The interaction of two homeobox genes,BREVIPEDICELLUS and PENNYWISE,regulates internode patterning in the Arabidopsis inflorescence

Plant architecture results from the activity of the shoot apical meristem, which initiates leaves, internodes, and axillary meristems. KNOTTED1-like homeobox (KNOX) genes are expressed in specific patterns in the shoot apical meristem and play important roles in plant architecture. KNOX proteins interact with BEL1-like (BELL) homeodomain proteins and together bind a target sequence with high affinity. We have obtained a mutation in one of the Arabidopsis BELL genes, PENNYWISE (PNY), that appears phenotypically similar to the KNOX mutant brevipedicellus (bp). Both bp and pny have randomly shorter internodes and display a slight increase in the number of axillary branches. The double mutant shows a synergistic phenotype of extremely short internodes interspersed with long internodes and increased branching. PNY is expressed in inflorescence and floral meristems and overlaps with BP in a discrete domain of the inflorescence meristem where we propose the internode is patterned. The physical association of the PNY and BP proteins suggests that they participate in a complex that regulates early patterning events in the inflorescence meristem.     

The Arabidopsis BEL1-LIKE HOMEODOMAIN proteins SAW1 and SAW2 act redundantly to regulate KNOX expression spatially in leaf margins

In Arabidopsis thaliana, the BEL1-like TALE homeodomain protein family consists of 13 members that form heterodimeric complexes with the Class 1 KNOX TALE homeodomain proteins, including SHOOTMERISTEMLESS (STM) and BREVIPEDICELLUS (BP). The BEL1-like protein BELLRINGER (BLR) functions together with STM and BP in the shoot apex to regulate meristem identity and function and to promote correct shoot architecture. We have characterized two additional BEL1-LIKE HOMEODOMAIN (BLH) proteins, SAWTOOTH1 (BLH2/SAW1) and SAWTOOTH2 (BLH4/SAW2) that, in contrast with BLR, are expressed in lateral organs and negatively regulate BP expression. saw1 and saw2 single mutants have no obvious phenotype, but the saw1 saw2 double mutant has increased leaf serrations and revolute margins, indicating that SAW1 and SAW2 act redundantly to limit leaf margin growth. Consistent with this hypothesis, overexpression of SAW1 suppresses overall growth of the plant shoot. BP is ectopically expressed in the leaf serrations of saw1 saw2 double mutants. Ectopic expression of Class 1 KNOX genes in leaves has been observed previously in loss-of-function mutants of ASYMMETRIC LEAVES (AS1). Overexpression of SAW1 in an as1 mutant suppresses the as1 leaf phenotype and reduces ectopic BP leaf expression. Taken together, our data suggest that BLH2/SAW1 and BLH4/SAW2 establish leaf shape by repressing growth in specific subdomains of the leaf at least in part by repressing expression of one or more of the KNOX genes.     

Regulation of compound leaf development in Medicago truncatula by fused compound leaf1, a class M KNOX gene

Medicago truncatula is a legume species belonging to the inverted repeat lacking clade (IRLC) with trifoliolate compound leaves. However, the regulatory mechanisms underlying development of trifoliolate leaves in legumes remain largely unknown. Here, we report isolation and characterization of fused compound leaf1 (fcl1) mutants of M. truncatula. Phenotypic analysis suggests that FCL1 plays a positive role in boundary separation and proximal-distal axis development of compound leaves. Map-based cloning indicates that FCL1 encodes a class M KNOX protein that harbors the MEINOX domain but lacks the homeodomain. Yeast two-hybrid assays show that FCL1 interacts with a subset of Arabidopsis thaliana BEL1-like proteins with slightly different substrate specificities from the Arabidopsis homolog KNATM-B. Double mutant analyses with M. truncatula single leaflet1 (sgl1) and palmate-like pentafoliata1 (palm1) leaf mutants show that fcl1 is epistatic to palm1 and sgl1 is epistatic to fcl1 in terms of leaf complexity and that SGL1 and FCL1 act additively and are required for petiole development. Previous studies have shown that the canonical KNOX proteins are not involved in compound leaf development in IRLC legumes. The identification of FCL1 supports the role of a truncated KNOX protein in compound leaf development in M. truncatula.     

Key players associated with tuberization in potato: potential candidates for genetic engineering

Tuberization in potato (Solanum tuberosum L.) is a complex biological phenomenon which is affected by several environmental cues, genetic factors and plant nutrition. Understanding the regulation of tuber induction is essential to devise strategies to improve tuber yield and quality. It is well established that short-day photoperiods promote tuberization, whereas long days and high-temperatures inhibit or delay tuberization. Worldwide research on this complex biological process has yielded information on the important bio-molecules (proteins, RNAs, plant growth regulators) associated with the tuberization process in potato. Key proteins involved in the regulation of tuberization include StSP6A, POTH1, StBEL5, StPHYB, StCONSTANS, Sucrose transporter StSUT4, StSP5G, etc. Biomolecules that become transported from “source to sink” have also been suggested to be important signaling candidates regulating the tuberization process in potatos. Four molecules, namely StSP6A protein, StBEL5 RNA, miR172 and GAs, have been found to be the main candidates acting as mobile signals for tuberization. These biomolecules can be manipulated (overexpressed/inhibited) for improving the tuberization in commercial varieties/cultivars of potato. In this review, information about the genes/proteins and their mechanism of action associated with the tuberization process is discussed.