Reinvestigation of grain weight genes TaTGW6 and OsTGW6 casts doubt on their role in auxin regulation in developing grains

The THOUSAND-GRAIN WEIGHT 6 genes (TaTGW6 and OsTGW6) are reported to result in larger grains of wheat and rice by reducing production of indole-3-acetic acid (IAA) in developing grains. However, a critical comparison of data on TaTGW6 and OsTGW6 with other reports on IAA synthesis in cereal grains requires that this hypothesis be reinvestigated. Here, we show that TaTGW6 and OsTGW6 are members of a large gene family that has undergone major, lineage-specific gene expansion. Wheat has nine genes, and rice three genes encoding proteins with more than 80% amino acid identity with TGW6 making it difficult to envisage how a single inactive allele could have a major effect on IAA levels. TGW6 is proposed to affect auxin levels by catalysing the hydrolysis of IAA-glucose (IAA-Glc). However, we show that developing wheat grains contain undetectable levels of ester IAA in comparison to free IAA and do not express an IAA-glucose synthase. Previous work on TGW6, reported maximal expression at 20 days after anthesis (DAA) in wheat and 2 DAA in rice. However, we show that neither gene is expressed in developing grains. Instead, TaTGW6, OsTGW6 and their close homologues are exclusively expressed in pre-emergence inflorescences; TaTGW6 is expressed particularly in microspores prior to mitosis. This combined with evidence for high levels of IAA production from tryptophan in developing grains demonstrates TaTGW6 and OsTGW6 cannot regulate grain size via the hydrolysis of IAA-Glc. Instead, their similarity to rice strictosidine synthase-like (OsSTRL2) suggests they play a key role in pollen development.

Auxin Regulation and MdPIN Expression during Adventitious Root Initiation in Apple Cuttings

Adventitious root (AR) formation is critical for the efficient propagation of elite horticultural and forestry crops. Despite decades of research, the cellular processes and molecular mechanisms underlying AR induction in woody plants remains obscure. We examined the details of AR formation in the apple (Malus domestica) M.9 rootstock, the most widely used dwarf rootstock for intensive production, and investigated the role of polar auxin transport in post-embryonic organogenesis. AR formation begins with a series of founder cell divisions and elongation of interfascicular cambium adjacent to vascular tissues. This process was associated with a relatively high indole acetic acid (IAA) content and hydrolysis of starch grains. Exogenous auxin treatment promoted cell division, as well as the proliferation and reorganization of the endoplasmic reticulum and Golgi membrane. By contrast, treatment with the auxin transport inhibitor N-1-naphthylphthalamic acid (NPA) inhibited cell division in the basal region of the cutting and resulted in abnormal cell divisions during early AR formation. In addition, PIN-FORMED (PIN) transcripts were expressed differentially throughout the whole AR development process, with the up-regulation of MdPIN8 and MdPIN10 during induction, an up-regulation of MdPIN4, MdPIN5 and MdPIN8 during extension, and an up-regulation of all MdPINs during AR initiation. This research provides a deeper understanding of the cellular and molecular underpinnings of the AR process in woody plants.

Auxin regulation involved in gynoecium morphogenesis of papaya flowers

The morphogenesis of gynoecium is crucial for propagation and productivity of fruit crops. For trioecious papaya (Carica papaya), highly differentiated morphology of gynoecium in flowers of different sex types is controlled by gene networks and influenced by environmental factors, but the regulatory mechanism in gynoecium morphogenesis is unclear. Gynodioecious and dioecious papaya varieties were used for analysis of differentially expressed genes followed by experiments using auxin and an auxin transporter inhibitor. We first compared differential gene expression in functional and rudimentary gynoecium at early stage of their development and detected significant difference in phytohormone modulating and transduction processes, particularly auxin. Enhanced auxin signal transduction in rudimentary gynoecium was observed. To determine the role auxin plays in the papaya gynoecium, auxin transport inhibitor (N-1-Naphthylphthalamic acid, NPA) and synthetic auxin analogs with different concentrations gradient were sprayed to the trunk apex of male and female plants of dioecious papaya. Weakening of auxin transport by 10 mg/L NPA treatment resulted in female fertility restoration in male flowers, while female flowers did not show changes. NPA treatment with higher concentration (30 and 50 mg/L) caused deformed flowers in both male and female plants. We hypothesize that the occurrence of rudimentary gynoecium patterning might associate with auxin homeostasis alteration. Proper auxin concentration and auxin homeostasis might be crucial for functional gynoecium morphogenesis in papaya flowers. These results will lead to further investigation on the auxin homeostasis and gynoecium morphogenesis in papaya.

Constitutive auxin response in Physcomitrella reveals complex interactions between Aux/IAA and ARF proteins

The coordinated action of the auxin-sensitive Aux/IAA transcriptional repressors and ARF transcription factors produces complex gene-regulatory networks in plants. Despite their importance, our knowledge of these two protein families is largely based on analysis of stabilized forms of the Aux/IAAs, and studies of a subgroup of ARFs that function as transcriptional activators. To understand how auxin regulates gene expression we generated a Physcomitrella patens line that completely lacks Aux/IAAs. Loss of the repressors causes massive changes in transcription with misregulation of over a third of the annotated genes. Further, we find that the aux/iaa mutant is blind to auxin indicating that auxin regulation of transcription occurs exclusively through Aux/IAA function. We used the aux/iaa mutant as a simplified platform for studies of ARF function and demonstrate that repressing ARFs regulate auxin-induced genes and fine-tune their expression. Further the repressing ARFs coordinate gene induction jointly with activating ARFs and the Aux/IAAs.

Seasonal variation of auxin in stem cambial region of Pinus sihestris L.

Investigation of auxin from the stem cambial region of adult <i>Pinus silvestris</i> trees by diffusion to agar, and by extraction revealed both seasonal variation and gradients along the stem. Variation in dynamics of auxin efflux and the effect of the size of tissue blocks are used as arguments in the critical discussion of the concept of auxin regulation of cambial activity by way of simple seasonal concentration changes.