodal root primordia develop initially in situ from an extensive region of dedifferentiated stem cortex. A sleeve-like extension of the stem encloses the base of each root formed at aerial nodes. At each node the complexity of vascular interconnections results in all of the framework roots being indirectly linked to each other and to the vascular traces from all of the leaves.

We conducted kinematic and cytological studies on “between vein” epidermal cells of the gibberellin (GA)-deficient M489 dwarf mutant of barley (Hordeum vulgare L. Himalaya). GAs affect radial and axial components of cell expansion and cortical microtubule orientation. Adaxial cells in particular expand radially after leaving the elongation zone (EZ), probably as part of leaf unrolling. Exogenous gibberellic acid corrects the mutant's short, wide blades, short EZ, and slow elongation rate. Cell production rates increase more on the adaxial than on the abaxial surface. Cells spend equal periods of time elongating in dwarf and tall plants, but relative elemental growth rates start to decline sooner in the dwarf. GA increased the rate at which longitudinal wall area increased because the increased axial growth more than compensated for reduced radial growth. In dwarf leaves, increased radial expansion was detected in basal parts of the EZ before cortical microtubules lost transverse orientation in the distal elongation zone. We conclude that loss of microtubule orientation is not required for low GA levels to reduce growth anisotropy.

SUMMARYBacteria isolated from proteoid roots and non-proteoid lateral roots of Telopea speciosissima (Sm.) R.Br. seedlings were able to acidify the medium and solubilize calcium v^hosphates when grown in culture in the presence of ammonium .salts and an appropriate carbon source. In general this activity was not detected when NO3" was substituted for NH,,', and it is proposed that protons were secreted in exchange for ammonium ions. Cation exchange between these protons and calcium in the medium is a possible cause of the calcium phosphate solubilizing activity. The relevance of these data to phosphate-solubilizing activity in natural environments is discussed.

Development of the primary and early nodal roots was studied in Zea mays L., Zea mexlcana (Schrad.) Soil or sand sheaths encasing roots of grasses have been described as features found in desert and dry environments (13). More recently they have been described on corn (12) and mesophytic grasses in humid environments (3). Striking illustrations of such sheaths will be found in these three papers. The sheathed regions of corn roots coincide with the regions of immature xylem where LMX2 elements are still alive (1 1). This region stretches much further back (up to 50 cm from the apex) than has been supposed. The present paper extends the association of soil sheaths with the immaturity of the LMX to four other C4 grasses, and explores the likely effect of the nonfunctioning xylem on water supply to the shoot.It is widely believed that the permeability of roots to water is much greater in the axial direction (P,) than in the radial direction (P,). The finding (11) that, in axile roots of field-grown corn, maturation of LMX is long-delayed, raised the question when in the development of the seedling the full axial permeability became operative. Each nodal root goes through a stage when it bears a coherent soil sheath for up to half a meter proximal to the bare elongating tip (2-5 mm), and has inside immature LMX (with cross walls and cytoplasm) which cannot engage in transporting water more efficiently than any other file of living cells (7). The longitudinal water permeability of such roots is thus limited to the outer ring of small (20 ,um diameter) EMX elements, which are open vessels to within 10 cm from the tip (2). When the LMX elements lose their crosswalls and become open, and the soil sheath disperses leaving the root bare, the longitudinal permeability ofthe root will increase by a factor of at least 100 (1 1). If the first formed seedling roots showed a similar slow maturation oftheir LMX there could be a stage when the primary and early nodal roots all had only their EMX vessels open. The time at which one or more of these early roots opened their LMX would herald a dramatic change in the availability ofwater to the developing shoot. The sequence and numbers ofdeveloping roots in corn have been described (4) for the variety Seneca Chief. The primary root is followed closely by a tier of four roots from the coleoptilar node, and a little later by four to five roots from the second node. Roots ofeach tier are even-aged. Roots of successive tiers (usually seven tiers) go through similar stages ofdevelopment to the earlier lower ones, from sheathed to bare. We determined to follow the development of the primary and early tiers of nodal roots, recording the presence and extent of soil sheaths, the presence or absence of crosswalls and cytoplasm in the LMX and the stage oflignification of the vessel sidewalls. These measurements would be related to the number and size of the leaves. Measurements of stomatal resistance on days of high evaporative demand might reveal whether at any stage the young seedl...